Critical fire weather conditions, strong winds through Tuesday evening

Red Flag Warning until 7 PM Tuesday and Wind Advisory in effect through this evening. Critical fire weather conditions late tonight through Tuesday evening due to strong north winds and low humidities. Lighter winds and a warming trend mid to late next week, except for locally breezy north winds Thursday. Light showers possible over the Sierra Monday.

Discussion

Anonymously deep (-3 to -4) 5H closed upper low is forecast to develop and dive SSEwd across the Great Basin today and Tue. This synoptic pattern is recognized and documented by Parish (1982) and Staudenmaier (1994) as resulting in the formation of a northerly barrier jet and the implications for extreme fire weather behavior given strong winds, lowering humidity, and dry fuels.

This has led to the issuance of a Red Flag Warning and public wind advisory. Probabilistic guidance from the NBM (4.1) also indicates possible local gusts =>50kts in a narrow corridor at the interface with the North Coastal Range and Sac Vly and the Nrn Diablo Range and Nrn San Joaquin Vly, generally from Wrn Colusa/Wrn Yolo and into Solano Co, and also extreme Wrn San Joaquin Co in the hills W of Tracy.

Northerly winds in this area will be perpendicular to major interstate travel along I-80 and I-205/580. This area is forecast to be under the NAM peak 925 mbs winds thru 15z Mon and then again around 03z Tue. This timing is just a bit outside of peak downward momentum transfer at the Valley sites and thus have held off on High Wind Warning headlines for now. Breezy winds will decrease on Tuesday, especially during the afternoon, as upper support weakens.

A strong vort max was over NorCal at press time which will be moving SEwd into SoCal thru evening. Clouds and isolated to scattered light rain/snow showers will occur over the Sierra zones in deference to this feature. 850 mbs temps and thickness values collapse in the wake of the vort max and behind the surface cold front yielding much cooler temps today and an upcoming cool nights in the Valley and cold nights in the high elevations and cold mountain basins. Although temps will gradually recover in the short term, they will remain below normal through mid week.

A much weaker trof and cold front is forecast to bring another chance of some light showers to the Nrn mtns Tue night and Wed as it moves into the Pac NW behind this deep 5H low as it moves from the Desert SW into the Nrn Plains. Pacific high pressure then builds Ewd into our forecast area late in the week.

Extended Discussion (Friday through Monday)

Upper ridge over the area Friday shifts east next weekend as upstream Pacific frontal system approaches. Models suggest baroclinic zone weakens as it moves onshore and associated precipitation remains outside of the CWA. Upper ridging returns early next week. High temperatures expected to be near to slightly below normal through the extended forecast period.