December 23, 2020 – Fog will be present once again this morning in the Central Valley from about the Sacramento area southward into the San Joaquin Valley, but will break up by the afternoon. Otherwise, dry weather with locally breezy north to east winds will persist today. High pressure will begin to weaken Thursday, then more significant precipitation will develop on Christmas and continue into the weekend with winter weather travel impacts likely over the mountain passes by Friday evening.
Clear skies, other than the footprint of Central Valley fog, cover the region early this morning under strong high pressure. Moderate to strong surface pressure gradient remains in place, though the north gradient has weakened overnight (MFR-SAC is down to 7 mbs) as the east gradient continues to tighten (RNO-SAC approaching 7 mbs). The strongest northeast winds have been occurring over the northern Sierra west slopes where gusts of 30-40 mph, locally higher, are being reported. In the Sacramento Valley, gusts in the teens to mid 20s mph are still occurring over the western portions while return southeast flow is drawing dense fog northward from the northern San Joaquin flow into the Sacramento metro area.
The dry pattern will continue today and Thursday resulting in mild days (highs around 5-10 degrees above average). The north to east breezes will decrease this afternoon as the surface pressure gradients relax, but some lighter north wind will linger into Thursday.
The subtropical wave to our southwest will continue to move northeast and approach the region tonight and Thursday as the upstream trough evolves over the eastern Pacific. It will spread mid and high clouds across the area later tonight and Thursday, and may bring some virga or sprinkles, but may potentially also affect overnight low temperatures.
The bigger news remains the upcoming pattern change late in the week! Rain and mountain snow is forecast to move into the area on Friday (Christmas Day) and continue through Saturday morning. Still some uncertainty on overall QPF, but amounts have been trending up with most of the valley expected to see around a quarter to half an inch with locally higher amounts approaching an inch possible. Heavy snow is expected to develop on Christmas over the northern mountains, spreading into the northern Sierra by the evening impacting travel over Interstate 80 and the other higher northern Sierra passes. 8-18 inches of snow possible across the higher passes with up to 2-3 feet over the peaks. Winter storm watch is out, but stay tuned as forecast details are adjusted over the next few days.
Extended Discussion (Sunday through Wednesday)
An upper level trough will be digging out of the Gulf of Alaska and track down the California coast for the start of the extended period. This will bring the return of rain and snow to the area. This system does look to favor a more southern track and that will keep the best chances for precip over the souther half of our area. Light showers look to develop Sunday afternoon with the best chances for precip later Sunday evening into Sunday night as we see some weak warm air advection across the area along with vorticity advection. Showers will continue into Monday morning and will start to diminish during the afternoon as the trough pushes over SoCal. With cold air in place from the Christmas Day system we will see low snow levels with this Sunday system. Currently snow levels are looking to be between 3500 and 4500 feet and some light snow does look to be possible down to 3500 feet Sunday night into Monday morning but the majority of the accumulation should stay above 4500 feet. Current snow totals are looking to be 6-12″ above 4500 feet with a few inches possible down to 3500 feet. QPF totals will range from 0.25-0.50″ in the Sacramento and San Joaquin Valley to around 0.25″ in the northern Sac Valley and 0.50-1.00″ over the higher elevations mainly south of Lassen Park. These totals will likely change as model zero in on the track with this system.
Upper level ridging builds back in on Tuesday. A weak short wave trough will bring the return of light shower chances to northern areas late Wednesday into Thursday. Temperatures will generally be near average with the chilly overnight lows returning Monday night as high pressure builds back in.