April 5, 2018 – Rain spreading over the region north of interstate 80 today and over all of northern California tonight and Friday. Heavier precipitation Friday night and early Saturday with high snow levels. Some local flooding possible. Showers and possible valley thunderstorms Saturday with drying Sunday. A wet pattern returning by Tuesday. Flood Watch from Friday afternoon through Sunday morning.
Discussion
Weak upper level ridging over the west coast is bringing dry conditions to the CWA this morning. A weak shortwave trough riding over this ridge brought cloud cover to most of the forecast area earlier this evening but the trace amounts of precipitation seemed to have stayed just north of the forecast area.
A Pacific cold front associated with a cold upper low in the Gulf of Alaska will push into NorCal later this afternoon spreading relatively light precipitation southward across the region this afternoon and tonight. Snow levels with this first system remain fairly high and with the fairly light precipitation expected, the impacts should be minimal. Precipitation rates are expected to increase on Friday as plume of subtropical moisture pushes into NorCal. GFS progs show TPW values with this plume at over 1 1/2 inches so precipitation amounts associated with this moisture could be significant.
Upper level ridging remains in place over the west coast on Friday so snow levels will climb to well above pass levels. The possibly heavy precipitation amounts combined with at least some snowmelt could bring river rises and some localized flooding and a Flood Watch remains in place for the entire forecast area from Friday afternoon through Saturday night.
The Gulf of Alaska low shifts coastward on Saturday with a strong for the season cold front expected to push across the north state during the day on Saturday. Gusty winds are anticipated during this time. Snow levels will drop significantly by Saturday afternoon as the cold front moves through but the precipitation rates will drop off quickly as well. There may be enough higher elevation snow left over Saturday afternoon and night to warrant a snow advisory and this situation will need to be analyzed as the time period draws nearer.
Other issue for Saturday will be the threat of valley thunderstorms. Instability proggs show instability down to the central part of the state so expanded valley thunderstorm threat to cover all valley locations.
Rebuilding upper ridge will bring a shift in the precipitation threat northward on Sunday with dry conditions expected forecast area wide by afternoon. More sunshine will bring warming temperatures over the weekend with daytime highs expected to top normal by Sunday.
Extended discussion (Monday through Thursday)
Upper ridging and increasing subsidence will result in drier weather Monday. The next series of Pacific frontal system begins to spread precipitation over the forecast area beginning Tuesday. These systems have some resemblance to the upcoming system this week, but will not have the same strong moisture plume so QPF will be less. Snow levels will be around 4000-6000 feet, so potentially more travel impacts.