January 16, 2018 – A weak and warm weather system passes through the region this morning, followed by stronger and colder storm system Thursday- Friday. Moderate snowfall below pass levels for mountain travel impacts. Next system moves through Sunday-Monday.
Discussion
Weak frontal band now moving through northern California bringing generally light precipitation. So far, precipitation amounts have been less than about one quarter inch. The passage of this front over the Sacramento valley has mixed the atmosphere enough to break up the fog a bit and visibilities have improved so was able to cancel the dense fog advisory. The light precipitation should end most places by late this morning as the weak front moves eastward with just a few showers over the Sierra lasting into the afternoon. With light precipitation amounts and snow levels at or above pass levels, mountain travel impacts should be minor. Upper level ridging will end the shower threat after sunset tonight. Fog will form in the valleys again tonight but amount of cloud cover will determine how dense this fog becomes. Warmer airmass and a bit more sun should allow for a little warming on Wednesday afternoon with daytime highs remaining a little above normal.
Next Pacific storm system pushes onto the north coast Wednesday night then pushes across the north state on Thursday. Precipitation amounts will be a little heavier than the system moving through this morning with heaviest amounts expected over the mountains. This will be a colder system with snow levels dropping to pass levels by Thursday afternoon. Snow levels continue to fall in the cold air behind the frontal system Thursday night. Snow levels well below pass levels and moderate precipitation will bring likely mountain travel impacts. General troughing over the west coast will keep showers going on Friday. Total snow amounts over the Sierra could add up to several inches to over a foot so Winter storm watch now in place Thursday to Friday afternoons still looks good. Colder airmass will bring down daytime highs Friday to below normal for this time of year.
Extended discussion (Saturday through Tuesday)
Short-lived upper level ridge will build across the area on Saturday, resulting in generally dry conditions. The ridge of high pressure will be displaced as an upper trough deepens along the West Coast. Forecast models similar in bringing widespread precipitation across interior NorCal Sunday into Monday as associated Pacific frontal boundary pushes through. There are still some minor differences in timing as well as precipitation amounts this far out. Regardless, mountain travel impacts could be possible given the cold nature of this system. Breezy to gusty winds could be possible as the system moves through.
Lingering showers will continue early Tuesday as the system exits the area. Guidance indicates a brief period of dry weather before the next system moves in around the middle of next week, but models solutions diverge significantly. Nevertheless, it appears that active weather could continue through the extended period. Temperatures will gradually warm up by the end of the forecast period, but should remain near or slightly below normal.