October 31, 2017 – Dry weather with a cooling trend through mid-week. Precipitation possible mainly over the mountains Thursday, spreading over all of Interior NorCal Friday and Saturday. Rain could be moderate, to locally heavy at times, with potentially significant accumulating mountain snow through Sunday morning.
Weak upper trough is shifting south into central California with clear skies in its wake across interior NorCal. Onshore flow has weakened and is transitioning to light, to locally moderate, north to northeasterly flow as ridging surface and aloft noses in from the north. Current temps are running cooler across the area and range from the upper 20s in the colder mountain valleys to the mid 40s to lower 50s in the Central Valley.
Upper ridging builds inland today with some locally breezy northerly wind possible over northern portions of the forecast area. Resultant downslope flow will push high temperatures near 80 degrees over the Northern Sacramento Valley. Elsewhere, max temperatures expected to be up slightly compared to Monday.
High pressure begins to weaken Wednesday allowing for some minor cooling across interior NorCal, except northern areas which may cool around 4-8 degrees.
Low pressure system develops over Vancouver Island Thursday and approaches northern portions of California Thursday night. Models suggest potential for some pre-frontal light precipitation over Shasta county Thursday afternoon and across much of the foothills and mountains Thursday night with snow levels remaining above 7000 feet. Remainder of Interior NorCal expected to remain dry Thursday as temperatures continue to cool to below seasonal normal.
Precipitation gradually spreads south across interior NorCal on Friday and Friday night as the trough deepens off the PacNW. Southerly winds will also increase across the area as the front moves into NW California.
Extended discussion (Saturday through Tuesday)
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Main forecast concern for the long term period is the incoming storm that will bring cooler conditions, breezy winds and widespread chances for rain and mountain snow. Model forecasts continue to be in good agreement with timing and amplitude of the upper level trough this weekend and into early next week. Temperatures will range between 10 and 15 degrees below normal. Rainfall amounts could range between 1 and 2 inches for the valley and 2-4 inches for the foothills and mountains.
Snow levels will start above 7000 feet and will drop rapidly through the weekend to 4000 feet by the start of next week as the upper level low moves toward northern California. Heavy storm accumulations are possible through the weekend. The heaviest areas of precipitation will move out of the area late Sunday into early Monday, but lingering light precipitation will continue through mid week, especially for the Sierra.