Periods of moderate to heavy rain and mountain snow this weekend with isolated thunderstorms. Strong wind today in the Central Valley. Dry early next week with cold nights and below normal daytime highs. Periods of heavy rain today may cause minor debris flows within the Mosquito Burn Scar area.

Discussion

Pacific frontal system beginning to move into NW California this morning with bands of light pre-frontal precip out ahead of it indicated on 88-D. IVT analysis showing associated weak to moderate AR (~500 kg/ms or less) with baroclinic zone as it moves through today. Tight pressure gradients ahead of front and 50 KT LLJ at 925 mb will result in a period of strong wind in the Central Valley today, mainly during the morning to early afternoon hours, where wind advisory is in effect for this.

Moderate to heavy rain and mountain snow expected today as front moves through with snow levels around 4500 to 5500 feet. Heavy snow continues tonight in the Sierra Nevada with front and LFQ of polar jet providing strong upper level divergence in conjunction with SWly 7H 70kt favorable orographic flow. White-out conditions likely with possible road closures.

Moderate snow expected elsewhere in the interior mountains of NorCal overnight as snow levels drop to 2500 to 3500 by Sunday morning.

Strong dynamics with front and colder unstable air behind it tonight into Sunday could result in isolated thunderstorms in the Central Valley and eastern foothills. With 5H temps in the lower -30s C, potential exists for storms with ice pellets or small hail. Looping hodographs with favorable 0-1km and 0-6km shear also suggest potential for storms with rotating updrafts which could produce funnel clouds or brief short lived tornadoes.

Main upper low moves through Sunday night into Monday morning with decreasing showers from north to south. Storm total liquid QPF over the weekend looks to be around 1-2 inches in the Central Valley, 2-4 inches in the foothills with higher amounts in the mountains.

Snow totals of 1-4 feet will be common with locally higher amounts in the Sierra Nevada.

Drier weather sets in early next week under upper level ridging. Areas of low temperatures below 32F expected in the Central Valley, especially Tue/Wed with widespread morning frost.

Extended Discussion (Wednesday through Saturday)

Ensembles are still advertising an upper-level ridge will be off the CA coast following this weekend’s storm, keeping weather quiet in the forecast through much of Tuesday. Cluster Analysis and the latest ensembles are advertising another shortwave trough off the west coast which may impact the region sometime mid to late next week.

However, there are still significant differences between the ECMWF and GFS ensembles in location, timing and strength of this shortwave. The latest ECMWF ensemble mean has the ridge over closed low remaining further west and in the Pacific with weak ridging remaining over NorCal through late week – depicting more of a Rex Block in the eastern Pacific. Whereas, the GEFS ensemble mean is depicting a more progressive low off the coast, impacting the region by as early as Tuesday afternoon. The Canadian ensemble is most similar to the ECMWF, with keeping the low off the west coast. Regardless, this low does not appear to be associated with ample moisture. However, if the Rex Block does develop, long- duration southwest flow may churn up some sub-tropical moisture.

Either way, the temperatures forecast through much of next week looks fairly consistent in bringing slightly cooler temperatures following the potent weekend system. Valley low temperatures will fall to near to below freezing each morning. Fog will also be a concern most nights/mornings with limited cloud cover.