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December 24, 2020 – Dry weather continues today with dense morning fog again across portions of the Central Valley. A few sprinkles will be possible tonight as a weak weather system moves north across the region. More significant precipitation will develop on Friday/Christmas Day and continue into the weekend with winter weather travel impacts likely over the mountain passes by Friday evening.

Discussion

High clouds continue to stream northward into the region ahead of the weak subtropical low to our southwest. The clouds did not arrive early enough on Wednesday evening to inhibit dense fog development once again across the southern half of the Sacramento Valley. IR difference imagery is no help in tracking the fog as the high clouds mask the fog/stratus layer below, so we’ll rely on surface observations and traffic cameras.

The locally gusty easterly winds over the northern Sierra from 24 hours ago have subsided. Still seeing a few gusts in the teens and 20s mph as the RNO-SAC gradient remains above 5 mbs. These winds will drop off by later this morning.

Lots of mid and high clouds will continue moving north into the region during the day. The high temperature forecast for the valley will be tricky and will depend on how much mixing develops to clear out the fog/ stratus, and how early that occurs.

The weak system will lift northward tonight ahead of the approaching northern stream trough. A few sprinkles, or light showers will be possible, especially to the north and west of Sacramento. Uncertainty continues on whether we’ll see yet another night of fog development in the Central Valley as some of the guidance is suggesting.

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. Most recent model runs are tapering amounts a bit from previously, but it’s still shaping up to be a decent winter storm with mountain travel impacts increased due to increased holiday travel.

Most of the Valley is expected to see around a quarter to half an inch of rain with locally higher amounts approaching an inch possible. Heavy snow is expected to develop on Friday (Christmas Day) 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 is possible across the higher passes with up to 2 feet over the peaks Friday through Saturday. Winter weather warnings and advisories have been posted.

A brief respite in the weather will be possible later Saturday into early Sunday, but then another system is forecast to dig southeast into NorCal from the Gulf of Alaska Sunday into Monday. There is more uncertainty on the track of this system, but another round of moderate to heavy mountain snow will be possible. Stay tuned.

Extended discussion (Monday through Thursday)

By Monday morning an upper level closed low will be overhead of interior NorCal bringing continued unsettled weather through the afternoon. Still a bit of uncertainty with location and amounts for precipitation on Monday, although the better chances for additional rain and snow will be in the southern portion of the CWA. Snow levels are low on Monday morning around 2-2.5 kft, rising to 3.5 kft by the afternoon. Mountains and the upper foothills may see a couple more inches of snow before this system moves southeast and out of the area Monday night.

Models vary quite a bit with the synoptic pattern after the Sunday into Monday system. Upper ridging builds back into the area on Tuesday, although a weak wave may bring some additional chances for precipitation Tuesday mainly to the coastal and northern mountains. There is a chance unsettled weather returns to the area mid-week, although ensemble guidance suggests a ridge may be over the area which would suggest drier weather. Have kept the NBM PoPs in the forecast for now.