February 13, 2018 – A slight chance of light rain/snow showers over the mountains Wednesday and Wednesday night, otherwise dry through the end of the week. A chance of snow showers over the mountains again early next week. Temperatures warming this week, then cooling to near normal early next week.
Upper low dropping into southern California with high pressure pushing in over the north state. Most of the forecast area is seeing clear skies this morning with a decreasing northerly breezes keeping fog out of the valley. Northerly surface gradient is forecast to decrease quickly this morning with northerly breezes expected to drop off significantly by this afternoon.
An inside slider type trough is forecast to drop through the Pacific Northwest and northeast California on Wednesday. Models differ a bit on the strength of this system but generally agreed on at least a threat of showers over the north and northeast portion of the CWA Wednesday afternoon and evening.
As the upper trough digs to the southeast Wednesday night, this precipitation threat will shift southward into the northern Sierra. Only very light precipitation is expected from this system so impacts should remain minimal.
Upper ridge rebuilds on Thursday bringing clearer skies and warming daytime temperatures back up to above normal. Northerly breezes develop behind the departing trough on Thursday but are not expected to become especially strong.
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Warming airmass under the building ridge will bring warmer temperatures on Friday up to about 5 to 10 degrees above normal for this time of year and likely the warmest temperatures of the week.
Extended discussion (Saturday through Tuesday)
Medium range models continue to differ in the extended period. EC showing higher amplitude upper ridge off the West Coast with vort maxes digging south through the area Saturday into Monday. GFS showing more westerly component to upper flow with short wave troughs moving through the PacNW. Forecast leans more towards EC since pattern is more similar to what we are currently seeing. Thus a threat of mountain showers is possible over the weekend, then drier weather early next week as short wave ridging returns. Cooling trend Saturday into Monday to near or slightly below normal temperatures.