HMT discussion for Sunday 17 March 2008 (entered 19z Sunday)
Summary: As we saw yesterday the plume stalls north of the ARB Tue and then a shortwave could bring something on Wed into Thu, another wave possible Fri. Both fast-moving, the first warmer but unlikely to produce >0.5" of pcpn at BLU, although some signs noted by HPC that the new ECMWF was stronger with the Wed wave, so maybe more precip is possible. Bigger storm Sun into early next week but lots of timing differences.
Discussion: Big upper low and surface low over the Aleutians with trailing moisture plume and jet back to north of Hawaii. Upper low will remain in the vicinity of Alaska spinning off shortwaves with some timing and strenght differences amongst the models and ensembles for this week. The plume itself, similar to earlier forecasts, drifts down the coast and gradually gets thinner, though still maintains a nice connection all the way back to southern latitudes. It then stalls near the OR/CA border on Tue, then lifts north slightly ahead of the first shortwave. When this wave passes later on Wed this pushes moisture back to the south and this is when the ARB could get some (up to 0.5" in the 00z GFS) and should be warm enough for rain at BLU, at least for a good portion of the event. Other models do not produce as much moisture. A few ensemble members have more, most have something Wed late into Thu (19-20 Mar). The 12z/GFS was weaker with this wave and did not produce much precip at all for the ARB, so way less than the 00z run. HPC noted on the telcon that their quick look at the ECMWF 12z run just in had a stronger wave for late Tue into Wed, so opposite the trend on the GFS. This variability is probably to be expected for small features in the Pacific rotating around an upper low positioned over Alaska.
Another wave in the Fri time frame is stronger in last night's GFS with about an inch near BLU (but over to snow) but this GFS run seems to be high compared to most ensemble members and other models. And in fact the latest 12z GFS is much weaker with this wave with NO precip at all from it. Quite a lot of variability in the ensembles as well. At any rate, any system would likely be fast-moving and probably cool enough for precip to go over to snow after a possible start as rain at BLU.
Lots of timing differences seen in the ensembles and individual model runs for a potential bigger storm late in the weekend into next week with the 00z/ECMWF the slowest. The 12z GFS has reversed its faster solution from yesterday and from 00z and now is more in line with a system midweek (beginning maybe 26 Mar). Have not seen the 12z ECMWF yet.
ed szoke NOAA/CIRA/GSD