March 18, 2021 – Rain and snow spread over region today and Friday. Lingering snow showers possible over the mountains into Saturday, then dry and warmer on Sunday.
A narrow band of WAA precip ahead of the frontal precip band is crossing the coastal range at press time. Most of this precip will be weakening/dissipating early this morning as it runs out ahead of the main large scale ascent associated with the deep closed upper low over the NErn Pacific.
However, precip should enhance over NorCal during mid to late morning and into the afternoon as the upper low moves Ewd towards the Pac NW and large scale ascent and cyclonic flow enhances precip along the frontal band. Just tweaked the onset of the WSW in zone 68 to account for timing, otherwise continued previous headlines.
Initially, snow levels will be high ahead of the frontal band as warmer air associated with the moisture plume (weak AR) moves into the region.
The Cazadero snow profiler indicates the snow level is above 7 kft, but cooler air behind the frontal band will drop snow levels to around 3000 to 4500 ft in Shasta County late tonight and 4000 to 6000 ft in the Sierra on Friday.
Snow amounts are still expected to fall within bounds of an advisory with the NBM Viewer suggesting about 8-11 inches of snow for Donner Pass through Fri, Fri and about a 50/50 chance of >4 inches of snow, and about a 1 in 3 chance (~33%) of 6+ inches, and about a 20% chance of >12 inches. For BLU, the amounts drop off drastically, mostly just a couple of inches.
Meanwhile, Valley rainfall amounts will range from around 0.25 to 1.25 inches with foothill/mtn amounts 1 to 2 inches. Looking at the HREF probability of >40 dBZ the higher reflectivities over the Sierra will be this evening and later tonight when the heaviest precip is expected, although this could also be an indicator of the melting level.
There has been some lightning associated with the offshore closed low. 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 of thunder in later forecasts.
Precip chances decrease after sunset on Friday as the only remaining shower chances for Saturday are mainly over the mountains. This is due to the upper trof axis, elongated trof axis and continued cyclonic flow over Norcal.
Extended discussion (Monday through Thursday)
Short wave trough riding the EPAC ridge drops SE across eastern portions of CA into the Great Basin Monday into Monday night. Main threat of showers expected in the Shasta and Plumas mountains, and Sierra Nevada. Snow levels progged around 3500 to 5500 feet with little to no accumulations expected.
Drier weather returns Tuesday into Wednesday with warming temperatures and some breezy north to east wind. Forecast uncertainty increases Thursday on track of next short wave trough. EC and ensembles support a more westward track into NorCal than GFS and higher POPS than NBM. For now, will add a threat of light showers over higher mountain terrain.