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HMT Forecast made 1930Z F 28 Dec 07


Ridge off W Coast surface and aloft is bringing NW flow aloft over the
American River Basin (ARB) with weak s/w trofs embedded.
These have little vigor at the sfc and as John M. indicated yda,
are xpectd to produce total pcpn in the 1/4 to 1/2 inch range
at best. The last of this series is forecast to cross the coast midday Sunday with confluent flow aloft behind it.

Overall, trend is for a fairly zonal pattern across the Pacific next few days with progressive features
Upper ridge is xpctd to amplify off W Coast Sunday and move over the coast Monday,
and then gradually shift inland ahead of trough now in extreme western Pacific.
This is a major change in the longwave pattern, at least in its implications for the West, and shows
good promise of bringing storms into the W Coast.
The first of these will affect mainly the Pacific NW on Wednesday,
but then by Thursday the GFS indicates a good possibility
of a rain/snow event into the Nrn Sierra.
I expect there will be considerable cool,
dry air near the surface getting into the circulation of this
first system to affect the area, making for tricky prediction
of snow levels and also for good low-level flow blocking by
the Sierra and a barrier jet.
This mrng's GFS is indicating 2 other potential events
immediately behind this one, so midweek travel in preparation
for a few days stay in CA should be considered.
These later events, if they materialize,
are likely to be warmer than the initial event with higher
snow levels. Uncertain at this time is whether these will tap into the extensive subtropical moisture vicinity of HI, but this seems likely.

HMT folks in CA should be aware of Valley travel hazards
due to fog likely developing over the weekend and continuing into
early next week.

John B.