November 22, 2017 – Mild temperatures this week, with weak weather systems possible on Thanksgiving and Saturday. Limited impacts through the holiday weekend from slick roads due to rain as warm, weak systems pass through.
An upper ridge will keep most of the precipitation to areas mainly north of I80 through Saturday, and bring very mild temperatures to the area. More widespread rain is possible during the day Sunday. A cooler, wetter system could bring more extensive precipitation Sunday night into Monday. This could bring accumulating snow to pass levels, impacting travel. Forecast snowfall amounts remain uncertain at this point and are likely to change.
Discussion
Dry weather continues across interior Northern California today with strong high pressure to our south. Some high cloudiness is spilling over the top of the ridge, but hasn’t prevented areas of fog formation across much of the region’s valleys. Many observation sites have been reporting quarter-mile or less visibilities across the Central Valley, and GOES16 satellite products show these areas of fog expanding overnight. As such, a Dense Fog Advisory has been issued for the Sacramento and Northern San Joaquin Valleys through late morning. Fog may be slow to burn off today, which may help to suppress afternoon temperatures.
The stream of moisture currently off the California coast will finally push inland on Thanksgiving, weakening as it does so. A few showers and a bit of cooling is expected, but shouldn’t be a significant impact to holiday activities.
Dry weather returns on Friday. The GFS and ECMWF have begun to hint at a shortwave moving through Northern California Saturday, possibly into Sunday. Forecast confidence is low on the timing and intensity of this weather system.
Extended discussion (Sunday through Wednesday)
Weak shortwave will pass through the area early Sunday ahead of a larger low pressure system and associated trough. This will bring some showers to the area, with majority of the activity expected over higher elevations. Snow levels with this initial wave look to be above pass levels, so minimal impacts. Aforementioned low pressure system will then swing through the area later Sunday into Monday. Models have sped up timing a bit since yesterday, with precipitation occurring overnight before likely tapering off Monday morning. System doesn’t look overly wet with a tenth to half inch in the valley and up to 1.25 inches at higher elevations. As colder air wraps in, snow levels will drop below pass levels Sunday night, down to around 4000-5000 feet Monday morning. Several inches of snow accumulation possible at pass levels which could be enough to cause a few delays overnight Sunday into Monday. Dry weather returns for Tuesday into mid-week as ridging builds into the area.