Widespread rain and heavy mountain snow through today, along with gusty winds. Scattered showers lingering through Wednesday afternoon. Drier conditions and colder temperatures return by late week with morning frost and possible patchy fog. Winter Storm Warning until 10 AM Wednesday.
Discussion
The broad upper-level trough currently spinning off the WA coast will continue to provide unsettled weather across NorCal through Wednesday. As of 2am this morning, satellite and radar is showing the showers starting to fill in across the Bay Area and the Sacramento Valley. Mesoanalysis indicates a vorticity maxima near the Bay Area is allowing for a few embedded thunderstorms. As these dynamics and upper-level divergence continues to create enhanced lift across the region through Tuesday afternoon, expect heavier precipitation across the Sacramento and San Joaquin Valleys.
Widespread heavy snow with 1-3″ snowfall rates is expected through Tuesday, continuing to create hazardous conditions across the Sierra. Snowfall accumulations of 1 to 3 feet above elevations of 4,000 feet will be possible through Wednesday morning, with the bulk of the snowfall expected to fall today. Hires is highlighting snow showers with snowfall rates 0.5-1″ will linger across the Sierra through Wednesday morning, so extended the Winter Storm Warning through 10 am PST.
As the current low-level vorticity max moves northeast into the central Sacramento Valley around sunrise, expect some embedded thunderstorms accompanying the more expansive band of precipitation. This band will continue to fill in and move eastward through 19-22Z (12pm-3pm PST). Following this band, more scattered showers are expected through the remainder of the afternoon-Wednesday morning. There’s still some uncertainty in how much instability will be available this afternoon, following the widespread precip band. Regardless, instability will still be fairly limited with only ~200- 300 J/kg between Chico and the northern San Joaquin Valley. HREF is showing 20-30% with limited coverage of seeing 400+ J/kg this afternoon. Forecast soundings indicate there may be a brief period where southeast 0-1km shear of 15 knot shear may allow for a cold- air funnel. Confidence is low though, considering the lack of instability, and the very brief window for marginally favorable low level shear. With any thunderstorm that develops, expect brief heavy rain, gusty winds, and possible hail.
With the lack of instability, rain rates are not expected to be significant enough to be a burn scar threat today. Showers on Wednesday will be fairly light, with best chances across the Sierra.
Some patchy fog may develop across the northern Sacramento Valley Wednesday morning, but confidence is fairly low considering lingering cloud cover and weak winds of 3-6 knots.
As the upper low quickly digs southeast Wednesday afternoon, precipitation chances will quickly diminish north to south.
By Thursday morning northwest flow will develop over NorCal with a ridge just off the CA coast, allowing drier air to filter in. Cold air, clearing skies and lighter winds will allow temperatures temperatures to plummet Wednesday night-Thursday. Following multiple wet days, patchy fog and frost will be possible Thursday and Friday mornings across much of the Valley.
Extended Discussion (Saturday through Tuesday)
Short wave ridging will be in place for the start of the extended period. We will see a frosty start to the day Friday with continued below normal daytime highs. Ensembles then diverge with timing and placement of two short wave troughs over the weekend into early next week. Moisture will be low with these troughs with little to no impact expected with them but maybe a few isolated mountain showers. Overnight lows will remain chilly along with the daytime highs.