March 19, 2021 – Rain and snow showers linger over the region, especially over the over the mountains into Saturday, then dry and warmer on Sunday.
Cold, deep upper low forecast to spin off the VCRISL/WA coast today, opening-up and moving onshore with the trof extending southward tonight. Bands of showers will move through NorCal during this time, mainly over the northern zones and southern zones, while the mid section of the CWA will apparently remain within the mid level dry (or drier) slot.
The west side of the Valley and lee-side of the coastal range will be rain-shadowed for the most part. Snow levels will mostly be AOA 3000-3500 ft over Wrn Shasta Co today, limiting the impact on travel there, while ranging from 5500-6000 feet over zone 69 today, lowering to 4000-4500 ft this evening as the QPF winds down. With the colder air from trof axis directly over NorCal around 12z Sat the snow levels will lower to 2000 ft over Shasta Co to 3000-3500 ft over the Sierra as the QPF winds down.
The heaviest QPF overnite in Shasta Co was mainly along and east of I-5 with even a brief C-G strike east of O’Brien around 09z. With the forecast band of showers moving through today, have leaned to the higher WPC QPF forecasts which mainly affected Shasta Co and Srn zone 69. Based on the QPF forecast today and this evening will continue the WSW for zone 69. And, it looks as if only the Srn routes will be impacted the most, mainly Hwys 50/88 as Hwys 4/108 are closed at the passes.
Forecast soundings show several hundred J of CAPE with a strong capping inversion just above 700 mbs early Fri afternoon in the Nrn half of the Sac Vly, but favorable low level wind shear profiles. Will continue to monitor for the chance for brief thunder, but the dynamics look more favorable this evening as energy within the trof axis enhances ascent in the Nrn Sac Vly and Shasta Co/NErn Tehama Co mtns.
Shower chances linger mainly over the mtns on Sat due to the upper trof axis, elongated trof axis and continued cyclonic flow moving through Norcal. Snow levels will moderate (rise to 3500 Nrn mtns to 4000-4500 ft Srn Sierra) during the day as the trof axis moves through the area.
Dry wx is expected on Sun as upper level ridging moves back over Northern California bringing warmer temperatures, mostly clear skies, and dry weather. As skies clear early Sun morning, some frost is possible in the out-lying areas in the Valley.
Extended discussion (Tuesday through Friday)
Dry weather Tuesday into Wednesday with warming temperatures and some gusty north to east wind as EPAC upper level ridging extends inland over NorCal. Upper ridge retrogrades late Wednesday into Thursday as short wave trough digs SE from GOA into the Great Basin.
Main threat of associated light showers over CWA looks to be in the mountains with snow levels initially above 6000 feet Wednesday evening, lowering to 4500 to 5500 feet by Thursday afternoon.
Drier weather returns Thursday night into Friday with some locally gusty north to east wind as EPAC upper ridge rebuilds inland.