HMT Forecast Discussion for Friday 22 Feb 08 (entered 2030z)
Bands of precip still coming thru the ARB as with weak upper low and surface low now lifting north off the WA/OR coast, with ill-defined 500 mb trough axis extending to the southeast. More westerly flow now in
place from the ARB westward at 500 mb and within this flow are still acouple of small-scale shortwaves, the last of which is still approaching the nw CA coast. Flow still southwesterly at 700 mb down to the surface
with the temperature at KBLU hovering around 32. Some precip totals so far include, as of 15z, ~1.4" at Alta, about the same at BLU (update as of 20z is close to 1.6" at BLU) but the ESRL gage showing ~10" of snow. The precip should continue until the final shortwave passes a little after 00z/Sat, with the NAM having about another 0.5" at KBLU. A look at the 12z run of one of our hi-res models shows the echoes starting to end by 00z and pretty much totally over by 03z/Sat, but still with local accumulations of 1.5-2" in the forecast.
The ensemble mean was not in yet; probably additional precip will be somewhere between the NAM and
the hi-res forecast for today, maybe another inch or so at most but probably less, in the form of snow at KBLU, with additional cooling beginning later today.
The big weekend storm is starting to take shape in the Pacific. Satellite imagery showing rapid enhancement near 35N/150W over the last 6 h (thru 12z this morning) and continuing now through 18z as the ridge to its west amplifies ahead of the very strong wave that has now moved east to 40N/170E. A look at the
various models indicates not a lot of change from what we were thinking during yesterday's telcon. Maybe a tad slower with when the system moves onshore, now consensus being ~15z on Sunday, with the GFS and ECMWF now in quite good agreement on moving the weakening system into northern CA and then as an open wave eastward. The forecasts then would have an open wave and trough axis passing the ARB around 21z/Sun to 00z/Mon. Here are some preip totals for the storm: 00z/ECMWF, 2.2" at KBLU
(more to the southeast) with 1.3" at KSAC; 12z/ECMWF, 1.8" at KBLU and 1.2" at KSAC; 12z/NAM, 3.75" at KBLU and 1.2" at KSAC; 12z/GFS, 2.9" at KBLU with 1.1" at KSAC. Precipitation looks to begin on Saturday afternoon, likely by 21z. Our hi-res runs increase the echoes by 22z/Sat and then things really pick up after
00z/Sun. Strong period of precip then overnight on Saturday night but then again on Sunday as the upper low gets closer, with totals by late Sunday in the 4-5 inch range. There will be some rapid warming as the sw flow increases to 50-60+kts from 850 mb through 700 mb by late Saturday into early Sunday. Consensus on the telcon is there should be strong warm advection Saturday afternoon and this may be enough to have the precip begin as rain (or snow over to rain) at KBLU, with the snow level rising to maybe 6000 ft or so. As noted yesterday a tricky call for this, if it is rain probably 6 h or so at most and then transitioning to snow by around 06z/Sun. The NAM BUFR soundings never really get much above freezing at KBLU, but the GFS model soundings by 00z/Sun bring the freezing level up to near 800 mb and then lower it towards 850 mb by 06z and then certainly by 12z/Sun. Our local model shows strong warming late Saturday afternoon with the freezing level rising to ~800 mb then gradually lowering after ~06z/Sun. So this would support a period of rain at the beginning of the event. Then heavy snows through Sunday with gradual cooling as the trough
approaches. With the weakening of the trough and the remnants of it passing well to the nw of the ARB probably not anywhere near as dramatic a trough passage as in the early Jan IOP, instead winds by later Sunday shifting to more westerly and the snow at KBLU gradually coming to an end overnight Sunday night, with increasingly lighter density snows as Sunday goes on.
As for the longer range, still looks dry next week as upper level ridging moves over the area. But the next wave now in both the ECMWF and latest GFS for ~Sat/1 March timeframe, with a growing consensus of something possible by then in the ensembles. After that for the following first week of March quite a bit of spread in the ensembles at this point.
ed szoke NOAA/GSD/CIRA