HMT Forecast 2/20/2007
Today's GFS looks on track for the IOP beginning 08Z Wednesday Feb 21. Some
light orographic precip ahead of the main front should gradually be working
down over the ARB from the northern Sierra between 00Z and 12Z Wed. The heavier
frontal precipitation picks up around 03Z Thu Feb 22. The GFS yields about
1.6" up until a lull in the action on 02Z Fri Feb 23.
The NAM is similar in timing and intensity with a start time of light precip
03Z Wed and the heavier precip picking up around 04Z Thu Feb 22. The 06Z
WRF-NMM highlights the light precip event commencing over the NRN ARB by 00Z
Wed with a max of 1.0" over the NRN ARB by 16Z Wed. The WRF-NMM is consistent
with the other models bringing in the main frontal precip to the WRN ARB
by 02Z Thu. By 06Z Fri the WRF-NMM has widespread areas of 2-3" of precip
with pockets of 5" over the NRN ARB. Snow levels look to be dropping from
5000' to 3000' during the main precip event. The PW plume in the GFS looks
respectable at almost an inch 12Z Thu connecting to the tropics.
The second colder wave cuts off SW of the ARB limiting precip from this event
to just around 0.1" between 06Z Fri and 00Z Sat Feb 24. This cutoff moves
SSE from SFO at about 13Z Fri, so any precipitation after that would have
to battle some NVA and low-level downslope winds. Snow levels could make it
down to 1000' or so early Friday morning.
The weekend system is still below IOP criteria with the GFS having 0.6" on
Sun Feb 25, some increase from yesterday's 12Z run.
The following system for Wed Feb 28 bears watching as it could be strong enough
for a marginal IOP. Confidence is low at this point on the GFS ensembles.