Cooler with widespread showers tonight through Monday. Dry and gradually milder by the middle of next week. Breezy northerly winds possible late next week.
Discussion
Dry conditions are forecast across interior NorCal through this afternoon ahead of an approaching upper level low situated well offshore per latest satellite imagery. This system will bring cooler temperatures as well as widespread precipitation chances tonight into Monday. In addition, breezy south winds will pick up ahead of the system this afternoon and evening. This system will have a chance to tap some moisture with the GEFS IVT plume forecast depicting a weak AR.
Hi-Res guidance shows precipitation spreading in from west to east tonight into Monday morning. The heaviest precipitation is expected tonight through Monday morning with showers persisting the rest of the day Monday. Precipitation totals could range from 0.50″ to 1.25″ along and south of I-80 and the Coastal Range; elsewhere, 0.15″-0.75″ are expected.
Snow levels will initially start around 7000 feet, and then lower to around 5500-6000 feet by Monday morning. Snow amounts north of I-80 are forecast around 1-4″, with 5-10″, locally higher, for areas along and south of I-80. NBM guidance gives a 15-20% chance of at least 4″ of snow at Donner Pass and Echo Summit. HREF shows the heaviest snow beginning early Monday morning through mid- morning.
A Winter Weather Advisory remains in effect for the northern Sierra south of I-80 from midnight to 8 PM Monday above 6000 feet. Travelers should be prepared for slick roads, chain controls and reduced visibilities at times.
Thunderstorm potential remains on track from mid-morning into the early evening on Monday from around the Central Sacramento Valley and into the northern San Joaquin Valley and Mother Lode areas. Any thunderstorms that develop could produce small hail as well as locally brief heavy downpours.
A dry pattern returns by mid-week as ridging builds in. Forecast highs in the Valley will be in the low to mid 70s.
Extended Discussion (Thursday through Sunday)
As the mid-week trough pushes east, long wave troughing will develop over the central part of the country with ridging over the Pacific. This will set up northerly flow over the area. Ensembles are unsure on how far west that long wave troughing will set up and that will have a big impact on how strong the north winds will be. We may see breezy north winds late week but confidence is low at this time. The forecast looks to remain dry for the extended period with above average highs.