Light rain and mountain snow tapers off today. The next storm will move in Friday bringing significant mountain travel impacts that will continue through the weekend. Additional storms expected early next week. Winter Storm Watch from Friday afternoon through Tuesday afternoon for West Slope Northern Sierra Nevada.
Discussion
Satellite imagery shows the upstream trough digging over the eastern Pacific. The frontal band ahead of the trough remains off the coast and is becoming more north-south oriented and slowing its eastward progress. Meanwhile, out ahead of the system over NorCal, warm-advection is continuing to generate some light precipitation early this morning (mainly over the north end of the Sacramento Valley, northern mountains and northern Sierra where some pretty good amounts fell yesterday (1.5-3 inches)).
It looks like Sacramento will have a pretty good chance of ending our streak of consecutive days with measurable rain. The remaining light precipitation is forecast to taper off this morning, then most of the region will see a break into Friday as precipitation with the front remains to our west.
The next incoming trough will begin to nudge the front inland on Friday and precipitation is forecast to begin in the morning across the Coast Range and western edge of the Central Valley, then spread east across the remainder of the valley and into the Sierra during the afternoon.
Looks like there will be a band of moderate to heavy precipitation with the front as it moves quickly through, and accumulating snowfall will be likely again over the northern Sierra by mid to late afternoon. Precipitation will turn more showery for a time behind the front on Friday night before warm-advection increases ahead of the next wave leading to more widespread light to moderate rain by Saturday morning.
Another wave moves through on Sunday. At this point, the Saturday system looks to have the most vigor with another round of gusty southerly winds and heavier precipitation.
The main impacts over the weekend are likely to be focused on winter weather over the mountains where several days of heavy snow will lead to very difficult travel with several feet of snowfall accumulation.
However wind impacts can’t be completely discounted as it looks like we’ll see gusts at a minimum in the 35-45 mph range again in the valley on Saturday, though recent GFS runs have been developing a deep surface low which races across the northern Sacramento Valley Saturday afternoon which would portend stronger gusts up there.
We’re also closely watching how the trough evolves behind the second wave on Saturday with potentially another incoming negative tilt short-wave bringing thunderstorm chances Saturday evening. We’ll continue to monitor the latest developments.
Extended Discussion (Monday through Thursday)
Cluster analysis and ensemble guidance generally in good agreement for troughing to remain over the region through much of next week.
Additional rounds of precipitation are expected over most of the extended forecast period. Periods of widespread rain and mountain snow will bring continued flooding concerns and dangerous mountain travel impacts early to midweek.
Beyond Tuesday the models begin to diverge and confidence is low for amounts and timing for precipitation the middle of next week. The Extreme Forecast Index (EFI) highlights wind speed and wind gust for much of interior northern California Monday, supporting gusty southerly winds returning on Monday.