December 15, 2017 – Weak system brings a slight chance of light showers over the mountains Friday night into early Saturday, otherwise dry through early next week. Breezy to windy conditions forecast this weekend, resulting in increased fire concerns.
Red Flag Warning from 10 PM this evening to 10 AM PST Sunday for Carquinez Strait and Delta-Central Sacramento Valley in Glenn, Colusa, Yuba, Northern Sutter, and Butte County Below 1000 Ft- Eastern Mendocino NF-Lake County Portion of Lake-Napa-Sonoma Unit-Northern San Joaquin Valley in San Joaquin and Stanislaus Counties Below 1000 ft-Southeast Edge Shasta-Trinity NF and Western Portions of Tehama-Glenn Unit-Southern Sacramento Valley in Yolo-Sacramento Far Western Placer, southern Sutter and Solano County Below 1000 Ft.
Wind Advisory from 4 AM to 4 PM PST Saturday for Carquinez Strait and Delta-Central Sacramento Valley-Clear Lake/Southern Lake County-Mountains Southwestern Shasta County to Northern Lake County-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Southern Sacramento Valley.
Discussion (Today through Monday)
Upper level high pressure over the region overnight began to show signs of degradation as a shortwave trough approached in the eastern Pacific. Water vapor imagery showed increasing amounts of moisture streaming ahead of the approaching disturbance with increasing high level clouds for northern California.
Short range models continue to be in good agreement to tracking a shortwave trough of low pressure aloft through northern California tonight and exiting by early Saturday. This system will bring increasing cloud cover and some chances for light rain for the northern mountains and high Sierra elevations.
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The most impactful weather will be as the system passes and northerly winds develop Saturday. Pressure gradients across the valley and mountains will allow gusty winds to develop. The combination of dry fuels with a lack of recent rainfall, low relative humidities and windy conditions will provide critical fire weather conditions for portions of the valley and west valley foothills/mountains. Outdoor burning is discouraged.
Calmer winds will return Sunday into Monday as another upper level high pressure system develops and returns to northern California to provide dry conditions and above normal temperatures.
Extended discussion (Tuesday through Friday)
Models continue in agreement with a cold Pacific storm system dropping south mid-week. This system breaks down the ridge rather rapidly and brings a few impulses of precipitation across Northern California. Current forecast reflects precip chances Wed-Thur with possible pattern change. This system appears to be rather dry with only light rain and snow amounts expected. Relatively low confidence remains with specific system details.