Moderate to heavy rain and snow expected Saturday

January 3, 2019 – Dry with chilly nights and near normal daytime temperatures through Friday. Wet weather returns this weekend with hazardous mountain travel impacts likely from heavy snow and gusty winds. Periods of wet weather continues through at least the middle of next week.

Discussion

Upper level high pressure ridge over the southwest U.S. bringing fair skies to the north state although some high cloudiness is spilling over the ridge and into NorCal this morning.

Fairly light pressure gradients bringing light winds this morning. These relatively light winds are allowing for another cool morning. Many central valley and delta locations seeing low temperatures at or below freezing.

Some light mist appearing from about Marysville south this morning but temperatures and dewpoints showing enough spread that fog should not become significantly dense. Even with high cloud cover today, daytime highs should come in a little above normal.

Upper ridge axis shifts east of the state on Friday but NorCal remains under the ridge so only minor changes are expected in current dry pattern. Cool morning lows and slightly above normal highs in store for Friday with light pressure gradients bringing relatively light winds.

Significant changes are in store for Saturday as a Pacific Frontal band pushes into northern California. By 18z Saturday, most forecast area locations should be seeing some precipitation. Heaviest precipitation currently looks to occur Saturday afternoon and evening as the main cold front pushes through. This fairly cold system will drop snow levels down to between 3000 and 4000 feet or well below pass levels.

Even the coast range passes like Buckhorn Summit on highway 299 west of Redding could see significant snowfall so have added a winter storm watch for the coast range Saturday through Sunday morning in addition to the winter storm watch already in place for the Sierra Cascade range.

In addition to the snowfall and valley rain, increasing pressure gradients Saturday will bring gusty winds. GFS surface progs indicate better than 14 mb surface gradient between KSAC and KMFR with 925 mb wind forecasts topping 40 knots. Combination of wind and heavy snow will make for very hazardous mountain travel conditions.

Passage of upper trough axis on Sunday will bring continued showers. Should see decreasing winds as gradients relax with cooler air behind the front bringing down high temperatures Sunday to a little below normal for this time of year.

Extended Discussion (Monday through Thursday)

Wet weather will be the story next week as a series of disturbances pass over the region; however, timing continues to remain a conundrum. Monday, a large trough will advance towards the West Coast with a warm front bringing another round wet weather. Models then begin to differ vastly with the placement of the incoming low pressure system. The ECMWF lifts the system towards the Pac NW while the GFS keeps offshore, but closer to the OR and CA area. Precip looks possible for all areas of the CWA with the ECMWF keeping it to the north while the GFS has it more widespread.

Come Wednesday, models are suggesting another day of wet weather as they bring a cold front CA. Again, models differ on the timing and strength with the GFS being the most aggressive. Beyond that models disagree with each other with the GFS suggesting a quasi-stationary front and the ECMWF building a ridge over the area.