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February 7, 2019 – Central valley frost this morning. Light rain and mountain snow spreading across the north state Thursday night and Friday. Moderate snow over the mountains over the weekend with relatively low snow levels and more snow down into the foothills. Gusty winds on Saturday night and Sunday. Decreased showers Monday and Tuesday then more rain and mountain snow middle to end of next week.
Small scale upper ridge over the west coast bringing mainly clear skies over the forecast area this morning. Early morning temperatures running about the same as 24 hours ago which means there will be areas of frost in the central valley again this morning. An upper low off the Pacific Northwest coast will edge towards NorCal today and will bring a threat of light showers to the coast range by this afternoon. Cloud cover associated with this system will bring up overnight lows tonight so should not see much in the way of frost in the central valley again tonight. The offshore upper low continues to drop to the southeast tonight and Friday but center remains off the Bay Area coast through the day Friday so precipitation should remain light over the forecast area through then.
This first disturbance and associated frontal band pivots through central California Friday night bringing increased precipitation. Snow levels are again fairly low so snowfall will once again impact the coast range and foothills. This initial band of precipitation shifts east of the forecast area Saturday morning but showers likely to continue over the Sierra while central valley sees a break. This break will be quite brief however with models in good agreement in dropping a cold arctic low into the Pacific northwest by Saturday afternoon. Therefore expect an increase in valley rain and mountain snow by Saturday afternoon. This cold system will bring another round of relatively low snow levels so mountain winter weather impacts continue. Precipitation increases Saturday night and early Sunday as the low drops into NorCal especially over the northern Sierra and southern Cascades where orographics are good. Surface and upper levels gradients max out Saturday night and early Sunday as well so expecting windy conditions most areas to go along with the fairly heavy snowfall over the mountains.
Heavy snow and wind expectations Friday night through Sunday have prompted issuance of a winter storm watch for all CWA mountains above 2500 feet. Winds drop off Sunday afternoon as the upper trough axis centers over the north state but precipitation continues. Upper trough axis shifts east of the state overnight Sunday into early Monday bringing decreasing showers most areas. Kept Winter storm watch up through early Monday for Sierra Cascades to handle lingering showers and snow impacts there. Although winter impacts are likely over the mountains this weekend due to moderate snowfall, the colder and drier source region of these weather systems will limit moisture and valley rainfall. Therefore flood or debris flow impacts are not expected at the lower elevations.
Extended Discussion (Monday through Thursday)
For Monday-Tuesday, light precipitation could be possible in the mountains, with dry weather returning for the rest of the area. Active pattern is expected by mid-late next week, with storm strength and timing differences leading to low confidence in specific details. The latest ECMWF and FV3 solutions indicate widespread precipitation across interior NorCal on Wednesday, while the 00z GFS run shows a slower progression. Current forecast leans more towards the ECMWF / FV3 solutions.