First of two storms moves in tonight, main cold front moving through Friday evening through Saturday

November 2, 2017 – Light rain over the northern mountains as early as this afternoon. Precipitation spreads over interior NorCal tonight and Friday. Significant rain and mountain snow, mainly Friday night through Saturday with gusty winds especially over the mountains. Decreasing showers Sunday, followed by another storm Sunday night into Monday.

Discussion

Update: Lower level Sly flow has increased over CA with cloud bases roughly 4-6 thousand feet spreading to the N from SocCal into interior NorCal. Over the Sierra, the cloud deck has slammed into the terrain with LIFR conditions there. So a much cloudier and cooler day expected today.

Forecast on track for Fri-Sat with storm #1 affecting Norcal as frontal band drops Swd from the Pac NW into/across NorCal. Snow levels near Sierra pass levels on Fri lowering to near 5000 ft as precip ends Sun. Brief break in the precip on Sunday before storm #2 moves across the area from NW to SE Sun-Mon. This is a little weaker system with precip developing over the Sierra again Sun night into Monday. Snow levels remain around 5,000 ft with storm #2.

Previous Discussion

Just a few clouds spreading into the far north state this morning ahead of a significant upper low pressure system dropping southward into the Pacific Northwest. For the most part, the CWA should remain dry today but a few light showers may be possible over the northern mountains as the system continues southward. The northern Sierra could see a few light showers as well as orographics and moisture improve there.

Temperatures will see a significant drop today as the region sees airmass cooling and winds will likely become more breezy as pressure gradients ahead of the system tighten. Warm frontal precipitation is forecast to spread across the forecast area tonight but precipitation amounts should be fairly light. Thickness progs imply cold front will move into northwest California Friday afternoon bringing heavier precipitation across the northern mountains before continuing southward Friday night and Saturday.

Snow levels will drop as cold air behind the front drops in with the heaviest snow at Sierra pass levels late Friday night and Saturday morning. Breezy to gusty south winds can be expected as the front moves through creating hazardous mountain travel at the higher elevations.

Much colder air will bring down daytime highs still further with daytime max values 10 or more degrees below normal. Much of the forecast area will see a break in precipitation early Sunday although orographics will keep showers going over the northern Sierra. Another frontal system is forecast to drop into northwest California by Sunday afternoon so any break in precipitation will be brief.

Extended discussion (Monday through Thursday)

Positively tilted upper level trough centered over the Pacific northwest will slide through northern California and bring chances for rain and snow mainly for the mountains/foothills Monday. A shortwave ridge is expected to be pinched between the exiting disturbance and another incoming trough. The short lasting ridge will bring dry weather for Tuesday before more chances for rain and mountain snow for mid week. Temperatures will remain near to slightly below normal through the long term forecast period.