November 28, 2018 – Wet pattern continues through the weekend with several weather system forecast to affect the region. A stronger system moves through tonight and Thursday with valley rain, mountain snow, thunderstorms and gusty southerly winds. That will be followed by a colder system over the weekend.
Discussion
Yesterday’s frontal system has moved south and stalled across central California. Next wave is moving ashore south of the Bay Area where heaviest QPF is expected today across the southern forecast area as it interacts with the front. Occasional showers along with a few thunderstorms are expected across the remainder of the forecast area today.
Snow will continue today, heavy at times, across the high Sierra. Snow levels are expected to remain fairly high, but low enough for continuing travel impacts through the passes including Donner on I80 and Echo Summit on US50. A winter storm warning is in effect through early Friday.
The next potentially more impactful system will move into the region tonight into Thursday. This next system will have heavier precipitation, thunderstorms, gusty winds and heavy snow over the mountains. Due to the potential for thunderstorms, have issued a flash flood/debris flow watch for the Camp, Mendocino Complex and Carr, Hirz-Delta fires for late tonight through Thursday morning.
This system will continue to gather strength offshore today which will lead to tightening southeasterly surface pressure gradient. Initial surface low is forecast to move inland north of Sacramento late tonight with gusty southerly winds expected to be strongest to its south. Deeper main surface low forecast to be filling as it moves through the Bay Area by 00Z Friday. So primary wind impacts for the valley expected late tonight into Thursday morning mainly from the Sacramento area southward.
Somewhat drier and more settled weather returns Friday, but shower chances linger over the mountains.
Extended Discussion (Sunday through Wednesday)
Models fairly consistent for start of extended period Sunday with NorCal between a large upper low over the northwest U.S. and a high amplitude ridge over the eastern Pacific. Shortwaves dropping down the back side of the low may bring a few showers over the north state Sunday but likely only over the higher elevations.
Cold air out of the arctic and western Canada will bring down snow levels to below 3000 feet so the upper Sierra foothills may see a little snowfall but precipitation amounts should be fairly light keeping impacts relatively minimal. Daytime highs Sunday will be several degrees below normal in this cold airmass with overnight minimums fairly chilly considering moderate cloud cover.
At this time, Monday looks dry for interior NorCal under upper level ridging. More sun will mean a slight warm up but daytime highs will still remain below normal. Extended models aim the next Pacific Frontal system into NorCal around Tuesday of next week but differ on timing a bit with ECMWF being a little faster. Have brought precipitation threat back into the forecast during the day on Tuesday continuing into Wednesday. Snow levels come up a bit as a Pacific low starts to move onshore but still fairly low for this time of year at between 3000 and 4000 feet. QPF and snow levels would indicate mountain travel impacts again by mid week but poor model run to run consistence makes for low confidence in details.