October 29, 2017 – Dry and continued warm weather today. Near normal temperatures return beginning Monday, and precipitation is likely by Friday or Saturday.
Persistent upper level ridge has begun to erode as a weak, closed upper low approaches Southern California and as a shortwave trough moves into the Pacific Northwest. Light winds prevail across the region under clear skies, the exception being a slowly growing marine layer near the Bay. Temperatures at this hour are within a few degrees of 24 hours ago.
Fair weather will continue today, and through the short term. Weakening high pressure will bring about a cooling trend starting today. High temperatures today will still be a bit warmer than normal, with lower to mid 80s expected across the Valley, and 60s to 70s over the mountains. Highs will be closer to normal Monday through Wednesday, as temperatures will reach the 70s across the Valley.
The aforementioned Pacific Northwest trough will continue to drop southward, and eventually form a closed low off the SoCal coast midweek. A brief period of breezy north to northeast winds are expected mainly over the mountains Tuesday and Wednesday after this system passes.
Extended discussion (Thursday through Sunday)
Significant weather pattern change starts on Thursday as an upper level low pressure system off the Pacific Northwest coast digs southward. Increasing clouds, cooler temperatures and breezier south winds will result. There may also be enough instability for some light rain in the afternoon over the northern mountains. Snow levels should start out high enough to avoid any surface impacts.
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The upper low continues to dig southward off the Pacific Northwest coast on Friday bringing a chance of precipitation to the entire CWA by afternoon. Current modeling indicates wetting rain possible over nearly all of the forecast area. Cloud cover and cooler airmass will bring down daytime highs to 5 to 10 degrees below normal for this time of year. Breezier southerly winds will also result as pressure gradients tighten.
Extended models in fairly good agreement in moving a frontal band across the north state on Saturday. Moderate precipitation is forecast but a strong subtropical moisture connection is not indicated so heavy rainfall is not forecast at this time.
The highest precipitation amounts will be over the northern Sierra where good orographics will assist in higher amounts.
Snow levels will be dropping on Saturday and could get low enough by afternoon to start impacting pass levels with light snowfall. Timing differs a bit between models but there is general agreement in upper trough pushing through NorCal on Sunday for a continued shower threat.