Widespread showers and cooler weather today. Dry and gradually milder by the middle of next week.

Discussion

Sly flow ahead of the closed low near 35N/128W has tapped an .80″ to 1″ TPW plume and precip frontal band oriented meridionally over NorCal. This TPW plume (mainly categorized as a weak AR, but marginally classified as a low end moderate AR) is forecast to but shunted well to our SE into the LA/SoCal area by 11 am so the initial band of precip should move through our CWA this morning. After which, the precip should turn to scattered showers as the closed low moves inland over central CA and south of our CWA, preceded by a narrow mid level dry slot behind the TPW plume.

Satellite imagery indicates convective cellular cloudiness with the seasonably cold closed low, and developing C-G strikes. With the minus 24 deg C 5H isotherm over the extreme Srn portion of our CWA this afternoon we can’t rule out the chance of thunderstorms. Some of the CAMs suggest a narrow convective band developing in the Nrn SJV this afternoon, and possibly the Srn Mother Lode.

Snow profilers show a 6 kft snow level this early this morning at Oroville and Colfax, and is a good starting point at the onset of the precip. The St Helena profiler farther W and relatively closer to the low center shows about 5.5 kft and would be a good approximation for the snow level this afternoon as the colder air moves over our CWA. This will not be an orographically favored storm for the Sierra due to the Sly flow backing to SEly flow this afternoon as the closed low moves inland. Thus snow amounts will not be excessive, but given the time of year with be marginally within the snow advisory criteria. Marginal impacts are expected from the snow although there are winter closure of highways 4, 108, 120 in zone 69 where most of the snow should fall.

Precip will diminish this evening as the closed low moves deep into Socal and then across the Desert SW on Tue. Models suggest some lingering light showers are possible in zone 69 near the Sierra Crest as cyclonic flow lingers over the Sierra, while short wave ridging brings dry wx and mild wx to the rest of the CWA. Short wave ridging will flatten some on Wed as a trough moves through the Pac NW. This will bring additional cloud cover and slightly cooler daytime highs compared to Tue. Milder wx expected Thu as the trof moves Ewd and Ern Pac ridging moves towards the W Coast.

Extended Discussion (Friday through Monday)

Short wave upper ridging progged through interior NorCal Friday into Saturday with some locally gusty north to east wind. Short wave trough brushes across northern and eastern portions of the CWA Sunday into Sunday evening bringing a slight chance of some light showers, mainly over the Shasta mountains and Sierra Nevada. Drier weather returns Monday as EPAC upper ridge builds inland. Above normal high temperatures expected through the extended forecast period with minor cooling Sunday into Monday.