Looking ahead the overall scenario outlined yesterday still looks good: the next system a fast-moving one arriving late Wed night into Thursday followed by a series of waves with progressively colder conditions before the overall upper low sinks south of the ARB by about Mon/13 Mar bringing an onset of what looks like an extended period of drier weather as the mean ridge moves over the West Coast for next week and the mean trough into the CONUS. The ensembles (00z GFS) are in pretty good agreement with this through day 10 (next Friday/17 Mar, and the mean 8-10 day forecast from the 00z ECMWF also supports this ridge position), then more spread develops with the ridge potentially moving inland, opening the West Coast to possible moist systems again for later this month, though right now not a lot of the ensembles show much heavy precipitation even for days 10-15. The 12z GFS deterministic forecast does have such a moist system by day 15 (23 Mar), a long way off.

As for the upcoming action from Thu-Sun (9-12 Mar), the Thu system looks to be moving in quicker and with somewhat less moisture, so likely Wed night through midday Thu. This is followed by a series of shortwaves as an overall upper low strengthens off the Pacific Northwest Friday and moves slowly southward down the West Coast. Timing is tricky with each wave and precipitation amounts would be less than the current moist system, with progressively colder temperatures. By Sat/12z (11 Mar) thicknesses in the ARB are down to the mid-520's which would drop snow levels quite low, but most of the more significant precipitation may be confined to higher areas. Total precipitation for the whole Wed night through weekend period on the GFS is about 2" in the higher areas, so the amount of moisture certainly the main issue (though, as noted yesterday, there currently is a tie to the Hawaiin moisture as the system is moving across the Gulf of Alaska, so perhaps there could be more). The biggest characteristic of this period would be how cold the conditions become with time.

Decision was made to end the HMT field exercise at the end of this IOP. It's been enjoyable, we look forward to next season! ed szoke ESRL/GSD

The models seem to be coming into somewhat better agreement on the
overall broad pattern for the next week plus, with less spread than was
shown in yesterday's runs. The general trend is to shift the longwave
trough inland over the next week so that by early next week an upper
level ridge will be near the West Coast which could set up an extended
period of dry weather. There is still some spread in the GFS 00z
ensembles but not as much as yesterday, and the 12z GFS run is also in
agreement in showing dry weather for the most part in the ARB area from
about Sun/12 March through Tue/21 March. The mean 8-10 day ECMWF
pattern is similar to the GFS in showing this trend.

In the interim between the current system and next week there is active
weather as waves continue to affect the region in the NW flow. The
first such wave now appears to be more in the Thursday timeframe with
the bulk of the precipitation from about 12z/Thu through 06z/Fri 10 Mar.
The GFS shows only about an inch or so accumulation but, as noted
yesterday, there is a connection to the tropical plume extending north
from the system west of Hawaii as the next wave rides eastward near the
Aleutians, so there could be more of a heavier burst of precipitation
with this first wave, and, additionally, it is not all that cold to
start. After this relatively fast moving wave timing of any further
identifiable systems gets a little trickier, with the best guess Friday
night into Saturday (11 Mar), as the overall trough tries to close off
and shift south and east, with mostly scattered precipitation around by
about Sunday. Whatever happens from Friday and thereafter it will be
very cold, with thicknesses down into the mid 520s (dam), so very low
snow levels but moisture amounts could be an issue.

Beyond the current system things appear somewhat more uncertain today then they did yesterday in some regards. The next system is still on track to quickly move in probably Wed night into midday Thu/9 March with a quick shot of potentially heavy precipitation as it does seem to have a connection to the Hawaiin tropical plume. Snow levels start out around 4000 or so feet then plunge quite low by late Thursday. The main issue after Thursday is how quickly the overall trough position shifts into the Rockies, with the 12z GFS slowing this process down as it deepens an intense closed low off the Pacific Northwest next weekend that then slowly retrogrades slightly and finally moves through Northern California midweek (around 16 March). This is much slower than what looked like an end to precipitation on yesterday's runs (and last night's GFS) more like very early next week. The 12z GFS solution is supported by the 00z Canadian and 12z NOGAPS, but the 00z ECMWF is more progressive. The ensembles from the 00z GFS have some spread though are not as dramatic at holding the trough back as the latest GFS. Either way what all this means is that following the initial next system late Wed-Thu we enter an extensive period, at least through the weekend and possibly well into next week, of waves of precipitation moving in with timing and amount very uncertain. One relative certainty is that at least for later this week snow levels would drop to extremely low values not yet seen, I believe. If the latest GFS is correct they could then rise again and there would be the potential for a more moist episode or two of precipitation from say Sun/12 March through about Wed/15 March. All this may be moot if resources have run out, but wanted to point out that 1) uncertainty in the late Wed-Thu system, 2) potential for VERY cold weather with anything later in the week or weekend.

Ed Szoke - ESRL/GSD

The next system does come rapidly behind the first one but may be more an extended system and overall colder and not as wet, though there could also be a more distinct first punch to the system and then a longer lull. It looks like precipitation would begin in earnest by 00z/Thu (wed night) and continue for about 15-18 h with dropping thicknesses to the lowest seen so far as this wave dives down the western Rockies. About an inch or so of precipitation forecast by the GFS, though see note below. Then some uncertainty with the latest GFS keeping off and on precipitation and cold temps the rest of the week before the final wave Sat night into midday Sunday (11-12 March), while the 00z GFS had a bit less off and on precip (so more of a distinct break) but still the Sunday system though not quite as organized. Looking at some of the other models (Canadian, ECMWF, NOGAPS) indicates some uncertainty with the timing and strength of the midweek system, though all have something. I think the best that can be said about the midweek system is it might be interesting, it will be colder, could produce a good burst of precipitation say later Wed into midday Thu though not as much as this coming event, and that how it winds down is uncertain, as is whether there is a further distinct system by the weekend. One thing that should be noted is that, examining the nice U of Hawaii web site with its PW forecast in the Pacific from the GFS, there IS a decent connection setting up with the tropical moisture west of Hawaii as the system moves southeast down the west coast of Canada, and this would be part of the precip that hits later Wed into Thu. It is, though, a fairly fast moving system, but the point being there could be a decent burst of precipitation, moreso than might be anticipated by such a system moving from that direction, and a colder system.

Looking further ahead, both GFS runs are consistent with the idea that there will be a shift in the overall longwave trough position by this weekend and this creates a ridge closer to the West COast by early next week that could lead to a drying period for at least a week. The mean 8-10 day 00z ECMWF is also consistent in this idea. But the GFS ensemble shows a lot of spread, especially by day 9 (Mon/13 Mar) and continuing through the 15 day run (Sunday 19/Mar), so this would make one have less confidence in the extended dry forecast of the two GFS deterministic runs. Still, the end of resources might be nicely timed with an extended break in good events.

Ed Szoke - ESRL/GSD

Saturday should be quiet between systems but as noted yesterday the
next trough now crossing the eastern Aleutians is quickly diving
southeastward and intensifying for the next event. This system will be
stronger than the current one and, as noted yesterday, still looks to
be tapping into some of the moisture associated with the broad trough
near Hawaii. This moisture plume appears to be contained mainly in the
initial strong rainband that will arrive in the HMT area later Sunday
and persist until around 18z on Monday. It looks like todays model
runs are a little faster with the start of this event and significant
precipitation in the HMT area could well be underway by later (~21z)
Sunday, perhaps even 18z, with initial precipitation beginning in the
morning. In addition, strong SW to SSW winds are predicted to accompany
the first main rainband, with speeds forecast by the Eta and GFS to be
about 50 knots sustained near 925 mb and 55 to 60 knots at 850 mb.
This strong southerly flow will push warmer air in with the freezing
level rising to 850 mb so initially snow level will be fairly high
(4500-5500 feet) but eventually lowering to near what is happening now
as the trough passes on Tuesday, which probably happens around 12z or so,
with precipitation likely ending by later in the day on Tuesday. With
the system tapping into more tropical moisture than the current event and
a longer duration the 00z GFS is forecasting about 5" maximum precip
amounts for this event which look reasonable. The 12z GFS is a little
lower but still plenty wet.

Yet another wave closely follows the Sunday-Tuesday storm and this one
at least temporarily shifts the mean trough position over the Rockies
with upper level ridging shifting eastward off the West Coast extending
north to the Aleutians. This system is now forecast to arrive on
Wednesday/8 March and will be the coldest so far with thicknesses
forecast to drop into the lower 520's (they are near 530 dam with the
current event). After the initial Wed-early Thu burst of heavy
precipitation (perhaps aided with at least some contribution from the
same tropical source) there will be an extended showery period likely
perhaps into Friday. Beyond this somewhat uncertain with a number of
the GFS 00z ensembles indicating a deep trough setting up farther off
the West Coast mid-March while others regain something akin to the
current pattern, but there does seem to be a chance of a gap in the
events after the Wed-Friday storm of this coming week.

Ed Szoke - ESRL/GSD

The active pattern will continue as the mean trough position remains
just off the West Coast. Another strong wave deepens off the West
Coast by Saturday/4 March and will set up what looks like a fairly long
duration event beginning by Sunday/5 March. There is quite a mass of
tropical moisture lurking in the Pacific extending from southwest to
northwest of Hawaii in association with a broad and rather deep
trough near 175 W. Currently this connects northward all the way north
to the Aleutians...at least in the IR imagery...ahead of the next wave
that will be diving southeast and deeping off California. These two
systems don't end up phasing as the sourthern one is left behind...but
it seems apparent that some of this tropical moisture will get caught
up in this next wave and so would expect this next system to be wetter
than the current one. Best guess right now is that this next event
would begin as early as later Sunday afternoon (5 March), with higher
terrain precipitation perhaps as early as 18z but certainly by 00z/6 Mar,
and not end until later tuesday or tuesday night. During this period the
00z GFS generates between 4-5 inches of melted precipitation over
portions of the area and the 12z GFS is just a little less. Initially
snow levels should be rather high on Sunday night but lower to around
4000-5000 feet or so on monday and then possibly quite low by tuesday
when the trough moves inland...although it should be noted the 00z ECMWF
is slower with the end of the event although the latest 12z run has
similar timing to the GFS.

Looking farther ahead yet another wave could be heading down the coast
towards the end of the week for a potentially colder system though the
ECMWF is farther north than the 00z GFS with this one so obviously
uncertainty exists that far out. Nonetheless, the GFS ensembles
generally support a potential late week system, beginning as early as
late Wednesday, and beyond this a pattern that would remain very active.

Both NAM and GFS are fairly parallel on this next system however the GFS wants to keep the closed low together longer as it moves inland with the NAM opening it up earlier (9z 3/3) and bringing it in as a progressive wave, the GFS holds off on this and finally opens the wave at 00z 3/4. Neither model has a true surface fropa wind signature with this system however both indicate a weak upper level trof passage at about the same time between 21z 3/3 and 00z 3/4. The NAM has this as a stronger feature and like yesterday this corresponds with a drop in the precip intensity. Actually, in the NAM it looks like it might spell the end of the event. The GFS continues precipitation beyond this time. Both models have the start of this event at about 06z 3/2 with the precipitation on shore but not quite to the ARB at 03z 3/2. The precipitation in both models begins light with an early maximum around 12-15z 3/2. The precip in the NAM drops abruptly when the winds back to the SE between 21z 3/2 and 3z 3/3. QPF amounts for this system between the models is again pretty consistent like they were yesterday. The NAM has a mid-storm total eq liq. of 1.22 inches by 18z 3/2 before the lull and then continues through to 21z 3/3 with an ending storm total at that point of 2.06 with 0.84 falling in the latter half of the event between 3z 3/3 and 00z 3/4 when then NAM has the event ending.

The GFS continues precipitating a bit longer but with comparable QPF for the event of 2.09 inches ending by 12z 3/4.

Winds from the NAM show less than optimal direction throughout, with primarily south wind direction (as also progged yesterday) with speeds between 10-30 knots. It is only at the end of the event that winds veer to the SW for a brief time. This occurs at 12z 3/3 and 00z 3/4 and coincides with GFS precip rate of about a quarter inch per six hour period, of course the NAM is ending the event at about this time also.

The thermal profile appears to again be much warmer than progged last week, but maybe a tad cooler than yesterday near the end of the event. The freezing level starts at about 850 hPa and after 09z 3/2 begins falling in elevation, finally reaching abut 900 hPa at 15z 3/3, after that time it climbs back to 850 by 00z 3/5.

Looking at the ensembles for additional guidance on the end of this system and the following events it appears that the GFS ensembles are all dry by 09z 3/4. The normal GFS run has the next system beginning about 6z 3/6 (Monday). The GFS ensembles have the system well underway (about 100% agreement) by 12z 3/6 and the ensembles keep this going until 12z 3/8 (Wednesday) and there appears to actually be a surface fropa in the normal GFS run that occurs about 00z 3/8 which probably coincides with the event end, but the timing of course is the issue, putting all of this together, it could be any time between 00 and 12z on Wednesday.

Moisture plumes are looking a bit anemic in the future. The last vestige of a good tropical fetch is now departing the CONUS well south of the ARB and was associated with the last IOP. There is a minor surge from the NW 06z 3/2 associated with this event of about 2.0cm and then the next plume arrives at about 06z 3/6 and is very narrow extending back to a major N-S oriented plume at about 175w. This mid-pacific plume has 4.9cm of water to about 30N and then it falls off to about 3cm up to 40N. An appendage from the top part of this plume finally shears off and interacts with the ARB around 18h 3/9 (Thursday) with pw values of 2.8cm. After this episode, plumes really don't show much promise of making it to the ARB until 06z 3/16 (Thursday) about one week later.

Dan Birkenheuer - ESRL/GSD

This morning HMT run shows precipitation occurring up until 06h 3/1 utc after which it falls off rapidly, so this high resolution run also corresponds well to the NAM and GFS.

The forecast question of the day is really the onset of the next event and its forecast duration.

The NAM has the approaching precip band with the progressing closed upper level low moving inland by 06h 3/2 (Thurs). The first precipitation in the ARB from both NAM and GFS appears to begin about 09h 3/2 utc. Of the two, the NAM is slightly stronger with precipitation out to 12h 3/2 with a lull for a few time periods and picks up at the end with an additional 0.2 inches in the final 3 hours. As far as the NAM extends, it shows a total QPF of 1.58 inches between 09h 3/2 and 06h 3/3 utc.

The GFS begins the precipitation about the same time, likewise with a strong surge of moisture. Both models tend to show bands of precipitation that have been entrained in circulation around the low pressure system. As each of these bands moves over the ARB the precipitation appears to rise and fall accordingly. There is an initial strong precip event at 12h 3/2/06 utc much like the NAM followed by a tapering of precip until 9-12h 3/3 when there is another surge. This is then followed by a complete lull until 03-06h 3/4/06 utc afterwhich we see 0.84 inches in a 6 hour period. The event continues until about 09h 3/4 utc (Sunday). It is followed quickly by another system. Total QPF for the GFS for this system through Sunday is 2.82 inches with 1.14 inches falling in the same time frame as the NAM forecast termination (which is comparable).

The wind field is not as strong as this current system with winds staying mostly south at 20-30 kts and an upper level wind shift passing about 12h 3/3 utc possibly associated with the decrease (lull) in precipitation noted in the model. However, this feature has little affect at the surface being evident only above 600 hPa. After this upper wave passes it appears that precip at the surface resumes until the moisture in the system moves through. The GFS never runs far enough out to show a good surface wind shift associated with fropa.

The freezing level with this next system appears to start the episode at 850 hPa just above mountain top level and then drops to mountain top level for the duration of the event. It is definitely not as cold a system as was forecast a week ago, nor does it look as interesting to me as the event that is currently taking place.

Dan Birkenheuer - ESRL/GSD

The moisture plumes are arriving as progged in two sets. The first and heaviest is now interacting with the terrain and the second will arrive about 06h 2/28 utc. The first plume is roughly 2.5 cm TPW which increases to 3cm between 18h 2/27 and 3h 2/28. Then the second plume comes in from the NW to provide 2.4 cm of moisture for the tail end of the vigorous part of this precip event. The precip max for this event appears to be modeled between 18h 2/27 though 15h 2/28 utc. However, RH depth on model cross sections does not appear to be as deep as they did last week.

The wind field again is following prior forecasts with the NAM showing mostly southerly winds, ranging between 30-50 knots at times. At 9h 2/28 there is a fairly good wind shift from due south to more favorable SSW direction until 18h 2/28 at which time the winds turn WSW and the lighten. This probably coincides with the end of the "event" since at this same time the GFS modeled moisture plume is exiting the region moving to the south. Even so, as shown earlier last week in the GFS, the NAM today has precipitation continuing through the week with light amounts between the systems. Between 12h 2/27 and 0h 1/3 the total NAM QPF is about 4.85 inches (what I will call the "main event" here). Following this, light precipitation continues accumulating to 0.7 additional by 6h 3/2 and then another 0.88 inches by 3/3 utc. During the main event, the snow accumulates to approximately 40-53 inches in different areas of the higher terrain.

The freezing level again is following prior guidance with its mountain top presence for the major part of the heavy precip event starting there at 6h 2/27 and then falling later to 850hPa by 12h 2/28 utc. It then remains at that level for the duration of the NAM run well into Thursday afternoon local time in the ARB.

Dynamics from the NAM show the best PVA at 6h 2/28. The nature of the dynamics appears to be in waves that wax and wane throughout the event with this time period perhaps being the strongest.

The GFS model is very similar in most respects to the NAM on the first event with an almost identical QPF between 12h this morning and 0h 3/1 utc with 4.28 inches equivalent liquid. Following the first event, the GFS continues to have more of a lull, however, there now appear to be 2 moist surges coming into the ARB with the approaching closed upper level low. At this time the next event appears to begin at 9h 3/2 and continue at least to 00h 3/4 (Saturday) with the greatest precipitation maxes being at the start of the event and then a second max ending 12h 3/3 utc.

Unlike the earlier forecast runs, the temperature with the second system does not appear as extreme with the freezing level starting at about 850 hPa (where the first event left off) and then drops it to about 900 hPa at 18h 3/3 after which the level rises in altitude. Also at this time the GFS shows a SSW 30 knot wind field for the duration of the second event. The wind field never really veers to indicate Fropa between events one and two this week.

The first event is pretty well progged as far as its intensity and start and stop other than the NAM continues light precip between the two events. Both models seem to disagree slightly on when the second event begins with the GFS starting sooner and the NAM may starting the second event as late as 0h 3/3, but that time also coincides with the last panel I have for the NAM.

So looking at GFS ensembles for a maybe a better resolution of this question on the second event, it appears to be underway by 12h 3/2 utc with all panels showing precipitation, the event appears to end Saturday 12h 3/4 utc with all forecast panels dry.

Dan Birkenheuer - ESRL/GSD

However, this doesn't tell the whole story. There are lots of details. The trof offshore has at least 2 main pieces. A short-wave feature near 35N/130W at 15Z this morning was clearly associated with the main surface low off Nrn CA. Of significance is a large area of open cellular convection with cloud-top temperatures of -40C or so, between 30 and 40N, 142-149W at 15Z this morning. This has begun to turn cyclonically after heading mostly south earlier, and is currently on an ESE to E trajectory. It is also elongating and there is emerging a vorticity center estimated to be near 31N/145W at 2000Z. Beginning about 15Z there began formation of Ci to the E of this feature, and that has now expanded considerably, sgstv that there is appreciable dynamics (i.e., synoptic-scale upward motion) with this, separate from the main surface low and upper wave farther NE mentioned above. When I made a strenuous effort to do so earlier, about 16Z, I couldn't detect any separate cyclonic flow center at the surface with available data (tracking of low cloud tags, ships, buoys, etc.), but there
is clearly cyclonic shear in the sfc flow to E of the large patch of deep, open-cellular convection, probably concentrated along the surface front.

Closer to HMT, there is evidently good surface confluence just offshore Nrn and central
CA as the occluded/cold front approaches the coast. It should be noted that at the moment there is dry air over all CA, with dew points in the 40s at the sfc (35 at KACV!), and mostly offshore component to the surface flow along the coast also. However, GOES PW from RAMSDIS showed very strong precipitable water gradient just offshore and along the coast at 16Z. (This area has since become obscured by high clouds.) So, the picture emerges of a narrow wedge of high PW alg and just ahead of the cold front approaching the coast, with associated pcpn, and a wind shift behind the front into the WSW and decrease in wind speed.

At 16Z the pcpn offshore appeared to be in 2 batches: one from W of the Bay Area S
to Piedras Blancas, and the other from Pt. Arena to the CA-OR border. The southern of these has since moved NNE into the central coast and Delta areas, roughly with the 700mb flow. This morning the 12Z raobs had the freezing level near 700mb, but the wet bulb zero near 850, reflecting a very dry layer from near the surface to near 700mb. This sgsts to me that the snow level in the HMT may be closer to 5,000 ft with this IOP. There is also the likelyhood of significant evaporation initially that may delay the onset of significant surface pcpn. In the Valley, the surface dew points are highest in the rainy areas (e.g. the Delta), indicating this evaporation is already having a notable effect.

I believe the NAM may be 3-h or so slow with the onset of the pcpn over HMT, which I think should be underway by 03Z this evening. However, synoptic forcing of upward motion appears to not reach its peak till tomorrow.

Of phps more interest is what the NAM is doing with the overall sitn. It shows sfc wave formation offshore (near 30N/130W) tonight, and a new deepening surface low moving toward Nrn CA. The GFS is basically similar, but slower and slightly farther SE with the track of this wave. I am inclined to slightly favor the GFS in its handling of this feature, based on experience. This development will retard the eastward movement of the front now off the coast, setting up what looks to be a 24-30h period of pcpn over the HMT, with heavy pcpn ending before 12Z Tu as this new surface low moves rapidly ashore OR or Nrn CA and weakens. Frontal passage by GFS reckoning is anticipated to be about 08Z Tu (midnight LT), but I think this can at this point be considered only +/- 3h or more. Heaviest pcpn appears likely between 20Z Monday and frontal passage. Following the front snow levels will drop to 3500 to 4000' as low level flow continues with an upslope component and much colder air moves in aloft. However, synoptic forcing is likely to steadily diminish after frontal passage and be nil by 18Z Tuesday - 00Z Wednesday.

Then, there is a break of 36h or so before the next, colder system begins to affect the HMT. I expect this beginning will be late Wednesday, based on GFS indications.

John B.

GFS ensembles put about a 50% chance of the event beginning before 00h 2/27 utc Monday. Ensembles also show peak precipitation prior to 00h 2/28 and ending or tapering off significantly after 00h 3/1 utc.

The UKmet has the system starting a bit later than progged yesterday with the ARB receiving light amounts between 18h 2/26 and 00h 2/27, but not really getting going until after Sunday evening local time. The Canadian model still has the system coming in a bit earlier and is more consistent with its yesterday's run. In keeping with this scenario, the Canadian model has the best dynamics coming into the area sooner beginning not too much after 06h 2/27 utc.

The high resolution HMT 3-km run is still consistent with light precipitation starting about 3h 2/27 with rates really increasing after 06h 2/27. 1.0 to 1.5 inches of precip are forecast to have fallen by 18h 2/27 utc with the highest amounts near North Fork.

Total QPF from the NAM between 03h 2/27 and 15h 2/28 utc is 5.85 inches. Total QPF from the GFS between 00h 2/27 and 06h 3/1 utc amounts to 6.31 inches. So by both accounts this appears to be a good precip producing event. The GFS total snow accumulation in the higher terrain from this one event appears to be 20-30 inches.

Winds and temperature fields remain very similar to yesterday's forecasts with the winds primarily out of the south with speeds ranging from 30 to 55 knots in a deep layer of the atmosphere. The GFS does offer a brief veering to SW directions at 12h 2/27 utc, but this persists for only about 6 hours before the winds back again to the south. The temperature fields appear to begin with a very high freezing level (650 hPa this morning) with it descending to mountain top level at 9h 2/26 utc where it remains pretty much for the duration of the event. Then prior to the next and cold system coming in at week's end, the freezing level drops to about 850 hPa 00h 3/2 utc and rapidly extends to the valley floor 6 hours later setting the stage for a very snowy second storm.

This morning the GFS (along with the UKmet) looks to be more in line with the NAM and HMT runs as far as the onset of the event about 03ut Monday. The NAM and GFS have the event ending at about 03-06h 3/1 utc with minimal precipitation occurring after 00h 3/1.

It now appears that the next system will begin roughly at 6h 3/2 and run through 12h 3/3 utc.

Dan Birkenheuer - ESRL/GSD

Another difference today, both models do not have the wind direction as favorable as yesterday with both models showing South winds at 30-40 knots instead of the preferred SW direction. The GFS does have the winds shifting to the SW near the end of the first event at about 00h 2/28/06 utc.

Today the ensembles are not as enthusiastic about the probability of precip after 15h 2/28 utc. Precipitation totals for the first event are not fully covered by the NAM model run which has a total QPF of 2.42 inches up until 00h 2/28. The GFS has a run total QPF over the ARB of 4.62 inches between 18h 2/26 and 21h 2/28. The GFS shows the greatest intensity of precipitation falling between 06h and 12h Monday 2/27 with a secondary peak between 18h to 00h 2/28. The GFS moisture plume forecasts do not show as much promise for deep moisture making its way into the ARB compared to yesterday.

The freezing level begins the run pretty high at about 750hPa at 12ut today. It drops to mountain top level 9h 2/25 utc and then remains there throughout the first precipitation episode but drops to about 850hPa at 06h 2/28 utc. The level then drops rapidly to near valley floor levels after 18h 3/2 associated with the arrival of the very cold system discussed yesterday that is forecast to arrive late next week. Even with the warmer environment, the forecast snow totals for the GFS are on the order of 30-40 inches in the high terrain areas by the end of the "first event."

Since the GFS had the event starting earlier than we had discussed yesterday, I examined some of the other models for guidance on this key issue. The new GSD HMT run (we have made some modifications as to the 3km forecast duration), shows precip beginning in the ARB at 03h 2/27 utc, which matches today's NAM. The Canadian and UKmet models also hint at an earlier start perhaps around 18h 2/26, more in line with the GFS.

Dan Birkenheuer - ESRL/GSD

The GFS PW moisture plume situation (via the U of Hawaii website) shows that the system progged for early this weekend does not have a solid connection to tropical moisture but over time this evolves. At 6h 2/28/06 utc a major tropical plume originating at about 135w moves up the coast to merge with the smaller surge of moisture that we see arriving at the coast this weekend. This surge of moisture looks significant with PW values of ~4.5cm at its core. It briefly sweeps up the coast and the northern edge of the plume reaches into the ARB on about 00h 2/28/06 utc after which it swiftly drops south and out of the area. Following this brief surge, the long range GFS moisture forecast looks pretty grim for any additional significant moisture arriving until 06h 3/8/06 utc when a more anemic band of moisture moves inland with max PW values in the 2.8cm range. A long stretch of moisture that spans the northern Pacific after traversing high latitudes finally reaches the CONUS by the last frame of the GFS moisture run on about 6h 3/11/06 utc, with again light moisture values on the order of ~2.2cm. At the end of this current model run (last night's initialization), this last plume does not make it to the ARB but looks like its trajectory will bring it in inland within about 12h of this time.

The GFS model run has a weak wave over the ARB by 12h this Sunday. Supporting the re-enforcing moisture scenario, at this same time the greatest precipitation areas remain out over the Pacific. The real action begins between 06 and 12h Monday with a good QPF area progged over the ARB. The thickness associated with this event is pretty warm at 552. This continues until about 18h Tuesday 2/28/06 utc and is followed by a short interval of decreased scattered QPF. By Tuesday the thickness falls to the 546 range. By 06h 3/3/06 utc the cold system coming in from the NW Pacific backed by a 522 thickness core reaches the ARB with additional precipitation and a cold thickness of 534. Precipitation increases with this push of cold air and the event appears to finally end sometime Friday afternoon 3/3/06.

A quick look at the wind field associated with this system and RH cross sections along with temperature has the scenario playing out as follows:
12h 2/27/06 utc south winds at 30kts very deep moisture; this continues SW winds 20-30 kts 12h 2/28/06 utc after which there is a moisture advection lull. Resumed good moisture flow at 00h 3/1/06 utc winds SW 40 with a second but weaker moisture surge; 06h 3/1/06 utc winds SW 20 continuing to 18ut 3/1/06 utc with the winds dropping to 25 kts but still out of the S to SW. Then there is another moisture lull as the cold air approaches with the colder precip event starting 12h 3/2/06 utc until 06h 3/3/06 with SW winds at about 10-15 kts. So the initial event earlier in the week has stronger winds but the wind field with both precip episodes is from a favorable direction. Temperatures begin with the freezing level at about 800 hPa, dropping to 850 at 18h 2/28/06 utc, then dropping further to 900 6h 3/5/06 utc by the event's end.

Ensembles are also very favorable with the storm providing the following probabilities of precipitation:
0.75 12h 2/27, 0.83 00h 2/28, 0.91 12h 2/28, 1.0 00h 3/1, 0.91 12h 3/1, 0.41 00h 3/2, 0.91 00h 3/3, 1.0 12h 3/3, 0.75 00h 3/4, 0.25 12h 3/4, all times UTC.

It looks like a very active scenario coming up with the integrated QPF for the entire week working out to 6.7 inches of liquid with up to ~40 inches of snow accumulating in the higher terrain from this one event.

Dan Birkenheuer - ESRL/GSD

Paul Schultz

Paul Schultz