May 16, 2020 – Cool, wet, and windy weather expected tonight into Monday. Periods of moderate to locally heavy snow possible above 6000 feet Sunday night through Monday. Thunderstorms could develop on Sunday and Monday. Lingering showers expected Tuesday into Wednesday, mainly in the foothills and mountains.
IR satellite showing Pacific frontal system approaching the NorCal coast attm. Models suggesting associated precip spreading into Western Shasta county and the Coastal Range this afternoon. Mostly cloudy skies expected over the remainder of the CWA today. Cooler temperatures progged over the northern quarter of the forecast area and in the Coastal mountains. Highs elsewhere progged to be within a few degrees of Friday. Low level southerly flow increases ahead of the baroclinic zone in the Central Valley this afternoon.
Frontal precip spreads over interior NorCal tonight into Sunday morning. Snow levels progged to remain generally above 8000 feet during this period. Strong winds expected ahead and along the boundary late Saturday night and Sunday with potential for advisory speeds in the Sacramento Valley.
Frontal boundary progged to the Sierra by Sunday afternoon. Post-frontal instability depicted by models for thunderstorm potential tomorrow afternoon with highest CAPE over Shasta county, NE foothills, and the Motherlode.
Snow levels drop Sunday night to around 6000 to 7000 feet as snow ramps up. Strong winds with late season snow amounts support a Winter Storm Watch from late Sunday night through late Monday night. Potential for travel impacting weather to continue Tuesday in the Sierra. Current snow total amounts look to be range from 2 to 8 inches above 6000 feet with local amounts possibly up to a foot.
Post-frontal showers continue Monday as center of cold upper low remains offshore. Steepening lapse rates and deeper CAPE support more widespread thunderstorm development over the CWA. With 5h temps lowering to around -25 deg C over interior NorCal Monday, potential exists for some hail with stronger storms. Monday looks to be the coolest day with highs in the Central Valley in the 60s to lower 70s, and 30s to 50s for the mountains and foothills.
Lingering showers continue Tuesday as upper low tracks across Central CA. Best chances will be in the foothills and mountains where a slight chance of afternoon thunderstorms will continue. Highs rebound slightly Tuesday, but remain about 5 to 10 degrees below normal.
Extended Discussion (Wednesday through Saturday)
Ensemble guidance in relatively good agreement on Wednesday indicating the trough lifting out to the east while high pressure attempts to build into NorCal. Most of the forecast area will return to dry and milder weather except for lingering shower chances across the mountains.
Less confidence in the forecast late next week with differing solutions in guidance handling the next upstream trough. For now, have leaned heavily toward NBM with generally dry and mild weather forecast to continue.