HMT Forecast Thu 1/24/2008 2005UTC
Once again we've had heavier precip in the central valley in the past 24 hours
(approaching 1") with lighter amounts over the ARB closer to .1".
Our next cutoff low is sliding well off OR coast this afternoon, and off the
NRN CA coast tonight. It starts to elongate towards the south Friday into
Saturday, helping to provide heavy precip just W of the ARB. Some tropical
moisture gets into the act and feeds into a heavy precip event for SoCal early
Sunday before the system is kicked out. The 12Z ESRL WRF-NMM gives up 1" of
precip in the NRN ARB by 12Z Fri with lighter amounts to the south. 12Z Fri
to 12Z Sat the amounts range from 2" in the northern ARB to .5" in the south.
The 54hr GFS snow accumulation is 20-40" of snow over the ARB ending 18Z Sat.
Orographic enhancement is limited however with the S and SSE winds for the event.
There is a break in the action beginning Sat 18Z.
On Sunday the next replacement trough is coming in from the north to help
pick up the precip over the ARB. The first part of this event continues to
look on the warm side. Light rain/snow begins about 03Z Sunday and picks up
with a total of 1.3" by 21Z Sun at which time the 850mb temp drops from about
+3C to below zero. For comparison the ESRL WRF Ensemble also shows about +3C
at 850mb on 12Z Sun with a wet-bulb zero about 1600-1700m. The Ensemble snow
accumulation level is 1400-1500m at 12Z Sun. This suggests either wet snow or
rain at Blue Canyon for the first part of the event. The radar site being at 1610m
would favor a wet snow scenario. The winds on Sunday morning are from the SSE so
upslope enhancement would be limited.
After 21Z Sunday we get an additional 1" of snow liquid equivalent that tapers
off Monday afternoon with 850mb temps dropping to -6C. The tropical moisture
feed will be shunted away from the cold part of the storm. Monday night through
Tuesday morning represents a brief dry break.
The ECMWF looks similar to the GFS over the 96hr fcst period.
The system for next Tuesday continues to bear watching in the GFS. Between
18Z Tue and 14Z Wed we get about .8" of cold precip with 850mb temps in the -3C
to -7C range. There is however only a limited feed to sub-tropical moisture.
The ECMWF has a flatter appearance for this trough.
Later next week we continue a more progressive pattern with a wave train in the
NRN PAC. The PAC JET sags southward a bit in the WRN PAC during the period.