HMT Forecast Tue 2/19/2008 2020UTC
The PACJet we've been watching for a while is on track to roll into CA in
earnest on Thursday with a precursor wave coming in this afternoon and Wed.
The atmospheric river we've been watching gets into the act beginning late in
the week, at least with pieces breaking off from the main moisture feed.
The first wave undercuts the west coast ridge and gives some precip over the
ARB between 21Z today and 21Z Wed with about .6" in the GFS. The ESRL WRF-NMM
also starts at 21Z today and has similar totals, but ends earlier at 12Z Wed.
The model timing looks good as the LAPS radar analysis shows the main line of
precip just ready to move inland from SFO at 18Z.
The Thu/Fri event comes in as a negatively tilted open wave. The GFS gives
about 1.4" between 21Z Thu and 06Z Sat with the most intense precip about
03Z Fri, slightly better than yesterday's 12Z run. During this storm the 850mb
temps slowly drop from +1.5C to -0.5C so most of the precip at Blue Canyon
should be rain with perhaps some snow towards the end. The ESRL WRF-NMM has
similar timing on the precip start.
The weekend storm has a 500mb low riding the jet and filling in as it
approaches the SW OR coast. The GFS gives about 2.9" between 21Z Sat and
09Z Mon with the bulk of the precip on Sunday. This is a bit more than
yesterday's GFS and a bit slower as the ECMWF was hinting at then. 850mb
temps more gradually drop from +2C to 0C during this event. Over the weekend
the GFS now agrees better with the ECMWF timing as the wave approaches the
coast. The ECMWF has a slightly deeper trough.
We continue to see the long atmospheric river in the 12Z GFS at 06Z Thu
extending westward from 130W back to the Philippines. The details of the
pieces that subsequently break off change somewhat in successive model runs.
There is one piece with about 28mm IWV off the Santa Barbara coast on 06Z Fri.
Then on 00Z Sun we again have about 28mm off Santa Barbara. In comparison the
00Z GFS breaks up the river slightly sooner (after 00Z Thu) with 31mm IWV
reaching Santa Barbara 12Z Fri and 25mm on 00Z Sun Feb 24.
The ECMWF hi-res shows higher 850mb dewpoint values of +6C nosing into Santa
Barbara at 00Z Sun (with a more continuous plume) compared with the ECMWF
low-res and GFS.
Longer term on the GFS we see in interesting atmospheric river possibility
on Mar 3 with a secondary event on Mar 5.