Gusty northerly winds possible across interior NorCal Wednesday into Thursday

October 7, 2019 – Warm and dry again today with above average temperatures, then cooling Tuesday and Wednesday. Near normal temperatures return late week into the weekend. Breezy to windy conditions at times Wednesday into Thursday with elevated fire weather conditions.

A low pressure system digging into the Great Basin region will create gusty north to east winds over portions of interior Northern California Wednesday and Thursday. Daytime minimum relative humidities will be very low, mainly in the teens, with poor overnight recoveries Wednesday night. These will lead to critical fire weather conditions for the northern Coastal Range and foothills, the Sacramento Valley, and the Northern Sierra Nevada and foothills below 6000 feet.

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Discussion

Some high clouds associated with a weak mid/upper low are moving across the southern portion of the forecast area early this morning, otherwise skies are clear. Current temperatures range from the lower 30s in the mountain valleys to the 50s across the Central Valley.

Warm and dry weather will continue today with temperatures similar, or up slightly, compared to Sunday’s. Upstream trough presently along the northern BC coast will approach tonight and Tuesday bringing synoptic cooling to NorCal.

This progressive trof will be moving across the Nrn Rockys/Great Basin during the middle of the week and in its wake, tightening Nly and Ely pressure gradients over Norcal. Forecast SLPs some of the highest/strongest that we have studied in high wind events, so we look for some high wind signals on the WR Toolkit.

One, the climo percentile equals/exceeds 97.5 from 00z Wed to 18z Thu, with the 99.5 bulls-eye over the Sac Vly, and a 700 mbs bulls-eye over the NE foothills into the Sac Vly at 00z and 06z Thu. This offers some terrific upper support for widespread windy conditions, and a 1:10 yr event for this time frame. ECM ENS for BLU shows an ensemble mean wind gust of 50+ MPH at 10/10/12z, quite windy for BLU which achieves these wind speeds during winter storms and strong katabatic wind events only. Best combo of strong winds and dry fuels will be along the western edge of the Sacramento Valley into the adjacent Coast Range foothills.

Less windy and slightly warmer conditions return Thursday as short-wave ridging moves overhead.

Extended Discussion (Friday through Monday)

Upper ridge axis moves over the region, bringing light north winds and decreasing fire weather concerns. Temperatures will rise a little, with Valley highs around 80 Friday and Saturday. Models continue to show a weak trough moving in from the Pacific Northwest late Saturday into Sunday. Model ensembles and NBM keep precipitation chances north and west of our area, so dry weather is expected to continue. The main effects of the trough will be that high temperatures cool a few degrees early next week, with increased cloud cover.