January 4, 2018 – Unsettled pattern will continue through Saturday. A brief break Sunday then more wet weather Monday through mid week. Moderate to high snow levels.
Current satellite and radar imagery indicates back edge of weak frontal band now working its way through the central valley. Models forecast associated upper trough will shift north of the state by mid day today so should see a transition to showers by late morning or early afternoon.
Precipitation amounts have generally been a half inch or less but the front did bring brief periods of heavy rain. Snow levels with this initial system will remain above 7500 feet so travel impacts will be minimal. Cooler air filtering in behind the front will bring a slight cooling today but daytime highs are still forecast to come in a little above normal.
Another upper trough off the northern California coast will be trekking into northern California by late tonight or early Friday morning bringing another round of precipitation. This system looks to be a bit stronger than the current system. Wind gusts along the front Friday afternoon should push into the 20 mph range with precipitation amounts up a little as well. Snow levels with this system will stay relatively high through the main frontal passage then drop slightly by early Saturday as the upper trough shifts east of the state.
Upper ridging builds in over the west coast on Saturday so showers will be ending from the west during the day. Upper ridge will bring dry conditions Saturday night but variable cloud cover should reduce the threat of significant valley fog. A still more significant Pacific storm system will begin to move onto the coast by early next week. Previous model runs brought a threat of light prefrontal precipitation to the northern mountains by the end of the day Sunday but latest runs keep Sunday dry. Either way, Sunday should see a little warming with most of the forecast area remaining dry.
Extended discussion (Monday through Thursday)
Latest operational runs of the models show the GFS trending toward the faster ECMWF solution with the trough moving through NorCal early next week. Best chance of precipitation will be Monday into Tuesday before renewed ridging returns drier weather by mid-week. Preliminary indications are that QPF will be more significant than with this week’s systems, though high snow levels through most of the event will limit potential winter weather impacts to mainly the highest elevations.