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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 yesterday, are expected to produce total pcpn in the 1/4 to 1/2 inch range (water equivalent) 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 (Global Forecast System) 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.

It should be noted that during the telcon there was some opinion expressed that these events will not be extremely warm (though warmer than the current weak events, in which a little light snow has fallen at Auburn), and that snow levels are likely to get to 6,500 to 7,000ft briefly, if at all, even after the first event.

Regarding the first event (anticipated Thursday next week) precipitation may be relatively heavier in the coast ranges than the Sierra, as it likely will be weakening as it approaches the coast and encounters the residual shallow cold, relatively dry air and high static stability at low levels noted above.

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.