Open wave Pacific frontal system with 1003 mb low center located about 300 NM W of KSFO this morning is producing widespread light to moderate precip over Interior NorCal. Chain controls are in effect for trans-Sierra passes this morning with snow levels ranging from around 5000 feet in the Shasta mountains, to around 6500 feet over the northern Sierra Nevada. Wave progresses ESE today and fills as upper level support weakens. 88D Mosaic showing main band of precip pushing through eastern portions of the Central Valley into the foothills and Western Plumas mountains and northern Sierra Nevada attm.
Widespread showers expected behind this to continue today through Sunday. 1 to 2 feet of snow is possible through the weekend over the higher elevations and Winter Weather Advisory (Zone 68), and Winter Storm Warning (Zone 69) are in effect. Models indicating some instability this afternoon in the Northern San Joaquin Valley to support isolated thunderstorm development. Storm total QPF through the weekend looks to be generally under an inch in the Central Valley with around 0.5 to 3 inches in the foothills and mountains.
Slight chance of mountain showers early Monday otherwise dry weather expected with near to slightly below normal temperatures. Another Pacific frontal system begins to impact the CWA by Tuesday afternoon. GEFS/GFS Integrated Water Vapor Transport showing southern portions of this extra-tropical cyclone will be entraining a higher PW moisture plume (AR). Digging secondary short wave rotating through base of main offshore upper low Tuesday afternoon into evening will help to direct focus of AR into central and southern portions California. Period of moderate to heavy precipitation still expected over NorCal Tuesday night into Wednesday, but less than previously advertised as bulk of AR remains south of the forecast area.
Extended Discussion (Wednesday through Saturday)
Another wet period expected during the middle of next week with significant rain in the valley and heavy snow in the mountains above about 5-6k ft. Travel and hydro impacts possible, but likely not to the extent we saw earlier this week. Unsettled weather with showery weather pattern continues into next weekend.