Drizzle and fog through the weekend, heavier precip forecast for next week

January 11, 2019 – Dry conditions this morning with areas of fog in the central valley. Light precipitation spreading across the forecast area this afternoon and tonight. Additional light precipitation is forecast for late Sunday and Monday with heavier precipitation expected for the middle of next week.

Discussion

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Current satellite image shows band of high cloud cover moving across the state this morning ahead of a Pacific cold front over the eastern Pacific. Much of the central valley reporting fog this morning but combination of high cloud cover and low stratus deck keeping most of this fog from becoming dense so will cancel the dense fog advisory now in place. Offshore cold front will push towards the coast this afternoon with light precipitation spreading into the coast range. Increasing cloud cover and onshore flow will bring a little cooling but daytime highs are still forecast to come in a little above normal today. Most of the forecast area will see some light precipitation tonight and Saturday but bulk of precipitation will move into Southern California as main upper low associated with the front digs southeast into the south state. Therefore, impacts from this system will be minimal. Saturday afternoon and night look mainly dry but can not rule out a slight threat of showers over the mountains under light upper level flow. Cloud cover Saturday night should be significant enough to keep fog from becoming too dense in the central valley.

Another Pacific frontal system pushes into the coast by Sunday afternoon bringing another round of light precipitation to the forecast area. Once again, bulk of energy and moisture with this system slip southward so Socal will see the most precipitation from this storm. Therefore, impacts for Norcal will be minimal. May see a few inches of snowfall over the Sierra Cascade range Monday with snow levels below pass levels but likely not enough to cause significant mountain travel issues.

Extended Discussion (Tuesday through Friday)

An extensive period of wet weather is likely to continue, although models still exhibit a great deal of uncertainty with timing and intensity of individual features. Starting Tuesday morning, recent guidance show an upper low evolving into an open wave while lifting through the Central Valley. Strong lift in response to the passage of the trough will spread a band of showers through the region with accumulating snows around 5,000 to 5,500 feet and above. Like previous days, the 06Z/00Z GFS solutions are quite a bit wetter than the 00Z ECMWF.

Trailing this initial system will be another wave primed to reach the coast by the following morning. Continued precipitation chances remain in the forecast into the middle of the week while the strongest system lurks offshore. Models have remained adamant about a more potent longwave trough bringing heavier low elevation rains and mountain snow, as well as gusty winds beginning late Wednesday night into Thursday. The pattern setting up features an anomalous upper low moving toward western Oregon while strong moist/warm advection surges toward the Northern California coast. The 00Z ECMWF depicts 925 to 850 mb winds reaching the 40 to 50 knot range with some of this action possibly making it to the surface if enough mixing occurs. Thus, there will likely be a steady period of windy conditions, particularly over higher elevations. Additionally, models do suggest some instability in play on Thursday which would favor a rumble of thunder or two somewhere across Northern California. Unlike preceding disturbances, snow levels may drop below 5,000 feet given much cooler temperatures aloft.

Looking beyond Thursday, the conclusion of the work week will feature a building upper ridge although this is likely going to be transient in nature. While still nearly 8 days away, the GFS and ECMWF models show a broad upper trough amplifying over the Eastern Pacific. This favors a return of rain to the forecast to start the weekend although details are nebulous at this point.