November 1, 2017 – Dry with cooling through Wednesday, then precipitation possible mainly over the mountains Thursday.
Widespread precipitation Friday and Saturday with locally moderate to heavy rainfall and heavy mountain snow. Decreasing showers Sunday with another storm possible by the middle of next week.
Winter Storm Watch from Friday afternoon through Saturday evening for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.
Discussion
Clear skies across interior NorCal early this morning under upper ridging. Northerly surface pressure gradients have weakened and winds are lighter across the northern portion of the forecast area. Current temperatures range from the mid 20s in the colder mountain valleys to the mid 50s across milder portions of the Central Valley.
Mid and upper heights fall a bit today as high pressure weakens. Light up valley wind expected today with highs in the Central Valley expected to be similar to Tuesday’s except across the northern Sacramento Valley where readings will be considerably cooler today with the loss of north winds.
Low pressure system develops over Vancouver Island Thursday and approaches northern portions of California by Thursday night. Models suggest potential for some light pre-frontal precipitation over Shasta county Thursday afternoon and across portions of the foothills and mountains Thursday night. Snow levels forecast to remain generally at or above 7000 feet during this period. Temperatures will continue to cool to below average.
Precipitation gradually spreads south across the region Friday and Friday evening as the trough deepens off the PacNW and the surface front approaches. Southerly winds will also increase across the area ahead of the front. Heaviest precipitation is expected Friday night and Saturday as the frontal system slowly moves through.
IWVT progs show baroclinic zone taping into higher TPW air Friday night into Saturday while orographic flow intensifies. This could lead to enhanced precipitation amounts over the foothills and West Slopes of the northern Sierra Nevada Saturday.
Extended discussion (Sunday through Wednesday)
Another quick moving upper level disturbance is on track to pass through the region Sunday with a general unsettled weather pattern continuing through the middle of next week. Additional snowfall of up to 3 inches for most of the Sierra and up to 12 for higher peaks south of Highway 50 are possible above 5000 feet Sunday through mid week. Rainfall amounts elsewhere could range between a quarter and a half of an inch. The general trough pattern will keep temperatures across the region below normal, although a slight warm up is possible for the start of the workweek with valley highs reaching into the 60s.