July 22, 2017 – Hot and dry weather today. Widespread triple heat across the valley this weekend. A little cooler next week. Slight chance of Sierra thunderstorms Sunday afternoon and into early next week. Heat Advisory from noon today to 9 PM Sunday.
High pressure aloft over the southwest U.S. bringing fair skies to the CWA this morning with temperatures running just a bit higher that 24 hours ago. Upper ridge is forecast to back eastward over the state today bringing a warm up of several degrees. The northern Sacramento valley could see daytime highs today near 110 degrees. Will leave the heat advisory up as is with heat risk criteria still being met.
Sub tropical moisture and instability appear to remain south of the forecast area today so have left any shower or thunderstorm out of the forecast for now. Models in good agreement in moving a weak upper low in off the north coast on Sunday. More onshore flow and a slight airmass cooling will bring slightly cooler temperatures. Bigger impact may be push of sub tropical air northward up the Sierra crest. GFS shows instability along the crest as well so an afternoon slight thunderstorm chance along the Sierra crest seems a reasonable forecast threat.
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Upper low digs a bit farther southward on Monday. Southeast flow aloft will continue to bring moisture and instability up the Sierra crest so thunderstorm threat continues. More airmass cooling should drop daytime highs most valley locations down to below the century mark. Upper low remains centered around the north coast on Tuesday bringing still more cooling.
Daytime highs next Tuesday are forecast to drop to near or even a little below normal. Upper level southerly flow will continue to bring afternoon and evening thunderstorm threat up the Sierra and Cascade crest.
Extended discussion (Wednesday through Saturday)
Computer models have come into better agreement with the handling of the offshore upper low, with the GFS and ECMWF now pushing the system inland on Wednesday. As such, thunderstorm chances should continue across NorCal mountains through midweek. Drier weather ensues toward the end of next week with a slight warming trend and forecast temperatures a few degrees warmer than normal.