January 15, 2018 – Dry with patchy dense valley fog this morning. Periods of relatively light precipitation late today and Tuesday with fairly high snow levels. Dry Wednesday then more significant precipitation Wednesday night through the end of the week. Lower snow levels with mountain pass impacts Thursday and Friday.
Discussion
Northern California under southwest flow aloft this morning ahead of a Pacific frontal system approaching the coast. Main weather concern this morning is central valley fog. High cloudiness associated with an upper shortwave has moved east of the CWA with several central valley locations reporting one quarter mile visibility. Fog could become locally dense but a low cloud deck across the valley could preclude widespread dense fog and will have to keep an eye on fog situation throughout the morning hours today.
Frontal band pushes into northern California late today spreading precipitation across the CWA. This will be a fairly fast moving system with all models clearing this system to the east of the forecast area by Tuesday afternoon. Precipitation amounts with this system appear will be relatively light ranging from about one quarter inch in the central valley to one half to three quarter inches over the higher elevations.
Snow levels through the heaviest precipitation period will remain above 7000 feet so combined with light nature of precipitation, impacts will be minor. Tuesday night and Wednesday look dry at this time under a temporary upper level high pressure ridge. Less cloud cover and more stable conditions along with surface moisture will make for a return of valley fog Wednesday morning.
Next round of precipitation enters the north state Wednesday night or Thursday morning as the next Pacific frontal system moves onshore. This system will move slowly across Norcal Thursday and Thursday night. Models differ some with QPF on this system with GFS being somewhat wetter than ECMWF. Snow levels start out fairly high again so mountain travel impacts will be minimal at first but Cold air moving in behind the front will bring down snow levels going into the end of the week for more significant mountain travel impacts.
Extended discussion (Friday through Monday)
Pacific front moves through Thursday into Friday bringing another round of precip to Interior NorCal. System will bring in colder air with snow levels dropping down into the upper foothills Friday into Friday night. GFS showing afternoon CAPE in the Central Valley and adjacent eastern foothills to suggest possible afternoon thunderstorms. With 500 mb temps in the lower to mid -30s C, stronger convection will likely produce some hail or graupel. Highs Friday will be near to below normal with 50s in the Central Valley and 30s and 40s for the mountains and foothills.
Scattered light showers possible Saturday, otherwise drier weather as upper ridging builds inland. This will be short-lived as models bring the next frontal system across Interior NorCal Sunday and Monday. This looks to another wet and cold system with relatively low snow levels.