December 21, 2016 – Cool and dry weather for the next couple of days except for a few showers possible over the northern Sierra this morning. A winter storm with rain, wind and significant mountain snow accumulations is expected Friday and Saturday. Winter Storm Watch from late Thursday night through Saturday afternoon for Burney Basin / Eastern Shasta County-Mountains Southwestern Shasta County to Northern Lake County-Shasta Lake Area / Northern Shasta County-West Slope Northern Sierra Nevada- Western Plumas County/Lassen Park.
Discussion
A weak weather system dropping southeastward through Norcal is bringing clouds and a few light showers to the northern Sierra this morning. This system should clear to the east by this afternoon. Some light fog has formed in the San Joaquin valley this morning but cloud cover should keep it from becoming dense. A moderately tight northeast surface gradient will continue to bring breezy north to east winds at times over the next couple of days. Northerly winds in the northern Sacramento valley are keeping temperatures up there this morning with temperatures running over 20 degrees higher than 24 hours ago. Fair skies today should allow for slightly warmer temperatures most areas. Week upper ridging should make for mainly clear skies tonight. Slightly cooler temperatures are forecast but lows in the central valley should remain mainly above freezing. Fair skies continue on Thursday under weak high pressure with daytime highs a little above normal.
Upper ridging begins to break down Thursday night as a northern Pacific frontal system moves on to the coast. Precipitation will spread across the north state Thursday night and then into the Sierra on Friday. Initially, snow levels will be fairly high until colder air behind the front moves into the state Friday evening. Have therefore delayed Winter storm watch for the coast range until snow levels drop to below local pass levels. The higher elevation pass levels of the Sierra Cascades are likely to be impacted during the day on Friday.
Snow levels drop quickly on Friday night as the main frontal band moves into the Sierra and colder air filters in. Heavy snow is forecast for the Sierra Cascades Friday night with lighter amounts over the coast range and northern mountains behind the front. Main upper low moves into the north state on Saturday bringing a transition to showers and decreasing snow over the mountains but with very low snow levels, impacts are likely to continue over the mountains. Cooler airmass will bring colder daytime highs Saturday with maximum temperatures several degrees below normal.
Extended discussion (Sunday through Wednesday)
Forecast confidence remains low through the extended period. A closed upper low is progged to move toward the West Coast early next week. These kinds of systems are notoriously difficult to model, and the GFS, ECMWF, and GEM are all over the map with the handling of this system. Behind this system, another shortwave may move through the northwesterly flow into Northern California.
Given the low predictability, we made little changes to the previous forecast package.