November 16, 2017 – Pacific storm brings rain, wind and mountain snow today with showers ending by Friday morning. Dry Friday and Saturday with morning valley fog. Another weather system moves through Sunday night through the middle of next week. Winter Storm Warning until midnight tonight.
Pacific frontal band continues to drop very slowly southward and currently located roughly along the interstate 80 corridor from Blue canyon to the Bay Area. Snow levels with the front remain relatively high and appear to be around 7000 feet over the northern Sierra. Radar precipitation estimates indicate the precipitation intensity so far has been light to moderate except over the Sierra crest where orographics is bringing a little higher intensity. Breezy winds are accompanying the front but so far have not been excessively strong.
Snow levels will drop slightly this afternoon as the cold front continues to move slowly southward. Models fairly consistent in placing the front along a line from the Monterey Bay to Tuolumne county by 00z this afternoon. Main upper trough digs southward into the Pacific Northwest while a shortwave pivots around the base of the low tracking through Norcal this afternoon bringing showers to most of the northern CWA.
Isolated thunderstorms may be possible this afternoon as this shortwave pivots through but instability is fairly limited so any thunderstorms will likely be quite isolated. Shower threat continues into Friday morning before upper trough axis shifts east of the state. If clouds clear enough by Friday morning, central valley could see some morning fog.
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Upper ridge builds over the west coast by Friday afternoon bringing an end to any shower threat. Upper ridging remains over the west coast over the weekend bringing dry conditions and a slight warming trend with daytime highs climbing to near normal by Sunday. Fair skies will bring a continued morning valley fog threat.
Extended discussion (Monday through Thursday)
The next Pacific storm will move across interior NorCal late Sunday into Monday. Forecast models still disagree regarding storm strength and track. The ECMWF shows a quick moving system that brings widespread precipitation across the area. Meanwhile, the GFS pushes the moisture plume into the Pacific Northwest, keeping precipitation generally north of I-80. Regardless, confidence has improved and there should be at least some precipitation across the area.
Confidence in the forecast rapidly degrades after that system. The GFS has another system right on the heels of the Monday system, whereas the ECMWF holds it off until mid-to-late next week. Thus, our forecast remains relatively unchanged. Additional waves of precipitation could be possible through the extended.