Light precipitation will taper off across the region by this afternoon. Dry weather with cold overnight temperatures is expected Friday into Saturday. More significant rain and mountain snow expected for the second half of the weekend and next week.


Relatively low-impact weather system is working its way across NorCal early this morning. Precipitation has mostly been on the light side with most areas reporting less than a tenth of an inch since late Wednesday. Forcing is beginning to pick up resulting in some areas of moderate precipitation over Butte County and portions of the northern Sierra where hourly rates have increased to between 1-2 tenths of an inch.

Precipitation has just switched over to snow at KBLU, so appears the snow level has lowered to around 4500-5000 ft along I-80, and chain requirements have been posted on the highways crossing the northern Sierra. Precipitation is forecast to mostly wind down over most of the area by midday, and end by later this afternoon in the northern Sierra.

Northerly flow behind the system will begin to usher cooler and drier air from the north into NorCal today. Breezy northerly winds of 15-25 mph can be expected in the Sacramento Valley.

Widespread frost is expected across the lower elevations tonight as skies clear and north winds slacken, especially along the east side of the valley and in outlying areas. Coldest temperatures may briefly dip into the upper 20s around sunrise on Friday.

A Protection for frost/freeze sensitive vegetation is advised and a Frost Advisory has been posted from 2 AM until 10 AM Friday.

The cool and dry weather continues into Saturday, though increasing clouds may temper frost/freeze potential in the valley a bit for Saturday morning.

The next series of much more impactful weather systems are forecast to begin affecting much of the area by Saturday night with more substantial valley rain and heavy mountain snow.

Extended Discussion (Monday through Thursday)

High confidence that a longwave trough will develop in the eastern Pacific from Sunday onward as windy and very wet weather returns to NorCal. This trough will slowly dig south toward California early next week, allowing for a multi-day period of precipitation chances.

While the specific details are yet to be determined due to some timing uncertainties, the cluster analysis of the various ensemble families yields a classic synoptic setup for an impactful storm.

For one, the cold origins of the parent upper- level low will allow for much lower snow levels compared to earlier storms this season (e.g., the late October atmospheric river).

This will result in the potential for very heavy mountain snow, including the potential for snow accumulation into the foothills and far northern Sacramento Valley. Additionally, strong southerly winds are expected ahead of the associated frontal boundary. Stay tuned for further details.