HMT Forecast

Today's model runs are not as exciting as yesterday's runs. The basic timing for the onset of the event is about the same with both the NAM and GFS starting things in earnest by 03h 2/27/06 utc however, the GFS has more precip by 00h (0.15 inches) starting sometime after 18h, a bit earlier. Also today the GFS appears to create a distinct gap between the two systems described yesterday with the warm event ending sometime on Tuesday and then a second system starting later in the week mainly driven by the approaching cold air. This appears to get started at about 18h 3/2/06 utc with minimal precip between the events (maybe scattered).

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