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February 17, 2018 – A cold weather system brings breezy winds and a chance of snow showers over the mountains and foothills Sunday into Monday followed by cold temperatures with a possible widespread freeze. Freeze Watch from late Sunday night through Wednesday morning.

Discussion

The weather pattern over northern California continues to be dominated by high pressure centered over the eastern Pacific. Mostly clear skies prevail across the region, with only a few high clouds passing over far northern CA as a shortwave moves through the Pacific NW. Conditions remain quite dry across the Valley, inhibiting fog formation. Temperatures across the region are mainly running a few degrees warmer than 24 hours ago.

Dry and mild weather will continue today under the influence of high pressure. The pattern will begin to change on Sunday as the aforementioned ridge begins to retrograde, allowing a series of shortwave troughs to drop south from western Canada. A chance of precipitation will begin across the northern mountains tonight, and spread south Sunday into Monday. This system doesn’t tap into much moisture, so the best chance of precipitation will remain over the mountains.

Snow levels are projected to drop down to around 1000 feet by Monday, with a few inches of snow accumulation possible at pass level, and a dusting down to around 1500 feet. This could potentially cause travel problems over the holiday weekend.

Potentially the bigger impact will be the widespread cold temperatures this system will likely bring. NAEFS ensembles are projecting this to be a 2-3 standard deviation colder than normal event, with a recurrence interval of roughly every 5-10 years. To maximize radiational cooling, winds will need to become relatively light. That isn’t a sure thing. The coldest nights across the region currently appear to be Monday and Tuesday nights. Given the potential agricultural impacts from recent warm temperatures and early bloom, a freeze watch has been issued for early next week below 2000 feet.

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Extended discussion (Wednesday through Saturday)

Extended period starts out dry and cool under upper level ridging and northerly flow. Daytime highs Wednesday will push upwards to near normal while mainly clear skies bring another morning of below freezing temperatures in the valley.

Warmer airmass finally brings morning minimums up above freezing in the valley by Thursday morning with minimum temperatures currently forecast in the lower 30s. Models drop an upper trough out of the Pacific northwest on Thursday but differ significantly on strength. GFS more shallow with this system and would keep Norcal dry while deeper ECMWF would bring light precipitation to much of the forecast area. For now, have kept precipitation chances to the mountain areas.

Friday could be dry under shortwave ridging but model timing with these shortwave systems will be difficult. Model discrepancies continue going into the weekend. Models show similar differences to Wednesday system with ECMWF again coming in deeper with a shortwave dropping out of the Pacific Northwest. For now, have kept shower threat over the mountains again with just a bit of daytime cooling until models come into better agreement.