HMT Forecast

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