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December 18, 2017 – Dry weather this week except for showers moving quickly through the area Tuesday night into Wednesday. Breezy northerly winds Wednesday night and Thursday.
Some high clouds are spilling into NorCal over the eastern Pacific ridge, otherwise skies are clear early this morning. Winds are considerably lighter and temperatures are considerably cooler compared to 24 hours ago. Current temperatures range from the upper teens and lower 20s in the mountain valleys to the mid 30s and mid 40s elsewhere.
Dry weather will continue across interior NorCal today and Tuesday as the ridge gradually flattens and begins to shift east ahead of the upstream trough presently located over the eastern Gulf of Alaska. This system is forecast to dig southeastward along the BC coast over the next 24 hours before forming a closed low over Vancouver Island/NW Washington later on Tuesday.
A potent, fast moving, short-wave is then forecast to swing across NorCal later Tuesday into Wednesday bringing a quick shot of precipitation to the region. The valley is expected to see less than a quarter inch of rain while the Sierra Nevada may see upwards of 3-6 inches of snow with gusty southwest winds through the passes.
Most of the precipitation will clear the valley by early Wednesday morning, though steepening lapse rates behind the front will continue showers across the west slopes of the northern Sierra into early Wednesday evening.
Dry weather returns for Wednesday night with breezy northerly winds and cold temperatures developing. Most valley locations are expected to see overnight minimum temperatures in the upper 20s and lower 30s later this week.
Extended discussion (Friday through Monday)
Model differences remain in the extended, lending to a lower confidence forecast. GFS builds ridging back in for the end of the week into the weekend, while the ECMWF keeps the area sandwiched between ridge over the Pacific and trough digging into the Great Basin. The later would bring fairly good pressure gradient over the area with another period of breezy north winds. Regardless, it looks like precipitation will remain to the north and east of the area with dry weather into early next week. GFS then hints at a trough and precipitation moving across the area by the Tuesday timeframe, but the ECMWF remains dry. Highs will be in the 50s for the valley and 30s-40s for the mountains with chilly overnight lows.