April 6, 2018 – A warm and wet system will move through NorCal today and tonight with a transition to showers by Saturday afternoon. Moderate to occasionally heavy precipitation is expected with breezy winds. Colder air will move in late Saturday with a few inches of snow at pass level. Sunday and Monday look dry with the next chance of precipitation over NorCal next Tuesday.
Light overrunning type precipitation has been falling over most of the CWA through the overnight hours. So far precipitation amounts have ranged from a few tenths of an inch over Shasta county to only a few hundredths of an inch over Stanislaus county. Snow levels remain elevated under upper level ridging at above 8000 feet so travel impacts over the mountains are minimal.
A Pacific frontal band has begun moving in to northwest California this morning and this front will bring with it significant precipitation thanks to a subtropical moisture tap spanning back beyond Hawaii. The period of heaviest precipitation appears will be from later this morning through Saturday morning. Storm totals during this 30 hour period still on track to range from 1 1/2 to 2 1/2 inches in the valley to several inches over the mountains. Snow levels will remain high during this period at above pass levels so travel impacts over the mountains will stay minor.
High snow levels will bring some snow melt however which combined with moderate to heavy rainfall could bring some localized flood issues and so the flood watch will stay in place. Winds will be breezy this afternoon and overnight and may even reach wind advisory criteria briefly but likely only with the passage of the cold front.
The rain will drop off fairly quickly on Saturday afternoon as the front passes east of the state. At this time the main upper low associated with the front will be pushing into the Pacific Northwest leaving NorCal under weak troughing. Therefore,showers will remain a threat Saturday afternoon with a slight threat of thunderstorms as well although stability progs are not showing a great deal of instability at this time. Snow levels will drop below pass levels Saturday in the cold air associated with the low but precipitation amounts will have dropped off enough that only an inch or two of snow is expected at Sierra pass elevations.
Building upper ridge over the eastern Pacific and west coast will lift the shower threat northward on Sunday with the entire CWA under dry conditions by Sunday afternoon. Clearing skies will allow temperatures to warm to above normal by Sunday afternoon. The upper ridge is forecast by amplify over the west coast on Monday bringing fair skies and warming temperatures with highs 10 degrees or more above normal and pushing back up to around 80 throughout the central valley.
Extended discussion (Tuesday through Friday)
Weak system forecast to move through the West Coast ridge Tuesday bringing a quick shot of showers to areas mainly north of I-80. A colder system may bring showers further south Wednesday night and Thursday before moving east by Friday.