May 15, 2019 – Light precipitation today followed by more significant rain and snow tonight and Thursday as a Pacific frontal system moves through. A chance of thunderstorms most areas Thursday afternoon and evening. A brief break Friday night then cool, wet and breezy weather expected over the weekend into the middle of next week.
NorCal currently under southwest flow aloft ahead of a large Pacific trough approaching the coast. CWA under extensive cloud cover this morning with radar showing some very light echos but so far no precipitation being reported. Light warm sector precipitation will be possible today across the entire CWA as a cold front approaches the coast. Warm air advection ahead of this front will keep snow levels high and mainly above pass levels. Current model timing passes the main cold front across NorCal this evening through Thursday morning.
Main impacts will be a significant increase in precipitation, gusty south winds and lowering snow levels with enough pass level snowfall to justify winter storm warning currently in place. Mos guidance puts wind speeds in the Sacramento valley up a little over advisory level for a couple hours just ahead of the cold front this evening. For now have not issued wind advisory due to short duration of the stronger winds.
Cold front moves east of the state Thursday morning with associated upper low pushing into NorCal on Thursday. Therefore, showers will remain likely Thursday with models showing enough instability for a threat of thunderstorms in the afternoon and evening.
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Precipitation threat shifts eastward on Friday as a small scale ridge pushes in over the west coast. Friday night looks mainly dry at this time but dry period ends quickly with next Pacific frontal system expected to push precipitation into NorCal by mid day Saturday. This cold front looks weaker and not as cold as tonight`s system but still could drop enough snowfall over the passes Saturday afternoon and night to create travel impacts.
Extended Discussion (Sunday through Wednesday)
Unsettled weather and cool temperatures look to continue from late weekend through mid-week as multiple weather systems traverse the area. Troughing moves over northern California Sunday bringing chances for showers and possibly thunderstorms to the area. Snow levels will be near pass level and may lead to mountain travel impacts over the weekend into early next week.
There may be a brief break in precipitation Monday before another system moves towards the west coast. Ensemble means are in good agreement that the continued wet pattern will continue into early next week, with the best chances Monday night and Tuesday. Valley highs continue to be in the upper 60s to low 70s through Wednesday.