October 30, 2017 – Dry weather with a cooling trend into mid-week. Rain spreads south Thursday and Friday. Rain will be moderate, to locally heavy at times, with potentially significant accumulating mountain snow continuing into the weekend.
Satellite imagery shows plenty of high clouds over NorCal early this morning as the weak closed low moves into SoCal and the upstream trough moves south through the PacNW. Current temps vary from the upper 20s and 30s in the mountain valleys to mainly the 50s to lower 60s elsewhere.
The marine layer has deepened to around 2K feet deep along the coast and surface pressure gradients are trending stronger onshore from the coast to the valley. Westerly winds at Travis AFB have increased considerably overnight and are currently gusting 25-35 mph. IR imagery shows a finger of stratus nudging inland to south of Travis AFB, and it looks like there will be potential for stratus to make it inland into the Sacramento region this morning.
Combination of synoptic cooling associated with the incoming trough along with local cooling from increased onshore flow will result in cooler temps today.
The trough is forecast to shift south Tuesday allowing ridging over the eastern Pacific to briefly return into mid-week along with locally gusty north to northeast winds.
Precipitation associated with the late-week system is expected to begin as early as Thursday afternoon across the northern mountains, then spread south to around I-80 by daybreak on Friday. Initially snow levels will be high limiting snowfall accumulation to just the higher peaks across the northern mountains.
Extended discussion (Friday through Monday)
The pattern shift to wetter weather should begin in earnest on Friday. At this stage, it looks like heavier precipitation should begin to spread from north to south during the day Friday. There’s good agreement that the main frontal band then shifts through the region Friday night into Saturday. The ECMWF and Canadian models hint at another wave that could bring showers across the region into Sunday.
Early precipitation estimates across the region for this timeframe include 1-2 inches across most of the Valley and 2-4 inches of liquid precipitation over the mountains.
Snow levels are projected to initially start rather high, but drop below pass levels Friday or Friday night, and down to 4500-6000 feet by Saturday night or Sunday.
At this stage, up to a foot of snow will be possible near Donner Summit along I-80, and Echo Summit along Highway 50. Our current projections have up to 2 feet of snow over the highest elevations of the Sierra, and 2 to 4 feet over Lassen Park.
Looking beyond Sunday, longer range models are hinting at the potential of some clearing behind these systems early next week. With a cold airmass already in place, any overnight clearing and periods of light wind could result in some cold nighttime temperatures.
Confidence is low in the forecast that far out, but current projections have lows at or near freezing across the northern Sacramento Valley early next week.