November 25, 2017 – Mostly dry and mild during the day today. Widespread rain spreads south Saturday night and Sunday followed by a colder system with mountain snow Sunday night into early Monday.
Strong ridge of high pressure has begun to shift eastward as a longwave trough moves toward the West Coast. Extensive cirrus shield has moved overhead, and the network of doppler radars has actually detected some light high-level returns (not reaching the ground). This high cloud cover is also inhibiting dense fog formation across the Central Valley, at least compared to the last few mornings. Expect some localized reductions in visibility toward sunrise. Most of the region will remain dry and mild today.
Precipitation associated with the aforementioned storm system will begin to move into the region early Sunday morning, then spread southward across Northern California during the day Sunday. Showers will continue over the Sierra Sunday night, with conditions largely drying during the day Monday.
At this stage we’re expecting 0.50-1.00 inches of rain across the Sacramento Valley, with totals tapering off drastically southward in the San Joaquin Valley. Locally higher totals will be possible in the Sac Valley, as there will be a slight chance of thunderstorms Sunday afternoon and evening. 1 to 3 inches of liquid precipitation is forecast across the mountains. Breezy south winds are also expected with this system, with gusts up to 35 mph possible across portions of the Sacramento Valley. Higher gusts expected over higher terrain.
Snow levels will start out rather high, but lower below pass levels late Sunday night or early Monday morning. There is some uncertainty with how much precipitation will continue to fall behind the frontal system. Current projections have 6 to 12 inches of snowfall possible at 7000 ft and above, with a dusting down to perhaps 5000-6000 feet.
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Drier weather settles in Monday into Tuesday with locally breezy north winds possible. Fog will probably develop in the valleys next week, but difficult to say at this stage how extensive the fog will be.
Extended discussion (Wednesday through Saturday)
Building high pressure over the west coast will bring mainly fair skies and a little above normal temperatures Wednesday. Stable conditions and clearing skies in the morning will allow for patchy fog in the southern Sacramento and northern San Joaquin valleys. Fair skies and warm conditions continue into Thursday under upper level ridging.
Upper ridging starts to break down by the end of the weak as an upper trough approaches the Pacific Northwest coast. Models differ on timing on dealing with a low moving into the Socal coast. GFS puts showers over the Sierra Friday afternoon so included a slight threat then. Upper trough pushes into the west coast around Saturday bringing a chance of rain to much of the CWA. The end of the week will also see increasing southerly winds with snow levels low enough to impact the major Sierra passes.