November 3, 2017 – Precipitation spreads over interior NorCal today. Significant rain and mountain snow, mainly tonight through Saturday with gusty winds especially over the mountains. Decreasing showers Sunday, followed by a weaker storm Sunday night into Monday.
Discussion
Upper level low pressure system now dropping into the Pacific Northwest with a cold front now moving on to the north coast. Eureka radar is now showing some precipitation across the northwest corner of the state with this band of precipitation expected to drop slowly southward during the day today. Orographics and increasing moisture have allowed for some scattered light showers this morning over the northern Sierra. Some light warm sector showers are also showing up over the northern and central Sacramento valley as a small shortwave disturbance rotates through.
The cold front appears to have slowed somewhat and is currently expected to move into the Sacramento valley this afternoon. The northern mountains and southern Cascades will see the most precipitation today as orographics play a role with lighter precipitation expected elsewhere. Snow levels will remain fairly high today along and ahead of the front and generally expected to remain above 6000 feet. Frontal band is forecast to drop into the southern CWA overnight with orographics again playing a role in bringing the heaviest precipitation to the northern Sierra.
Snow levels will be taking a drop tonight and Saturday behind the cold front first over the northern CWA this evening and then over most of the CWA by Saturday afternoon. Snow levels between roughly 5000 and 6000 feet will bring impactful snowfall accumulations to major Sierra passes tonight through Saturday. Brief periods of gusty winds will be associated with the frontal passage as well with strongest winds across the north this afternoon and across the southern forecast area Saturday afternoon as the front drops through this area.
Sunday looks like a break between systems with precipitation ending most valley locations. orographics may keep lighter showers going over the mountains but any pass level snow amounts should be light enough to have minimal additional impacts.
Another, weaker frontal system rotating through the Pacific northwest trough will drop into NorCal on Monday. Latest model runs appear to split this system as it moves into the north state so forecast precipitation amounts have been reduced a bit with main precipitation area over the Sierra Cascade range. Cloud cover and airmass cooling will keep daytime temperatures through the short term period several degrees below normal.
Extended discussion (Tuesday through Friday)
The storm system that will bring rain and mountain snow to northern California in the short term forecast will exit the region Tuesday as a shortwave upper level ridge of high pressure moves across northern California Tuesday. This ridge will bring a quick break in sensible weather with dry conditions and a few degrees of warming for the region. Another disturbance is expected to move into the region for the Wednesday/Thursday timeframe. There is a bit of disagreement in trough placement with the GFS and ECMWF, and both models remain consistent with the previous runs in which the GFS is further south and the ECMWF keeps the trough further north. Regardless of the correct solution, confidence is increasing in a another stormy pattern with near to slightly below normal temperatures through the end of next week.