WX Discussion and Forecast for 7-March-06
Precipitation picked up overnight and this morning a pretty good band of precipitation is moving through the ARB with heavy snow reported at Blue Canyon. Some storm total numbers as of 12z this morning include 2.27" at Blue Canyon, but in the heavy band that set up to the north of the area 4.56" at Yorkville and 5.68" at Venado, both north of San Francisco. Some missing times at Brush Creek and Bucks Lake, both having at least 3". Squaw Valley Ski area reported 12" of snow so far but Sierra-at-Tahoe has received 22" over two days. This morning's heavier precipitation is occuring ahead of the trough axis which will move across the ARB 18-21z today with winds shifting to the NW from the surface through midlevels and drier air pushing in after the trough passage. Precipitation will persist in higher areas for at least 6 hours or so after trough passage then become scattered this evening and end later tonight. The heaviest precipitation should mostly be over by the time of our conference call today.
Looking ahead the overall scenario outlined yesterday still looks good: the next system a fast-moving one arriving late Wed night into Thursday followed by a series of waves with progressively colder conditions before the overall upper low sinks south of the ARB by about Mon/13 Mar bringing an onset of what looks like an extended period of drier weather as the mean ridge moves over the West Coast for next week and the mean trough into the CONUS. The ensembles (00z GFS) are in pretty good agreement with this through day 10 (next Friday/17 Mar, and the mean 8-10 day forecast from the 00z ECMWF also supports this ridge position), then more spread develops with the ridge potentially moving inland, opening the West Coast to possible moist systems again for later this month, though right now not a lot of the ensembles show much heavy precipitation even for days 10-15. The 12z GFS deterministic forecast does have such a moist system by day 15 (23 Mar), a long way off.
As for the upcoming action from Thu-Sun (9-12 Mar), the Thu system looks to be moving in quicker and with somewhat less moisture, so likely Wed night through midday Thu. This is followed by a series of shortwaves as an overall upper low strengthens off the Pacific Northwest Friday and moves slowly southward down the West Coast. Timing is tricky with each wave and precipitation amounts would be less than the current moist system, with progressively colder temperatures. By Sat/12z (11 Mar) thicknesses in the ARB are down to the mid-520's which would drop snow levels quite low, but most of the more significant precipitation may be confined to higher areas. Total precipitation for the whole Wed night through weekend period on the GFS is about 2" in the higher areas, so the amount of moisture certainly the main issue (though, as noted yesterday, there currently is a tie to the Hawaiin moisture as the system is moving across the Gulf of Alaska, so perhaps there could be more). The biggest characteristic of this period would be how cold the conditions become with time.
Decision was made to end the HMT field exercise at the end of this IOP. It's been enjoyable, we look forward to next season! ed szoke ESRL/GSD