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January 24, 2020 – Patchy night and morning valley fog possible today and Saturday. Light showers possible today, mainly this morning. Wetter Pacific storm moves through this weekend. Unsettled weather continues early next week.

Discussion

Short wave trough pushing through NorCal this morning with associated band of mostly light precip moving across CWA, north of I-80. Wave progresses into the Great Basin by late morning with precip threat winding down into the afternoon. S-Band snow level radar and Mesowest data indicating snow level above 8000 feet attm, but will lower during the morning as trough ushers in colder air. 88-D mosaic showing light returns occurring attm in the Sierra and scattered light showers will continue to be possible today, but best chances are this morning. Areas of mist with patchy fog have developed in portions of the Central Valley and Delta this morning. Expect visibilities will fluctuate as baroclinic zone moves through and potential for dense fog remains patchy. Upper ridging begins to build inland behind the wave this afternoon. High temperatures today projected to be near to slightly warmer for most areas compared to Thursday.

Upper ridging continues to strengthen tonight into early Saturday. Increased subsidence and moist boundary layer will raise potential for night and morning valley fog. Visibilities likely to bounce up and down as upper level RH progs and NAM simulated satellite modeling showing significant and variable amounts of high level cloudiness tonight into Saturday morning. Vorticity maxima progged through upper ridging into the PacNW tonight with some associated light precipitation possible north of I-80, best chances over the Coastal Range and Shasta mountains.

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Stronger Pacific frontal system approaches Saturday and looks to begin spreading precipitation into our Coastal Range by the afternoon. Heaviest precipitation expected Saturday night into Sunday morning as baroclinic zone moves through. Secondary waves keep a threat of post-frontal showers over interior NorCal Sunday. Some CAPE indicated late morning into Sunday afternoon over eastern portions of the Sacramento Valley extending into the foothills. Will continue to monitor runs attm to see if isolated thunderstorms need to be added to the forecast. Storm total QPF for the weekend system looks to range from around a tenth of an inch in the Northern San Joaquin Valley to upwards of an inch in the Northern Sacramento Valley. Locally up to around 2 inches of liquid precip possible in wetter portions of the foothills and mountains. Snow levels forecast to generally remain above pass levels ahead of the front on Saturday and Saturday night, lowering to around 5 to 6k feet Sunday and 4500 to 5500 feet Sunday night into Monday in showery regime. Light to moderate snow amounts are expected ranging from a few inches to locally upwards of 4 to 9 inches over the higher elevations of the Sierra, locally higher over Lassen Peak.

Broad upper ridging sets up over NorCal Monday/Tuesday, however embedded vort maxes moving through will keep a threat of light showers. Highest probabilities of precipitation will be over the mountains and Central Valley north of I-80. Slightly above normal temperatures expected in the Central Valley early next week.

Extended Discussion (Tuesday through Friday)

A short wave trough will be tracking into the PacNW early Tuesday pushing a weakening cold front into NorCal. This will bring a good chance for showers especially in the mountains but some valley showers will also be possible. Snow levels look to be between 4000 and 5000 feet Tuesday morning but rising to above 5500 feet by the late morning. Overall the system will be low impact but some light accumulating snow will be possible mainly above 5500 feet. The trough will push to the east by Tuesday evening with just some lingering mountain showers. Upper level ridging will then build into NorCal and will remain in place into next weekend. This will bring dry conditions with the exception of the far northern mountains where some warm air advection showers will continue into Wednesday before drying out. We will also see a warming trend in the temperatures and they are expected to be 5-10 degrees above average by the end of the week.