HMT Forecast

Our "zonal" storm for mid-week starts in the GFS at 18Z Wednesday and yields
.3" before ending 12Z Thursday. Moisture from this storm is already yielding
some echoes over the NW CA coast. Once again higher amounts (about double)
appear in the model closer to the coast. The GFS ensemble goes up slightly
to .6" over the ARB. The UKMO and AFWA/MM5 models are similar to the GFS
precip amounts. The NAM starts much earlier (about 6Z) Wednesday
and does deliver 1.25" by 18Z Thursday with the possiblity of some
intermittent precip beyond that until short wave ridging shuts the precip
off on Friday. The HPC forecast has amounts in agreement with the NAM. The
narrow high PW plume is on track having amounts of about 1.5" reaching the
CA coast near SFO at 00Z Thursday. Thickness values are about 552dm during
this event. This is probably our best IOP opportunity for at least the next
week, particularly if we get some good orographic enhancement to supplement
the generally weak dynamics. As an example the NAM gives about 2" on the
mountain crests compared with the 1.25" more generally in the basin.
There are a couple of weak upstream waves visible on satellite north of
the moisture plume that might be of some dynamical assistance.

The Saturday storm begins about 21Z in the GFS with good dynamics in the
OR/WA coasts. This progressive shortwave brings in a 500mb low to the BC coast.
The ARB is on the southern edge of a short wave that breaks off from the
main low to the north, receiving about .2" precip. This could be somewhat
more if the wave digs slightly to the south.

In the long range the GFS goes one step further than yesterday's run
in developing an omega block just off the west coast by the middle of next
week. This may shut off our IOP chances for a while.

Steve Albers

Posted by Forecast Editor at 08:04 PM | Permalink

Today's system in the GFS still has about .6" of precip, with a shorter
duration as presaged by yesterday's NAM run. Precipitation is currently
starting up and should continue until about 04Z Tuesday. The NAM today
also has about .6".

The mid-week storm in zonal flow starts with light precip in the GFS about
09Z Wednesday, then picking up and continuing until about 15Z Thursday.
The GFS dumps out only .5" over the ARB, yet has closer to an inch just
to the west near Sacremento and San Francisco. This appears to be a
reflection of the narrow high PW plume (nearly 1.5") aimed right at San Francisco,
so if this moisture can make it inland we may get a bit more precip than the GFS
advertises. The CMC regional model does give a bit over an inch over the ARB.
Thicknesses are steady at about 552dm during this event. The RFC is going for
a 2" event (more in line with the NAM and AFWA/MM5 models) so this appears
to be worth the gamble for an IOP as it is likely to be the best event for a while.

The Saturday storm in the GFS shows a higher amplitude progressive short-wave
beginning about 15Z and continuing only until 22Z with a quick heavy shot
of about .6".

In the long range the following system may have to fight a developing west
coast ridge so next week may have less precip over than the ARB compared
with this week. The development of a Rex block over CA is even possible.
Details of this are different in the ECMWF however.

Steve Albers

Posted by Forecast Editor at 08:03 PM | Permalink

The parade of moderate intensity systems continues. The next one in the GFS90
starts about 12Z Monday and yields about .65" of precip ending about 12Z
Tuesday. Thicknesses are moderate and cooling from 552dm to 546dm during the
precip. The GFS ensemble has about the same amount of precip. The NAM has the
southern portion of the wave splitting and amplifying more markedly. It shows
the precip starting later and ending earlier with amounts around .80". We are
still a bit below IOP criteria.

The main mid-week system still starts about 00Z Wednesday and extends to about
18Z Thursday in the GFS90. Precip amounts are about .65" again agreeing
with the GFS ensemble. Thicknesses stay fairly warm at 552dm. This system does
have support from a narrow plume of PW over 1" extending from Central CA all
the way into the tropics so could still end up slightly wetter.

In the longer range we have the next system on Saturday. Here the GFS shows
a stronger wave compared with the ECMWF. If the GFS verifies we could get
a healthy shot of dynamics with 1.5" of precip, despite PW values hovering
just around 1".

Steve Albers

Posted by Forecast Editor at 08:01 PM | Permalink

Several systems of interest during the forecast period, the main question for each is one of intensity. Today's short wave is on track with forecast precip amounts around 3/8" in the 12Z GFS, yet closer to 1" in the 12Z NAM, still a bit below IOP criteria for a warmer system.

On Sunday the ARB is on the southern edge of an approaching stronger shortwave impinging on OR and WA. As the cold front sags south on Monday, 12hr precip amounts reach .75" over the ARB in the GFS run and even close to 1" in the NAM. This is somewhat wetter than some of yesterday's forecasts, though would still be below IOP criteria. This is the same wave that as mentioned yesterday, is minoring out as it splits off to the south on Tuesday.

We still are looking at the longer wave system in the EastPac Wednesday with a good fetch of WSW 500mb flow ahead of it carrying a plume of moisture. The GFS continues to suggest light precip beginning early Wednesday, then the long lived event reaches moderate intensities on Thursday. The positioning of the narrow plume will be important as a ridge can possibly poke up a bit from the south, especially compared with yesterday's model runs. This event will end Thursday night and may become strong enough locally over the ARB to warrant an IOP if it stays far enough south. Andy Edman points out that the GFS has not been bringing storms as far south as it should.

Steve Albers

Posted by Forecast Editor at 10:23 PM | Permalink

A weak wave is moving over the forecast area today with precip amounts
up to about .10".

A moderately strong short wave is on track to come in over the weekend
and still looks a bit under IOP criteria. The GFS has the timing
of the event from about 08Z Saturday until 10Z Sunday with about 3/4
inch of liquid precip progged. The GFS ensemble mean is closer to 1/2 inch.
The AWIPS derived snow product yields about 10" of snow in the highest
elevations from this model run. Thicknesses are running about 546dm during
the event. This is therefore a relatively warm event with the main thrust of
the wave passing to the north. Best dynamics are near Seattle with
a 500mb low sitting just offshore NW British Columbia. For comparison,
the NAM80 run has the timing from 15Z Saturday with lingering precip until
mid-day Sunday with only about 1/3 inch of precip..

Longer range 12Z GFS90 forecast run suggests a split wave whose southern
branch comes on in early Tuesday with light-moderate precip.
The second mid-week wave sets up more as a plume of moisture in a flat
wave for a more prolonged period of moderate precipitation from Wednesday
through Thursday night. The PW field shows a long narrow plume of PW over 30mm
reaching the San Francisco coast during this event.

Steve Albers

Posted by Forecast Editor at 07:43 PM | Permalink

Looks like a weak short wave moving east of the American River Basin
as of mid-day with occasional light open cellular precipitation of less than
.10" still possible. The WRF-NMM run here at GSD is going for a bit more
precip over the higher terrain this afternoon though we'll have to see how
this verifies. A similar wave passes by about 18Z Friday. The cirrus
cloud shield from this next wave is just now reaching the NW California coast.

A stronger system comes in over the weekend though it may be a bit
under IOP criteria. The GFS and NAM appear to be on a similar track for
starting the event about 13Z Saturday in both models and ending about 08Z
Sunday (for the GFS). The deterministic GFS has about 0.9" of precip for
the event with ensemble members showing perhaps a +/-50% scatter. This
should be a relatively warm event with the main thrust of the wave
passing to the north.

Longer range 00Z GFSX forecast run suggests a moisture plume and two associated
short waves reaching northern California and Oregon mid-week. The question then
will be if enough precip reaches the southern edge of this system over
the American River Basin. Perhaps late in the life-cycle of this system it
could push a bit south if it holds together. The overall pattern remains
with moderately good moisture plumes (as seen in the GFS PW animation)
heading to Oregon and Washington, then weakening as they move south.
The best surges of PW over the forecast area would occur on
Tuesday and Thursday of next week.

Steve Albers

Posted by Forecast Editor at 08:13 PM | Permalink

One of the things we ESRL forecasters like to look at for the HMT project is the GFS precipitable water image loops (U Hawaii has a real nice web page). Really good precipitation events in the ARB sometimes have a very distinctive signature of a “river” of water vapor, a long contiguous stream that can reach thousands of miles. During the excellent events of December we had a great setup that way, with the river connected all the way to the Phillipines. The good news is that river is getting re-established after several weeks of interruption. The bad news is that it’s aimed at Oregon, not the ARB.
For the next week or so, the forecast question is whether/when the action will sag south far enough to reach the ARB.

For the first time in a week or more the GFS ensemble members are in good agreement through the next five days or so. Today’s wave moves through without much excitement, and the next possible event would be starting overnight Saturday and into Sunday. QPF amounts in the ensemble members are up to .5”, although several of the members have no precipitation at all. Given the updated IOP criteria suggested yesterday (2” for a warm event, 1” for a cold/convective event), we should probably let this one go by too.

Paul Schultz

Posted by Forecast Editor at 07:45 PM | Permalink

Very dry conditions in the eastern Pacific are giving way to much healthier vapor amounts, leading to precipitation in northern California, starting overnight Wednesday into Thursday, but probably very little if any in the ARB on Thursday. The NAM model puts .10” or so in the ARB on Friday but it’s a quick event; the GFS ensemble has very few members that support even that much precipitation on Friday. Right now it looks like Oregon gets all the action, at least through the weekend, although some ensemble members put a little precipitation in the target area on Sunday.

Paul Schultz

Posted by Forecast Editor at 07:44 PM | Permalink

All indications point to dry weather today, Tuesday, and through the daylight hours of Wednesday. Models are finally coming to some consensus about the Wednesday night / Thursday event we’ve been pointing to for a full week, and now that it’s getting near it appears to be a quick shot borne on an open wave, with little more than maybe .1” of precipitation in the American River Basin. Fully half the ensemble members bring no precipitation at all. There is some suggestion, particularly in ECMWF model runs, that there could be an active period beginning this Friday / Saturday.

Paul Schultz

Posted by Forecast Editor at 07:50 PM | Permalink

The cyclonic disturbance now in/near California is associated with a large push of dry air off the continent and into the eastern Pacific, but that’s about to replaced by a return of the tropical connection and higher vapor content. The models have been consistent for days in suggesting that the next opportunity for decent precipitation in the American River Basin starts Thursday during the day. There is a variety of ways in the ensemble members for that moisture to get there, some have a nice smooth river of vapor, not unlike the glory days of December; others have it churned up in the trough that brings it in. So it’s a little too early to tell much about what type of event we’ll see on Thursday, but the percentage of “optimistic” model runs is increasing with each cycle.

Paul Schultz

Posted by Forecast Editor at 06:45 PM | Permalink

No precipitation in or near the American River Basin until Thursday at the earliest. Several GFS ensemble members indicate IOP weather Thursday, maybe Friday, but that is a minority opinion.

Paul Schultz

Posted by Forecast Editor at 07:34 PM | Permalink

No interesting precipitation events for the American River Basin in the next 72 hours. Still looks like the next chance for that is Thursday, 26 January. Several ensemble members want to set up a few days of zonal flow. Strangely, although the majority of ensemble members don’t bring in precipitation, those that do seem to agree on the Thursday morning timing.

Now, about that Santa Ana event shaping up. I’m sure our Californian cooperators have a pretty good idea about what kinds of unusual mesoscale events can happen when these events come along, but I don’t. So I’ll summarize the NWP results that seem relevant. On Sunday at noon an intense, dry short wave barreling straight south is centered at Sacramento. The NAM sets up a 1032 high in the Great Basin, with about 8 mb gradient between SLC and LAX. That short wave is NW of LAX by Monday morning, and the Great Basin high is up to 1040 mb. At that time the center of very strong downward vertical motion is the ARB, but downslope off the Sierras extends down to Bakersfield. Throughout this event the N-NE 850 mb winds don’t seem to get much higher than 30-35 kt anwhere in California. None of this has anything to do with precipitation in the ARB, but it’s all there is to write about today.

Paul Schultz

Posted by Forecast Editor at 08:43 PM | Permalink

Short wave exiting the area delivered an IOP. That’s all over now. Next chance for precipitation is Sunday 22 January. The numerical guidance is discouragingly diverse on timing, position, and configuration of a short wave trough. Of all the GFS ensemble members, the operational run is the most optimistic for precipitation, giving about .25” from about 09Z-21Z Sunday, but all the other ensemble members have zero QPF; only 2-3 others have non-zero QPF nearby. Local FO gives only token PoPs (10%) for Sunday.

After that, the next chance for precipitation in the Basin appears to be a week from today (Thursday).

Paul Schultz

Posted by Forecast Editor at 07:40 PM | Permalink

This morning the event is in progress with radar echoes over the ARB. At 05h 30dBz levels were seen that weakened at 07h. Then at 09h the reflectivities reintensified followed by other minimum at 12h. Following this, they again intensified up until the current time.

This morning we did generate a 12h 12ut high res HMT WRF run. It shows precipitation going full bore up to and including 00h 1/19. Precip amounts accumulated over the ARB domain during this forecast episode range between 1.0 and 2.0 inches. Pretty much in line with the NAM and other models as will be discussed below. Winds at 700 hPa are sustained for the entire run from the SW at 25 to 40 kts with the trof axis approaching the domain's doorstep at 00h 1/19 utc, with a pronounced windshift to the NW. This axis is roughly progged to be just west of Auburn at 00h 1/19 utc. This might spell the "end" of the event at about 06h 1/19 utc with minor precip falling after that point.

Just about all of the models seem to be in sync today having the episode end at about 12h 1/19/06 utc, ending with colder temps and lighter precip amounts. Integrated moisture both the GFS and NAM bring in additional precipitation of 1.38 and 1.34 inches respectively between 09h 1/18 and the end of the event.

NAM winds are SW from 15-25 kts between 12h this morning until 03h 1/19 utc at which time they weaken substantially and finally turn to the north with the trof passage at 12h 1/19 utc. Both the Canadian model and the UKMET end the event at the same time.

The freezing level is a bit warmer in the NAM than yesterday with levels beginning at about 830 hPa and then staying at 850 from 15h - 21h 1/18 utc then steadily increasing in pressure to about 880 hPa by the end of the event.

Long Range

This morning's Northern Hemisphere GFS shows the extreme cold air in Siberia breaking down and a new cold area forming in the Gulf of AK area. This would tend to go along with Klaus' remarks of the other day. The set up of a cold pool in this area might force a trof on the west coast which would bring the moisture in from NW instead of a direct tropical pipeline which is preferred. The GFS PW plumes have a weak (1.8mm) surge that is really detached from any tropical source entering the ARB around 18h 1/24/06 utc (Tuesday). This might coincide with the prior day's weak suggestion in the ensemble forecasts for a precip event in that time frame. Following this not much is evident. The plume that was inline for this coming weekend just doesn't quite make it to the area and the GFS precip forecast from the 06h run this morning shows precip well to the north of the ARB over the weekend which should not be a factor for the experiment.

In keeping with the other days, the following ensemble averages are what are suggested based on the 0h GFS ensembles out to 384 hours indicating the probability of precip ending at the stated times over the ARB (all utc): 0.16 12h 1/21, 0.25 00h 1/27, 0.33 12h 1/27, 0.25 00h 1/28, 0.33 12h 1/28, 0.41 00h 1/29, 0.33 12h 1/29, -dry spell- 0.41 12h 2/1, 0.33 00h 2/2, 0.5 12h 2/2, 0.5 00h 3/2. Given these low probabilities it looks like maybe something happens in early February. This was also indicated in yesterday's forecasts. In the interim, it appears that there may be a chance for something late next week. The episode for this coming weekend looks more dismal today than prior runs.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 07:41 PM | Permalink

This morning weak showers were noted in the forecast area as early as 13ut on Nexrad. BLU reported 41F and dropping temperatures. Auburn was 39F. The main area of precipitation remains off of the N. CA coast and of course the first forecast question is when and how intense will this system be?

There was a problem with the HMT run this morning in that the super cluster was unable to communicate data from this morning's LAPS run. As a result there is no high resolution guidance similar to what we did have with IOP7. This would have been very helpful in estimating the onset of the event and the early precipitation amounts as well as precip phase.

The NAM is again the winner with the most qpf from the system at 2.82 inches with the max precip falling between 15h and 18h 1/18/06 utc, lower than yesterday's estimate. The NAM has the event starting about 21h 1/17/06 utc and ending with light amounts in the 0.06-0.2 inch range after 00h 1/19/06 tapering to 0.0 inches by 06h 1/19/06utc. The NAM is also in better agreement with the other models today so perhaps we can derive more confidence from its output.

The GFS run begins the event slightly earlier with very light amounts in the 0.01 inch range (12h 1/17/06 utc) and really doesn't start putting down decent amounts of water (0.46 in 6hours) until 12h 1/18/06 utc. The GFS has the event tapering off after 0h 1/19/06utc and finally ending at 12h 1/19/06 utc with a storm total of 1.29 inches. The earlier GFS had a higher precip total of about 1.7 for the event so the d-prog/d-t trend is for less moisture at least in the GFS. As we saw this is also the case for the NAM.

The NAM shows favorable wind speeds and directions through the entire event with winds beginning SW at 10 at 12ut this morning. When the moisture reaches the slopes at about 09h today the winds are 30kts from the south at the mountain top levels. Winds then continue fairly strong in the 30kt SW range from 03h 1/18/06 utc until 15h 1/18/06 utc after which they fall in intensity and the RH, which up until that time is 100% on the slope, begins to lift and reaches the higher terrain at the end of the event.

Freezing levels appear to hang around 840 to 850 hPa for the entire event on the NAM with a drop in elevation only near the very end of the event. GFS freezing levels are slightly cooler but have a similar trend with the 0.0C level reaching mountaintop at 0h 1/18/06utc and dropping throughout the event to a slope surface pressure of about 860 finally falling to lower levels (900) at the end of the event. For the most part the two runs are very similar in this regard and not as dissimilar as IOP7.

The UKMET model has the precip event starting about 00ut Wednesday in line with other guidance. Unlike prior runs it appears to be progressing the wave slower and in more agreement than past UK runs. It has the best dynamics coming in aloft at about 18ut Wednesday. So in that regard it might be a tad slower.

The Canadian model has weak precip as early as 12ut today, ending the event about 12ut Thursday. This model appears to be slightly warmer than the UKMET. The Canadian model correspondingly has less dynamic support and puts most of the emphasis to the north of ARB with an abrupt breakdown in the vorticity advection late in the event.

Overall, this storm is progged to be very similar to IOP7 in temperature with less of a dramatic drop in temperatures arriving late in the event.

Max plume of moisture with good connection to the tropics is forecast to move through the forecast area today with values of 21mm PW. Overall, this event looks very similar to IOP7 with the exception that the GFS does not forecast as cold a situation late in the event but I think offers better deep layer winds and the overall event might produce more QPF.

Long Range

The global trend appears to have the intense Siberian cold air breaking down over time and along with the set of short waves that have been crossing the Pacific. The 240h GFS forecast has a broad Pacific ridge in place that brings incoming systems into CA from the NW. I would consider this trend to one that would interfere with tropical moisture events that we have been seeing to date.

The GFS ensemble forecasts play out to show a minor weekend event beginning after Sunday 00ut 1/21 and maybe going through late Sunday. Probabilities for this are low at less than 0.5. Following this there appears to be a suggestion of something near then end of next week perhaps starting Friday at 00ut 1/27 and extending into the weekend; a longer duration event with the max probability (again less than 0.5) on Saturday the 28th. Then an extended dry period is forecast with a potential event out near 2/2/06 (Thursday). All of this is very speculative however; the trend appears to be on par with yesterday's ensembles for this coming weekend and very weak after that. There is a weak surge of additional moisture that follows the main thrust with origins from the northern part of the plume in the western pacific.

Moisture plumes appear to show a similar trend as the ensemble probabilities. The plume that might get into the forecast area for the weekend is now forecast to turn into a closed circulation just off the CA coast and not make it inland. It moves south of the ARB latitude as it circulates.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 07:44 PM | Permalink

The next system is progged to begin around 00ut 1/18/06utc with the NAM and GFS agreeing fairly well on timing of the onset. GFS ensembles for this event have it beginning about the same time (03h) and ending about 09h 1/19/06. NAM and GFS qpf differ as far out as the NAM goes (for my situation working from home) with the NAM producing more moisture. GFS storm totals appear to be about 0.6 inches for the entire event while the NAM has a whopping 3.32 by 00h 1/19/06. My personal feel for the precip in this event will be on par or slightly higher than IOP7 going by GFS guidance on qpf which I think does a better job. Furthermore, HPC indicates a 3 inch precip in the vicinity. On the other hand the NAM is coming in faster starting the event at about 18h 1/17/06utc. The Canadian model is starting about the same time as the NAM but it has been a bit fast in the past as well. The UKMET is in more agreement with the GFS. Both the Canadian and NAM models show an early (after 21h 1/17 utc) strong precip period followed by a weakening with strengthening between 9 and 12ut on the 18th. The Canadian and UKMET models have the qpf amounts more in line with GFS. The NAM seems to again be putting more precip down than the other models, but given HPC guidance would appear that this system might be more vigorous than previously thought.

The consensus forecast has temperatures for this system colder than IOP7. The NAM appears warmer than the GFS.

Winds in both models appear to offer the best SW flow early in the system before the moisture really reaches the area with the GFS showing W winds about 12h 1/18/06utc.

Dynamics in both models appear weaker than the last system and actually show possible subsidence in the GFS at 12h 1/18/06utc with the upper level jet right quad exit region above the ARB. The NAM is also showing the major dynamics not in the best phase with the precipitation. This might be the factor in the dip in the precip amounts that then increase a bit later in the forecast runs with an increase in the upper level jet. This dip might also correspond with the drop and then increase in the minor moisture plume entering from the west.

GFS moisture plumes over the Pacific show the main pipeline of moisture with this event exists at 06h 1/18/06utc with values 26mm southwest of the ARB but advecting to the SW. As described in earlier discussions this feature however swiftly moves southward and is south of the area by 18h 1/18/06utc but possesses more moisture than IOP7.

Long-Range Discussion

As we move forward in time, the next major plume reaches British Columbia around 18h 1/19/06utc with a major source of water to its west near 30N/165W. As time progresses the plume in BC moves south but falls apart. Leaving the ARB dry until another minor plume reaches the ARB 06h 1/22/06utc (Sunday). This is a weak surge of moisture and has no real direct connection to the tropics but is more of a broken-off side appendage to the major plume discussed earlier as it has broken into two sections, this one and another reaching farther north in BC at this time.

Long range ensembles play out as follows indicating the probability of measurable precipitation in the prior 12 hours of the stated times as: 0.25 on both 12ut Sun and 00ut Mon of next week; then another greater chance mid-week with 0.41 12h 1/25/06ut and 0.33 00h 1/26/06utc. It appears looking at the anemic moisture plume for the coming weekend that that is reflected in the forecast, also in keeping with the GFS trending toward weaker systems as crunch time approaches, the ensemble fractions are lower for the weekend storm today over what they were yesterday. For this reason, I would not get too excited about the event progged for the middle of next week. During the telecom, discussion indicated that we are transitioning from a week to a stronger la Nina situation that has the potential of shutting off good moist flow into the ARB domain in the coming weeks, dominating the Western US with drier Arctic air.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 08:09 PM | Permalink

Last evening, the 00h 1/15/06utc 3km HMT WRF run showed the precipitation with an abrupt downturn in intensity at about 07h 1/15/06utc. This was right in line with our forecasts. The moisture out of this latter model run also produced another 0.5 to 1.0 inches of precip in this interval over the ARB. The highest core of precip was well south of the domain in line with earlier guidance.

This morning's radar refectivities at 15ut show little in the way of action so I will assume the event ended sometime between 00ut and 12ut this morning.

One of the major points brought out in yesterday's discussion was the temperature of the system coming in next week. The forecast question is whether this next system is just going to be a repeat of the current one that just ended or colder? Based on the GFS 850 hPa thermal fields, it appears at this time that the system slated for the middle of next week will be WARMER than the current system.

Also, it was brought up whether or not the Pacific Ocean temperatures were continuing a weak la Nina situation or whether there were changes in that area that might have implications for the nature of the precip events down the road for this exercise. Here I looked into these issues as well as examining the timing of the next event and possibly looking out after that one to see if there is something later worth tracking.

The current la Nina situation appears if anything to be strengthening with time based on the web site:
http://www.pmel.noaa.gov/toga-tao/data-access/EQSST_xt.gif
that shows both the actual equatorial temperatures and the anomalies or departures from expected mean temps. It appears here that we have been in a weak la Nina situation for quite a long time (I have not looked at this page for quite some time) and the more recent departures appear if anything to be trending to a stronger la Nina. The bottom line is that there are no significant changes here that have occurred during the time frame of the HMT exercise. I would therefore anticipate that southern oscillation driven effects will not be a factor in changing the nature of the weather systems that we have been witnessing other than a possible strengthening of the la Nina and I am not an expert as how to interpret this for the Pacific coastal area.

Looking at the moisture plumes over the Pacific some interesting changes have developed over the last 24 hours. On 12h Tues, the plume of interest for the mid week event is now impinging on the NW CONUS coast with PW values near 20mm. This plume intersects OR and extends now into WA state. There is a detached moist area to its south with 18-20mm just off SFO. Nothing of interest is indicated over the ARB at this time. By 0h 1/18/06 utc the plume has moved south to a location just north of a favorable position with the ARB, with PW values in the 20-22mm range. By 12h 1/18/06 the plume swiftly passes through the ARB with declining PW amounts after that time. This puts the main surge of moisture over the ARB around 06h 1/18/06 utc (Wed); fairly much in line with earlier forecasts. Now there is an interesting twist. A ridge of moisture out over 150W/50N that has a weak descending plume to 140W/40N injects a surge of moisture in a 6 hour period straight east into the ARB area. This appears pretty weak but noticeable raising PW values in the ARB to 18mm by 18h 1/18/06utc. This begins to leave the area by 06h 1/19/06utc.

Following this episode, the ARB PW values appear to stay dry as the mid Pacific moisture just hangs in place over 150W and does not appear to bring in any more moisture surges for the duration of the run out to 18h 1/21/06 (Sat) utc and that is very thin and weak. Following that slight surge, there appears to be unfavorable upstream moisture with the main plume extending straight north to southern AK.

Long-term forecast
The ensemble values for the next system to hit the coast play out with the following probabilities of qpf 12h prior to the stated time: 12h 1/17 0.16, 00h 1/18 0.25, 12h 1/18utc 1.00, 00h 1/19 0.75, 12h 1/19 0.25. With these values I would anticipate the next precip episode to begin around 9h 1/18/06utc and end near 3h 1/19/06utc. Given the weak surge of moisture entering the scene late in the game, there might be a drawn-out end to the event with scattered and light precipitation. Also reiterating what I mentioned earlier, this system appears warmer than the one we just experienced. But we can follow the temperature characteristics as we approach mid-week. The Canadian model has the system coming in a bit faster but with the intensity peak at about the same time (12h 1/18/06 utc). The Canadian model also looks like this is a warmer system.

The longer term looks fairly quiescent. I examined ensemble GFS runs out to 384 hours looking for a definite indication of a "next" system following the one mid-week. There are no systems the really jump out at you other than probabilities that hang slightly under 0.5. There is weak indication of an event near Sunday 1/22 12ut. Another possibly appears near 1/25/06 with a max probability 1/27/06 at 00ut. That might be a longer drawn out system since the probabilities though low stay fairly constant between Wed and Friday of next week. This guidance is highly speculative and should be taken to mean simply that there is nothing "big" on the horizon at this time.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 07:42 PM | Permalink

The GFS moisture plumes this morning support the precip event today with a weak band of PW extending just shy of the forecast area's higher terrain. Values are 18mm at 18h 1/14/06 utc, these values drop to 14 mm by 00h 1/15/06 utc and are very low after that time. The plume for the next event (possibly IOP8) appears now to be weaker than this event with a faster progression of PW southward. At 18h 1/18/06 utc the strongest PW amounts are seen impinging on the area and these drop to about 14mm by 00h 1/19/06 utc after that point the run shows dry air over the ARB.

Radar echoes this morning indicate 2 main areas of precipitation at 15ut, one 35 dBz area in the Squaw Valley area, and another similar echo north of Auburn.

Winds at 850 appear favorable until 00ut 1/15/06 utc which would indicate the last part of the event will be light extended precipitation as we have anticipated.

The forecast problem of the day is discerning the end of the current event and the timing of the system next week. The IOP7 event appears to have the following ensemble probabilities of prior GFS 12h precip in the near term: Sat 12ut 0.66, Sun 00ut 1.0, Sun 12ut 1.0, Mon 00ut 0.0. These values would indicate that the we are entering the period of greatest intensity now and this will quickly fall off after 03h 1/15/06 utc. The integrated GFS again has the NAM with greater amounts integrated from 18ut this morning with 1.5 inches while the GFS which has appeared lower showing 0.75 inches liquid eq. The UKMET model agrees more with the GFS as it did yesterday with the Canadian model a bit faster.

Today we did generate a 3km HMT WRF run out to 19ut and the precipitation values appear to be concentrated mainly to the north of the ARB with some increase in total moisture to about 1 inch in the North Fork region by 19h 1/14/06 utc. Another small area of the same magnitude appears to be NE of FHL at about 1300 ft. Other than these two max areas the model indicates wide spread values between 0.5 and 1 inch accumulated by 19ut. There is a 1 inch area just north of the ARB domain directly north of BLU. Model winds at 700 hpa show SW @ 40-50kts in the west domain area at 16ut today while at the same time SW @ 20-30kts on the west part of the domain. By 19ut, the winds drop to SW @ 40 east and SSW @ 30 to the west of the domain. Extrapolating the wind shift line it would appear that there will be a wind shift initially more to the south at 21ut as the mid-level trough axis approaches and then westerly to NW winds will move into the area about 02h 1/15/06 UTC. This might spell the beginning of the end for the event.

Given the above criteria I would estimate the event will taper off swiftly between 03h and 06h 1/15/06utc and finally completely terminate by 15ut tomorrow. The period after 06ut tomorrow will probably consist of light and scattered precipitation.

Looking ahead to the next system, this is how the GFS ensemble is playing out. The ensembles put the start time of the event near 09h 1/18/06utc and end the event at 3-6h 1/19/06utc. This next event appears to be weaker than forecast yesterday and weaker than the current event; one element that stands out is the speed of the forecast moisture plume as it appears to move more quickly southward through the area. The main issue of concern for the next system is whether it will be cold enough to warrant another IOP event. This will be studied more closely in tomorrow's forecast.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 07:53 PM | Permalink

The upstream troughs that we have been studying have changed slightly. The wave off of the Pacific coast this morning (520Dm) has the appearance of an imbedded shortwave to the north that may move off faster than previously thought detaching the wave affecting the ARB from Arctic flow. Furthermore, the low center mentioned yesterday south and west of the Aleutians has opened and is 511 Dm with a closed low to its north on the Bering Sea (507 Dm) which has retrograded a bit and moved further north. The open wave which has implications for IOP8 has begun to propagate across the broad Pacific ridge but appears to be moving slower than earlier forecasts indicated. This morning's GFS appears to be well analyzed with respect to satellite imagery. In addition, the moisture PW plumes on the GFS forecasts appear to have reasonable PW over ARB at 18h 1/13/06 utc prior to the near-term event, but this drops some in magnitude until the moisture plume to the north impinges on the CA coast later. This enhancement occurs about 12h 1/14/06 utc until 18h after which water levels drop drastically.

The temperature forecast from the NAM is much warmer for the system today as contrasted with yesterday's GFS with a 700 hPa freezing level at 12h 1/13/06 utc, dropping to 850 hPa at 06h 1/14/06 utc and then remaining at about 850 hPa through the event and then warming. This is significantly warmer than yesterday's GFS forecast. Today's GFS forecast continues to bring the cold air in and drops the freezing level to near 950 hpa late in the event about 18h 1/14/06 utc. The UKMET model indicates that the precip type will be snow for most of the episode favoring the GFS temperature trends.

Winds from the NAM appear most favorable from the SW between 09h 1/14/06 utc - 00h 1/15/06 utc with speed between 15-30 kts. Prior to 09h they are mostly Southerly and after 00h 1/15/06 utc they become more westerly then northerly by 18h 1/15/06 utc. Winds from the GFS are very similar.

Unlike yesterday's GFS, today's model is keeping the subtropical jet from interacting with any of the onshore progressing wave of interest for IOP7 until the wave is well past the forecast area. The same timing is seen in the GFS but stronger (and later). In either case, this interaction will NOT play a role in the IOP7 scenario. The wave continues to have weak upper level dynamics as it progresses through the ARB for the duration of IOP7.

The total precip for the NAM works out to about 2 inches given the earlier onset and heavy precip as early as 06ut, but the precip from the GFS is much weaker with amounts totaling about 0.64 inches, quite a disparity. The average of these two totals is 1.32 inches which is in line with what I was seeing yesterday from the NAM on total precip. I believe the major difference today to be due to the fact that the NAM moves the system into the area faster and stronger than the GFS. The UKMET model has the major precip at 18h 1/14/06 utc with a slow start to the event (precip hanging just north of the ARB prior) and ending about the same time as the other models. Looking at all of this seems to indicate that the NAM might be the odd-man out here and that the early QPF from the NAM might be erroneous. If one integrates the NAM moisture from about 18ut onward, the total is in the 1.5 inch range agreeing with the other models. On the other hand, the NAM's higher resolution might be doing a better job detailing the southern extent of the precip that the other coarser models are not be able to grasp.

Our super cluster "Ejet" was down this morning so the special HMT-WRF 3km high resolution runs will NOT be available. This is most unfortunate since they would have been very helpful to forecast the onset of the event.

During the telecom it was decided that the NAM guidance appeared the odd-man out and the basic start and earlier end times to the event were determined.

IOP8

The future looks as if the models are picking up on the delayed progress of the next system in the mid-Pacific mentioned early in this discussion. The ensemble GFS runs now appear to bring this next system in later and for a shorter duration. This is in line with the discussion on the telecom yesterday in that the GFS has been tending to overplay future events and as the forecast time nears, they are much weaker than initially forecast. Today's ensemble QPF probabilities look like this for next week: Wed 00ut 0.09, Wed 12ut 0.63, Th 00ut 0.82, Th 12ut 0.27. Based on this guidance I would move the onset of the next IOP from late Tuesday next week closer to Wednesday morning. Also the total duration of the event appears to be shorter ending sometime between Th 00ut and Th 12ut.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 07:57 PM | Permalink

Zonal flow over the Pacific continues to break down to form a major trough west of British Columbia and the Eastern Pacific (500hPa, 520 Dm low) and a cutoff (~500 Dm) south of the Aleutians at about 130 W setting the stage for the next two weather systems of concern. The ridge between the two systems appears to become positively tilted over the forecast area by next Monday-Tuesday and the current cutoff at 130 W becomes the next open wave to affect the ARB area near the middle of next week. GFS forecasts of moisture plumes from the central Pacific have been associated with each system and the trend of these had been to impinge on the US/Canadian coast further to the north each time. Then as the moisture makes its way into the forecast area, the precipitable water levels appear to weaken.

Today's forecast breaks down into two problems, the late Friday (local time) event that should dissipate later in the weekend and then the timing of the next system in the middle of next week.

The past "Wednesday event" possibly tapered off even as early as 21h 1/11/06 UTC based on radar reflectivities showing only scattered precipitation over the ARB at that time. As of this morning there were no radar echoes observed on the weather service site available to me. No short-term GSD HMT WRF 3km runs were operated today since the precip event is over.

The big question now is the next event starting time which was forecast yesterday to be ~06h 1/14/06 UTC. There are differences between the NAM and GFS on this event, but the start time appears to be consistent with 06UT. Both models show a very narrow, high amplitude trough forming off the coast. The dynamics in both models appears to weaken with time with the best dynamics off the coast later today. As the system moves east and reaches the ARB area of interest it will have weak upper level dynamics, the GFS shows a weak subtropical jet that then interacts with the stronger dynamics coming around the west side of the main trough of interest to initiate a cutoff scenario at 500 hPa by 12ut Sunday over central NV. This does not bode well for large precip amounts but rather has a weaker system moving through the forecast area with the main precip farther south. The event appears to begin at high elevations at about 6ut (the 14th) fairly in tune with yesterday's thinking and continues through 18ut on the 15th. Both models appear to indicate a weakening in precip about 3ut on the 15th with an increase though slight after that. The NAM indicates bands of precip oriented N-S late in the event, the GFS hints at this but is harder to discern with the lower spatial resolution. Winds for this event appear to be unfavorable except for maybe 6 hours. Winds begin S-25kts @ 6ut (14th) and then by 18UT appear to be SW @ 15kts until 00UT (15th) when they become more westerly and lighten in intensity. The freezing level is low to begin with (around 700 hPa this evening) but when the event begins, the level falls to about 850. Between 06UT and 21ut on the 15th, the freezing level drops to about 950 hPa. Precip totals for the event are estimated to be about 1.3 inches liq. eq.

The second issue is the longer range, well out after 96 hours. The GFS shows initial precip in the area starting as early as 00UT Tuesday. By Wed 00UT there is a max precip area just north of the ARB with good precip rates in the domain. Max precip appears to continue until about 06UT Thursday. Looking at the ensembles for this time period we see the following probabilities for precip in the ARB - 0.27 Tu 12ut, 0.63 W 00ut, 0.72 W 12ut, 0.54 Th 00ut, 0.36 Th 12ut, and 0.00 Fr 00ut. Given these probabilities I would speculate that the next event will begin as early as sometime around 21ut Tuesday 1/24/06.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 07:49 PM | Permalink

Zonal flow over the Pacific is in the process of breaking down. Major troughs will be impacting the ARB over the next week.

The Wednesday event panned out with precipitation underway today. The first burst of action from the radar looked to be Tuesday evening (11/03GMT) . A series of surges continued throughout the night with Blue Canyon recording 0.46 by 8am. Rainfall looks to be lighter than expected with rates this morning in the ARB at about 0.1/hr. High Resolution WRF (clickable on HMT web page) shows a total for the event ( 11/12-12/00GMT) of close to an inch before the event ends by late afternoon. The model supports rain rates of .08 to .1 inch per hour up until noon then tapers off to less than 0.05/hr till about 4pm PST (12/00GMT). This compared favorably to observed rain rates this morning at Duncan and Forest Hill. In evaluating the models the NAM 12 appeared to move things in too early with too much rain; the GFS model was better on timing and intensity.

Big question now is the event for Saturday. There is no doubt a major trough will move through the ARB. The precipitation with the event will be in the post-frontal, cold-advection regime. A moist plume is present but weaker than previous events with 20-24mm maximum upstream of the ARB. This moist plume rapidly dissipates with the intensification of the surface low. Moisture moves into the area Friday night with a freezing level of about 8000ft. the freezing level drops throughout the event reaching 4000 ft by Sunday morning. The GFS begins the precipitation Friday night (14/03GMT) with maximum activity early Saturday (14/06-12GMT). This rain band is likely to have imbedded convection with low static stabilities likely. Other models support the general evolution of the rain but have differing opinions on its initiation. Both Etas and MM5 are looking at Saturday morning (14/12-18GMT); CMC and NOGAPS like the GFS timing.
Half of the GFS ensemble members start the precip off between 14/00-03; almost all have precip underway by 14/12GMT. So a consensus would point at late Friday for a start (14/06GMT). Amounts for the event look to be about 1-2 inches. Very strong SSW winds will accompany the storm.

The next situation looks to be late Tuesday-early Wednesday, probably another cold-advection dominated event, and another major trough.

John McGinley

Posted by Forecast Editor at 07:56 PM | Permalink

Today (Tuesday) and tonight – Zonal flow brings a nice surge of vapor on west winds increasing to 25-30 kt overnight. Precip water amounts are over an inch during the precipitation period, which appears to start after dark and continue until midday tomorrow according to NAM; GFS holds on to precip until sundown. Temperature advection is weak negative throughout event. Snow level starts out around 7500 ft, lowers to around 6000 ft by the end of the event. Total model precipitation amounts .5 to 1.0 inches.

Next event starts Friday afternoon. Sharp, fast-moving trough moves in, strong warm advection ahead of it, 850mb winds southerly at 35+ kt bring precipitable water amounts around 1 inch. Precipitation amounts not large (.5 inch) before warm advection gives way to cold after dark Friday. Strong cold advection and frontogenesis in predawn hours appear to cause greatest precipitation rates, 1 inch in 6 hrs ending sunrise. Total precip 2 to 2.5 inches, event ends late Saturday afternoon. This is a colder event than today’s, snow could get down to 3000 ft.

Next event after that starts next Tuesday afternoon, looks to be a brief but intense warm advection episode.

Go easy on the rookie.

Paul Schultz, subbing for John McGinley

Posted by Forecast Editor at 08:12 PM | Permalink

Wave 1 still a dominant feature over the northern hemisphere with wave 3 also significant. Models are hinting at a big transition of energy from these longer waves to wave 6-7. This means that the shorter waves will gain amplitude by day 3. Thus, major troughs are in the offing for the ARB after Friday. Today the jet across the Pacific is very intense – 180-200kts – and to its south is a well formed IPW plume moving toward the NW Coast.

One more minor wave to go, this being the system we have been following, predicted for Wednesday over the ARB. Models (GFS, CMC, NOGAPS, ECMWF, UKMET) are in very good agreement on all features: arrival of the moisture, the warm to cold advection transition, the precipitation initiation. The GFS starts the warm advection precipitation around Tuesday mid day (10/20GMT), rather light, continuing to early Wednesday (11/08GMT). At that time a cold front moves through the area with precipitation increasing at and behind the front. The GFS has higher amounts today compared to yesterday with a total of 1.5 inches for the event, half of this falling in the frontal zone (11/06-12GMT). The GFS indicates a good link to the offshore IPW moisture plume which should max out just upstream of the ARB at values of 26-28mm late Tuesday (11/06GMT). The NAM 12 is even more excited by this event but starting things a bit earlier with precipitation commencing Tuesday morning (10/15GMT). During the warm advection period the NAM handles the arrival of the moist plume and upslope with more detail predicting 0.7 inches prior to the front. With the front and low static stability predicted by the NAM, there could be some convective cells imbedded. The NAM forecasts 1.5 inches at and following the front. So a 2-inch + event is in the offing. Freezing levels are high for this storm starting out at 8000ft and descending to 6500ft., so mostly a rain event. All signs are indicating that this storm will be stronger than forecasts over the weekend. Each set of model runs is more encouraging as we approach the event.

The next situation for Saturday, the GFS has a major full latitudinal trough reaching the coast with a deep cold front moving through the ARB. Precipitation really doesn’t get going until after frontal passage, so it is predicted to be a cold advection-dominated event. Peak precipitation looks to be mid-day Saturday. The GFS-predicted moisture plume is really cut off from a true tropical source. The weak N-S orientated plume seems to be dissipating as it crosses the coast. Values upstream of the ARB are predicted to be 20mm or so. The GFS ensemble is in better agreement with fewer members closing off the low, and none predicting a cutoff well south as they showed yesterday. However, the ECMWF and UKMET are hinting at this still. If these turn out to be correct the outlook for Saturday is less favorable. The bottom line is that this will be a moderate rain to snow event, primarily post frontal, and should clear things out for a number of days.

The long range outlook is for another event Wed Jan 18th.

John McGinley

Posted by Forecast Editor at 07:54 PM | Permalink

Strong zonal flow continues to progress into the eastern Pacific as wave one shifts eastward. This has eliminated the longwave ridge over the W. US and opened the door to fast moving minor troughs and ridges into the NW US coast and eventually the ARB area.

A pair of troughs, one minor and the other with a bit more amplitude are predicted by the GFS to make the coast on Tuesday (Jan 10/12GMT) and Wednesday (Jan11/12GMT) respectively. Unlike yesterday the models seem to agree on the timing and intensity of both these waves. The GFS, ECMWF, CMC, NOGAPS, UKMET predict that that the Wednesday event is stronger and more consolidated than yesterday’s forecasts. The GFS brings in an IPW plume (with offshore values of 35mm), with values of 24mm into the San Jaoquin Valley during the period of warm advection (10/18 to 11/09). Only light amounts of precipitation are forecast until the cold frontal passage sometime late Wednesday. At that time precipitation increases with nearly an inch expected over the 24-hr event. Heaviest precipitation is predicted Wednesday mid-day (11/12-18GMT)…about a half an inch. Freezing levels will start at about 8500ft and drop to 5500 ft by the end of the event.

The likely event for next Saturday-Sunday(14-15 Jan ) is still being handled in a variety of ways by the model suite. The ECMWF and GFS agree on the “v-shaped”, full latitudinal configuration, and hint that the southern end of the trough will cutoff off the S. CA coast. . The GFS ensemble supports this idea with 25% of the members having some sort of cutoff. The full latitudinal v-trough configuration is not good for advecting in a high IPW plume ahead of the wave. The GFS IPW forecast supports a weak N-S oriented plume that dissipates during the event. Precipitation is predicted only during the post-frontal, cold-advection regime.

The long-range outlook, as might be expected, is highly divergent, but suite and ensembles hint at a major trough some time near 18 Jan.
John McGinley

Posted by Forecast Editor at 07:42 PM | Permalink

The ridge over western US is rapidly collapsing with heights increasing across the mid-Pacific. This is speeding up the steering flow and decreasing the amplitude of waves in the Pacific. As a consequence the NW US is being hammered by a series of minor amplitude fast moving waves that will impact the coastal areas from Washington to northern California. These systems will flirt with the ARB, brushing by to the north. The short wave that moved through today will be followed by one this evening (8/03GMT). This longwave readjustment is playing havoc with the model’s handling of systems, so there is less consensus today than yesterday.

The wave we expected for Wednesday (11 Jan) from yesterday’s GFS is forecast to come in as two separate minor short waves that will drive an IPW plume toward the coast, arriving early Tuesday (10/09GMT) and another minor trough (imbedded in larger scale warm advection) and moist surge for Thursday (12/06GMT). Most of the dynamic forcing goes north of the ARB with orographic lift on the Coast range of northern CA and Oregon. Only minor precipitation amounts are predicted by the GFS for the ARB. This solution favors what the ECMWF was saying yesterday: weak system Wednesday, strong system Saturday. I was unable to look at the ECMWF for today. All the models (GFS, UKMET, NOGAPS and CMC) vary the timing on these minor waves, but all agree that the “big” event we were expecting for Wednesday probably won’t pan out. The IPW plume lingers off the coast from Wednesday to Saturday, with occasional surges into the San Joachin Valley, so RHs will remain elevated in the ARB area. The problem is no dynamic or orgraphic lift.

The event for next Saturday-Sunday(14 Jan ) is still present in some of the models although there are some differences in timing, and intensity. A majority of the GFS ensembles still support a major trough on the coast in that period. However, a few support a closed-off low and blocking pattern. If the model majority wins out, there may be a period of warm advection with an associated IPW plume late on Friday (13 Jan).
John McGinley

Posted by Forecast Editor at 07:49 PM | Permalink

Broad trough in the mid Pacific continues with a weakening shortwave at 42N, 139W. This will be the next system to impact the ARB area as this wave moves east but weakens significantly. It should arrive on the coast by early Saturday (7/08GMT). This wave begins the breakdown of the ridge over the western US. A series of waves lined up across the Pacific will be on a path to impact first, the northwest (WA, OR) coast late Saturday (08/00GMT) and Monday (09/18GMT), but will slide further south as the ridge gives way. Two waves will move into central California one on Wednesday (11/18GMT) and another on Saturday (14/12GMT).

The GFS and ECMWF look to be in good agreement for the event on Saturday indicating cold-advection dominated showery precipitation in the range of 0.3-0.5 inches. Freezing levels drop from 9000ft to 6000ft during the course of the event. An IPW plume is very weak with values of 16-18mm. The GFS has it dissipating during the event. Bottom line: too weak to be of interest.

As discussed yesterday a better event is shaping up for Wednesday. Here, however the GFS and ECMWF forecasts are quite different. The GFS has an event predicted for late Wednesday/Thursday(12/00-12GMT) while the ECMWF rushes a minor shortwave onshore early and consolidates most of the energy in a trough further west that will impact the coast on Saturday (14/12GMT). The diversity in the forecasts are also captured in the GFS ensemble as solutions range from trough to ridge scenarios on the coast during this time. However, the Canadian CMC and Navy NOGAPS support the GFS in timing and intensity. Still, we should be a bit cautious at this time in specifying timing for this event. The GFS primary run shows that the event could be a good one if it verifies with 1-2 inches of precipitation beginning with a well defined IPW plume with values 40mm offshore and 30-35mm working into the San Joaquin Valley. It is curious that even though the GFS forecasts this plume to move in to the ARB with good upslope flow in the warm advection period, precipitation is late in starting, not getting rolling until Wednesday night (12/06GMT) and mostly post cold front in cold advection. Most of the precipitation during the warm advection period is focussed on the Coast Range. Freezing level is forecast to fall from 8000ft to 4500ft during the course of the event. The bottom line: it looks like a potentially good ARB event but there is much uncertainty about this storm and close monitoring will be necessary over the weekend to see if a deployment is warranted

The further outlook from GFS and ECMWF indicates another event for Saturday 14 January. The ECMWF has more intensity in this wave than the GFS but both agree pretty much on timing, starting the event Saturday morning (14/12GMT).

John McGinley

Posted by Forecast Editor at 07:48 PM | Permalink

The telecon discusssion indicates that NWS and ESRL forecasts are in good agreement with respect to current and forecast weather for the ARB.

The Northern Hemisphere wave 1 is still a dominant feature today, although we have seen higher amplitude waves develop on the east side of the strong jet stream flow. This has built a significant ridge over the ARB region and is keeping cyclones and fronts off shore for today through Friday.

The ridge appears to be short lived however on the GFS. The ridge begins to flatten starting today culminating in a short wave trough that impacts the ARB on late Friday (6/06GMT) and Saturday. GFS supports a cold frontal passage late Friday. While there is a weak IPW maximum off shore that will push into central California Saturday morning (7/12GMT), Univ. of Hawaii GFS IPW values range only 22-18mm through the event. The IPW band dissipates through the day Saturday. The precipitation event is post frontal with the GFS predicting about 0.5 inches during the cold advection period. To summarize, this is a weak, cold advection dominated event without a clear cut IPW maximum.

A better event may be in the offing for Wednesday. The GFS ensemble has a variety of opinions on how strong this wave will be, but generally are in agreement within 12 hours of its arrival over the ARB. The GFS primary run shows a well developed moist plume in the mid Pacific that moves down the Washington and Oregon coast Tuesday reaching N. California by Tuesday afternoon (11/00GMT). Moisture increases over the ARB region late Tuesday into Wednesday with a neutral to slightly warm advection period Wednesday morning (11/08-14GMT). Precipitation will begin during this period. IPW amounts of 24mm are advecting into the central Sierras. A cold front moves through late morning (11/18GMT) with the heaviest precipitation appearing to be post frontal. This situation may be a good rain to snow transition with freezing levels dropping from 8500ft to 4000ft during the event.

The further outlook indicates another event for Saturday 14 January. However, this one too, looks to be cold advection dominated. .

John McGinley

Posted by Forecast Editor at 08:05 PM | Permalink

Ridge has been building strongly surface and aloft over the west coast and inland last 24h behind yesterday's weak system that gave only spotty light pcpn to the ARB. Offshore, a very intense storm is near 40N/140W and creating gales to within 200miles of the Northern CA coast as it moves NEwd toward BC.

Progs continue to forecast a return to more zonal flow beyond 60h as the present mammoth
central Pacific trof collapses. Thus the forecast becomes less certain from the weekend into next week. Most likely is the scenario discussed in previous days of trailing fronts down into the ARB area at 24-60h intervals, with storm totals from individual events < 1" in wetter areas. The first of these should pass to the north of the ARB late Friday, followed closely by another Saturday night. Neither of these appear to have much promise. The following one, scheduled (roughly) for late Monday or Monday night, has slightly more promise, if for no other reason than the longer forecast range being considered. However, the ensembles suggest a probability of maybe 1 in 4 that this will produce storm total pcpn >1" at Blue Canyon. I regard the timing on these events as also uncertain. Beyond this, there are hints of an interesting event the following weekend (14-15 Jan), but this is WAY out in speculative territory.

John Brown

Posted by Forecast Editor at 07:16 PM | Permalink

Yesterday's IOP5 system is falling apart over the Rockies as the next system invades Nrn CA with more wind and rain. The main impact of this system is being felt N of 40N, well N of the ARB, but periods of light rain (snow above 4500-5000ft) can be expected for the next several hours in the ARB.

All progs continue with the downstream amplification scenario of amplifying trough Central Pacific, West Coast Ridge and eastern US trough over the next 2-3 days, followed by return to more zonal flow across the Pacific during next weekend. The next threat of any pcpn is Friday, as a wave yet to form in the central Pacific reaches the Northwest coast. Present indications, even allowing for the systematic tendency for the GFS to overdo West Coast ridging the last few weeks, are that this will be more of a trailing cold-front scenario for the ARB. Strong dynamics or the large precipitable water fluxes we saw last week will be absent.

It is too early to say with much confidence about either the precise timing or intensity of the next pcpn threat to the ARB after Friday, but overall, it looks like the next action will be Sunday/Monday, and affect mainly the Northwest and extreme northern CA.

John Brown

Posted by Forecast Editor at 07:44 PM | Permalink

IOP5 situation will wind down next few hours as cyclonic circulation below 500mb has come ashore
central coast. Associated short-wave trough aloft will cross the coast before 00Z Tuesday. Although at present there is probably good warm advection forcing over ARB at mid-levels, orographic forcing is ending as flow backs at 700mb. To the south, heavy pcpn has moved out of the San Joaquin Valley into the Sierra, as another wave of pcpn comes across the Coast Ranges into the Westside of the San Joaquin Valley just ahead of the upper trough axis. I anticipate this new pcpn area will not wrap around the 700mb low sufficiently to affect ARB. With nil to negative orographic forcing, expect pcpn to be basically over by 03Z or before.

Next storm is coming in hard on the heels of the present one, but with a significant
pattern shift underway over the Pacific, the next storm is taking a more northerly track.
It should be noted, however, that the GFS in particular is showing this coming in a little farther south in the 12Z run from this morning than in earlier runs. Although pcpn can be expected, particularly over higher terrain, beginning by 18Z on Tuesday, this will be much lighter than the past two events and I put chances of Blue Canyon getting 1" or more melted pcpn between 12Z Tu 3 Jan and 12Z W 4 Jan at 10%.

As the long-heralded major trof deepens in the central Pacific next day or 2, indications from the NWP guidance continue to be for dry Wx Wednesday and Thursday as ridging, surface and aloft, dominates and storms spawned in the central Pacific affect mainly the Northwest and
BC. Even if the GFS is in substantial error at the 48-84h range, I think the ARB is safe from
pcpn of more than a few mm from 12Z Wednesday through 12Z Friday.

The big question is whether this long-anticipated dry spell will be sustained. In studying the ensembles, it is my impression that the GFS ensemble spread by 120h from last night and this morning is larger than usual, indicating that lower confidence should be placed on any given forecast. The overall trend, however, is toward a return to more zonal flow as the weekend approaches, with the central Pacific trough and downstream West Coast ridge losing amplitude.
This overall zonal flow, however, looks to be weaker and farther north than what we have been
experiencing. This argues that overall pcpn amounts will be less and heaviest pcpn will be to the north of ARB, associated with trailing fronts. The first of these is due Friday. The ECMWF from this morning suggests the next one will be late Sunday 8 Jan or Monday 9 Jan.

John Brown

Posted by Forecast Editor at 07:16 PM | Permalink

Things appear to be on track for the IOP5 storm. It is evolving much as progs indicated yesterday. The intense surface low is now moving northward off the Northwest coast, and the trailing wave is clearly evident in water-vapor imagery near 140W at 12Z this morning. Enhancement of water vapor tags and clouds indicate the mid-upper tropospheric upward motion 12-14Z this mrng associated with this wave was in a NE-SW oriented area from near roughly 40N/127W to 33N/133W. There is a strong barrier jet along the coast, and clear evidence of a front in the scatterometer oceanic sfc winds from 14Z this morning from near the CA-OR border at the coast in an arc extending SW - WSW underneath the aforementioned area of upward motion aloft. Rain is now moving into the central coast and Bay Area, but heaviest radar-indicated pcpn is offshore from about Pt Piedras Blancas to WSW of Pt. Conception.

It appears that GFS has better handle on current pcpn distribution than the NAM. Both models,
along with the GSD RUC13, continue to indicate focus of pcpn Central Coast and southern Sierra, with the GFS deterministic run from 12Z this mrng indctg about 2" upper portion of ARB. Amounts to the south will be at least twice this amount. Timing of 00Z 2 Jan thru 00Z 3 Jan for the IOP looks good based on models for bracketing at least 80% of the pcpn from this event. Snow level of near 4500 to 5000ft. Short-wave trof aloft scheduled to pass over the ARB between 00 and 06Z 3 Jan.

Looking farther ahead, consensus of models is that next deep low will take a decidedly more northerly track as it heads toward the coast. Pcpn from this will stay mainly in the NW, with the only
threat to the ARB being some light pcpn from the trailing front late Tu into W. At the moment I
would put chances of Blue Canyon getting >1" between 18Z Tu and 18Z W at 10%. With a major trough developing in the Central Pacific, it looks like further pcpn will not occur until this trough begins to flatten and shortwaves rotate around it toward the Coast. Based on the various progs I
have examined, I don't see this happening until Friday 6 Jan at the earliest.
John B.

Posted by Forecast Editor at 07:47 PM | Permalink