Showers and thunderstorms over interior NorCal through Wednesday. High temperatures well below normal through mid-week, returning to near normal by next weekend. Ash & debris flow concerns on recent burn areas and urban flooding concerns today due to heavy downpours.

Discussion

Low remains nearly stationary just offshore to the southwest of Cape Mendocino. Frontal zone to the east of the low center has only shifted slowly east overnight. The focus of heavier rain was along the Coast Range and the western side of the Sacramento Valley Sunday evening as showers and thunderstorms trained northward along the boundary, and 2 to 4 inches of rain occurred in some areas resulting in some flooding, slides and debris flow over the eastern portions of the August Complex burn scar. The boundary is now a bit further east, and training showers are now expected along the eastern edge of the valley up into the northern Sierra.

HREF probabilities of exceeding 1/2 inch per hour are now the highest in the Sacramento Valley this morning, then in the Delta and northern San Joaquin Valley later this afternoon and evening.

The northern Sierra, including the Mosquito Fire, will see an extended period of light to moderate precipitation with widespread 1 to 2 inch amounts expected through this evening, with more precipitation expected on Tuesday. High end HREF ensembles indicate potential for 2 to 4 inches over this area through Tuesday afternoon. We’ll go ahead and reissue the flash flood watch for the Mosquito Fire burn scar and begin the watch this morning as there will likely be some ash flows today.

Shower and thunderstorm threat will continue into mid-week as the low meanders in the vicinity, but deeper moisture will likely be pushed further south later tonight. We’ll continue the flash flood watch for the Mosquito Fire on Tuesday given the threat for afternoon thunderstorms.

Much cooler than normal temperatures will stay around through the middle of the week.

Extended Discussion (Friday through Monday)

Main upper low progged to progress east as NorCal comes under upper ridging, which amplifies and progresses through the area over the weekend into early next week.

As a result, dry weather expected with a warming trend. Below normal high temperatures Thursday into Friday, warm to near to slightly above normal through the weekend into Monday.