January 31, 2020 – Dry with above normal temperatures through Saturday. Pacific storm brings much cooler conditions with light precipitation Sunday into Sunday night, mainly over the foothills and mountains. Breezy northerly wind develops Sunday and persists into early Tuesday. Dry conditions expected throughout much of next week.
Upper ridge axis parked over NorCal as embedded vorticity maxima, and associated baroclinic cirrus, move through. T-Td spreads are becoming smaller with mist attm in the Delta and Northern San Joaquin Valley. Some potential for patchy valley fog this morning, but enough higher clouds moving overhead to hinder development. AMS warms slightly today as 1000-500mb thicknesses increase under stronger subsidence. High temperatures this afternoon expected to be around 10 to 15 degrees above average, but remaining below record levels.
Upstream Pacific short wave trough approaches Saturday as upper ridging weakens and progresses. Clouds will be on the increase Saturday into Saturday night. Patchy valley fog will continue to be possible Saturday morning in the Southern Sacramento and Northern San Joaquin Valleys, and Delta. Max temperatures Saturday will continue above normal with mid to upper 60s in the Central Valley, mainly 50s to lower 60s in the foothills and mountains.
Some precipitation possible after midnight over the Coastal and Shasta mountains with approaching baroclinic zone. Bulk of light precipitation is modeled over the foothills and mountains Sunday into Sunday night as Pacific short wave moves through. A few hundredths of rain possible in northern and eastern portions of the Sacramento Valley with upwards of about three tenths in the wetter mountain areas. System ushers in significantly cooler air with high temperatures Sunday upwards of 25 degrees lower than Saturday. Snow levels plummet during the day Sunday lowering into the foothills. Snow totals look light, given limited moisture, with local amounts up to around 5 inches over the higher elevations of Western Plumas and Northern Sierra Nevada. Locally up to one inch of snow could fall in the higher elevations of the foothills Sunday afternoon into night. Breezy to locally windy conditions expected Sunday with trough.
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Precip threat ends early Monday as high amplitude upper ridging rebuilds over Interior NorCal. Surface northerly wind ramps up in the Sacramento Valley late Sunday into Monday under strong upper level northerly momentum transfer downward, and increased N-S surface pressure gradient. Breezy north to east wind over portions of the CWA persists Monday into early Tuesday. Monday morning lows in the Sacramento Valley will be in the 30s and could locally be below freezing in wind sheltered areas. High temperatures Monday continue well below normal with lower 50s in the Central Valley and 20s to 40s for the foothills and mountains.
Extended Discussion (Tuesday through Friday)
Breezy northerly winds early Tuesday morning will begin to decrease by the afternoon hours as a trough over the Four Corners region continues to push southeast away from interior northern California. In general, light winds, mostly clear skies, and increasing temperatures will be seen for the work week as ridging builds offshore. A system grazing just north of the area will bring increased cloud cover Tuesday into Wednesday, although no precipitation is expected at this time. Ensemble guidance continues weak ridging across the area through Friday with temperatures reaching slightly above average by the end of the week.