HMT forecast discussion for Monday 17 March 2008 (entered 19z Monday)
Summary: Models, including now all the NAEFS ensemble members, continue to agree that the moisture plume currently impinging on the Pacific NW will stall out near the CA/OR border on Tue, with 3+" of precip forecast by the NAM just into nrn CA, but nothing south to the ARB. This leaves the Wed-Thu shortwave to bring in the precip, and pretty much all models do, but with wide variation. It looks though that the main precip falls with cold advection and would likely be over to snow at BLU for the bulk of the precip on Wed night into Thu. The late Friday shortwave is becoming less likely to do anything, and there continues to be lots of differences in something for the weekend and beyond. The 12z models paint a similar picture, so it does not look good for any IOP this week. Details are below, but based on discussion during the telcon, it was decided that the project would end today, and this will be the last formal forecast discussion.
Discussion: Upper low sitting over Alaska with a jet and moisture plume into the Pacific NW. The upper low will remain pretty much in place this week, with shortwaves moving around it and towards the Pacific Nw. The plume extends from the Pacific NW all the way back to the wsw to an unsettled area wnw of Hawaii. As we've been talking about for awhile, the plume will drift down the coast over the next two days, but all models and now all the 00z NAEFS ensemble members stall it near the OR/CA border. This ends up producing a nice precip max in this area with the 12z NAM predicting over 3" by late Tue in sw OR into extreme nw CA; less on the GFS but well over an inch. However, the moisture never gets farther south and the ARB stays dry, although our local hi-res models do put out some light precip in the higher areas of the ARB Tue night. Then a shortwave rotating around the upper low, that is now in good agreement amongst the forecasts, comes across the ARB late Wed, which forces the precip to the south. General timing would be 18z/Wed until 15z or so on Thu. The 06z NAM had 0.84" at BLU but the 12z is down to about 0.55", with less than a tenth at KSAC. Other models have less, with the 00z/ECMWF only predicting a few hundreths. Looking at the BUFR soundings from the 06z NAM, it appeared that most of the precip came very late Wed and then overnight during cold advection with the shortwave, so with good cooling the mild temperatures initially would drop and at BLU it looks like the bulk of the precip would fall as snow Wed night into early Thu. Our local model ensembles do have spots over an inch in the higher areas of the ARB with this system, and individual members areas of 1-1.5" by 12z/Thu. This may be a tad high based on experience so far with these 3 km runs. Looking at cross-sections of precip type and temperature, the local runs confirm the conclusion based on the larger scale models that the bulk of the precip at BLU would be in the form of snow with the freezing leverl lowering. Discussion during the telcon was focused on this feature, with the CRNFC suggesting snow levels in the 4-5 kft range by Wed night. Given then that: 1) the warm rain with the plume never makes it far enough south Tue-Wed, and 2) when the precip does fall it is with the shortwave that brings cooling and likely snow at BLU, the system was not of interest for a potential IOP. Therefore the decision was made to end the conference calls as of today as well as the field program for this season.
To finish out the discussion...the next shortwave would be on track for Friday but all indications are this is weaker than earlier forecasts and most of the models and even the ensembles predict nothing or only light precip with this in the ARB. Thereafter there remains quite a bit of dissagreement on the timing and strength of the next storm, perhaps by Sun/23 Mar, but overall the trend is for a weaker event and not a strong storm Sun-Mon that was indicated in earlier runs.
ed szoke NOAA/CIRA/GSD