Widespread rain and heavy mountain snow will continue through the morning, along with gusty winds. Continued heavy snow across the mountains with scattered showers and thunderstorms expected this afternoon across the Valley. Another period of widespread rain and heavy snow expected Tuesday, with scattered showers lingering through Wednesday afternoon. Colder temperatures, possible fog and frost Thursday. Drier conditions late week.
Discussion
The broad upper-level trough currently spinning in the Gulf of Alaska will continue to provide unsettled weather across NorCal through mid week. As of 2am this morning, satellite and radar is showing the initial precipitation band moving across I-80 with some upslope flow/upper-level lift allowing for light precipitation to fill in across the foothills and along the Sierra. Based on webcams this morning, snow levels appear to be between 3500-4500 feet, which is right around where they’re forecast to be. Snow levels will continue to drop through the morning behind the cold front, dropping to around 3,000-3,500 feet by late morning. Though, snow levels will increase a bit, and remain around 3,500-4,500 feet through Wednesday morning.
Heavy mountain snow is still expected to be the primary hazard with this system as several rounds of heavy, wet snow move through NorCal. Based on the latest HREF and NBM, heaviest snow is currently moving across the Sierra and the I-80 corridor, producing 1-3″ snowfall rates.
A brief break in precipitation is noted behind the frontal band, with more scattered precipitation rounding the base of the upper low which will fill in once again through the afternoon and evening.
With more scattered activity expected this afternoon-evening, post- frontal thunderstorms will be possible. Though, instability will be fairly limited, generally 100-300 J/kg, decent low-level lapse rates, and impressive shear will allow for the few stronger storms to become briefly organized.
Forecast soundings indicate there may be a brief period where southeast 0-1km shear 20-30 knots combined with 0-6km 40-50 knot southwest shear may allow for a brief, cold- air funnel this afternoon.
Confidence is low though, considering the lack of instability, and the very brief window for marginally favorable low-level shear. Best chance to see isolated to scattered thunderstorms this afternoon will be north of the I-80 corridor and east of the I-5 corridor. With any thunderstorm that develops, expect lightning, 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, and are generally expected to be < 0.25-0.40″ per hour. Based on the HREF probabilities of >40dbz, the only marginally ‘threatened’ burn scars would be the lower elevations of the Mosquito and North Complex burn scars this afternoon.
As the low center continues to dig down the West coast through Monday night-early Tuesday, better upper-level divergence will allow for better lift. This will allow precipitation to become less scattered and fill in across the Sacramento and San Joaquin Valley’s.
With better lift, another period of 1-3″ snowfall rates is expected Tuesday, continuing to create hazardous conditions across the Sierra. 48 hour snowfall totals continue to advertise 1-4 feet for elevations primarily above 4,000 feet.
As the low center moves overhead Tuesday night-Wednesday, another round of scattered showers and thunderstorms will be possible. Best instability will be mainly in the northern San Joaquin Valley and along the Sierra.
Precipitation chances will quickly diminish north to south Wednesday afternoon as the low digs southeast, allowing drier air to filter in.
As the low exits, cold air, clearing skies and lighter winds will allow temperatures to plummet Wednesday night-Thursday. Following 2 wet days, patchy fog and frost will be possible Thursday morning across much of the Valley.
Extended Discussion (Friday through Monday)
Significant model differences with progression of short wave late in the week and high variability in cluster analysis leads to low confidence in NBM pops Friday through Monday. Ensembles and deterministic models point to a drier forecast than NBM through the extended.