January 7, 2018 – Dry weather with patchy valley fog this morning. A storm will move across the area early this week, with light precipitation spreading into the area tonight and continuing through Tuesday. Moderate to heavy precipitation is expected across the area, with some concern for possible debris flow over recent burn scars. Snow levels will start out high, but drop below pass levels by early Tuesday, with enough snow to cause travel delays and chain controls. Exact timing of the snow level drop is the main uncertainty, and will impact final snow totals. Breezy southerly winds will also accompany the system.
Lingering moisture from yesterday’s system and clearing skies has lead to the formation of fog across the valleys this morning. Expect the area of fog to become more widespread by daybreak with patches of dense fog. Drivers should be aware and prepare for rapidly changing visibility. Upper level ridging shifts east today ahead of storm system brewing in the Pacific. This will allow for one more day of dry weather with clouds beginning to stream into the area throughout the day. Afternoon highs should be fairly similar to Saturday.
Focus remains on more significant storm system that will move onshore Monday morning and impact the area through Tuesday. Models remain in good agreement with only uncertainty remaining in how fast snow levels will drop. Therefore, forecast remains largely unchanged. Widespread moderate to heavy rainfall for the entire area Monday into Tuesday, heaviest Monday night into early Tuesday. There is a chance for some embedded convection within the main area of rain, but have left out of the forecast for now as model soundings show meager instability. If anything does pop up, this would enhance rainfall amounts locally. Storm totals will range from 1-2 inches in the valley with 2 to 5 inches across higher elevations. This will cause slick roads, ponding of water, and slow-going commutes. Concern remains for minor debris flow across recent burn scars, but confidence not high enough to warrant any flood products at this point.
Area will remain entrenched in the warm sector of storm for much of Monday into Monday night, therefore snow levels will start out high, at 8000 feet or above. As frontal boundary moves through sometime Tuesday morning, expect snow levels to drop to around 6000 feet. Winter storm watch remains in effect for Monday night into Tuesday night, with 6 to 12 inches above 6000 feet and up to 2 feet along the Sierra crest. Heaviest snow is expected Tuesday morning. Travelers should be prepared for delays and likely chain controls across mountain passes.
Breezy to locally gusty southerly winds will also accompany the system, especially as the front moves through. Breeziest conditions expected in the valley, south of Sacramento, and across higher elevations. Right now, valley gusts look below wind advisory criteria.
System departs later Tuesday into Wednesday with a few showers lingering over the northern mountains and the Sierra.
Extended discussion (Thursday through Sunday)
The eastern Pacific ridge will rebuild late in the week resulting in drier weather across northern California. A series of short- wave troughs moving through the PacNW may bring a chance for some light precipitation across the far northern portion of the state Thursday into early Friday. Temperatures will be above average.