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January 15, 2019 – Wet and windy weather is expected across the region through Thursday with periods of heavy snowfall in the mountains. Drier weather will return for the weekend.

Discussion

Radar indicates showers covering a good portion of interior NorCal early this morning as the first system of the week begins to affect the region. Current temperatures are milder compared to 24 hours ago and range from the 20s in the mountain valleys to the mid 40s to around 50 across the Central Valley.

The first couple systems affects the region today and tonight and will result in mainly light to moderate amounts of precipitation along with breezy southerly winds as the upper trough over the eastern Pacific lifts northward. Travel impacts will become increasingly likely across the northern Sierra passes later this afternoon and tonight as precipitation increases.

The main event is still on track for later Wednesday into Thursday as a strong Pacific storm moves across the region. Precipitation and wind will increase across the area ahead of the storm by Wednesday afternoon as surface pressure gradients tighten. Strong warm-advection will raise snow levels a bit to around 5500-6500 feet in the Sierra.

A large area of moderate to heavy precipitation is expected to accompany the cold front as it moves through Wednesday night, possibly with some embedded thunderstorms. Rainfall amounts of 1.5-2.5 inches are expected in the valley with 3-7 inches in the foothills and mountains through Thursday night.

The period of strongest winds for the valley is expected from late Wednesday afternoon until late evening. While the center of the surface cyclone will remain well off the Oregon coast, strong pressure gradients will cover the entire region. The MFR-SAC gradient is forecast to reach around 10 mbs Wednesday evening with another 6-8 mbs of gradient forecast from SAC-FAT. Southerly gusts of 50-60 mph will be possible. Gradients will relax behind the front, but gusty southerly winds will continue Thursday.

Heavy snow will develop at the higher elevations of the northern Sierra Wednesday night and continue in the post-frontal environment on Thursday. With strong southwest winds and heavy snowfall rates (exceeding 2 inches an hour), blizzard conditions will be possible into Thursday evening with several feet of snowfall accumulation through the passes. If possible, travel over the Sierra should be completed today or early Wednesday.

Precipitation expected to retreat northward on Friday as ridging from the eastern Pacific moves in.

Extended Discussion (Saturday through Tuesday)

Mainly dry weather is forecast for Saturday with upper level ridging over the region. Wet ground with light wind and clearing skies should be a good set up for morning fog in the Valley. Precipitation spreads into the northern Coastal Range Saturday afternoon as ridge shifts eastward and a Pacific trough moves in. This system will bring widespread precipitation but rain and snow amounts will be relatively light with little impact. Timing continues to be uncertain with the European model quicker than the GFS, which would mean precipitation across the area Sunday followed by a dry Monday. The latest GFS runs would keep much of the area dry until later Sunday into early Monday. For now, will lean towards the quicker solution.

Drying northerly winds are expected to increase Monday and Tuesday as the trough exits, with breezy winds on the west side of the Valley. A dry pattern sets in through mid week as the area remains on the east side of a Pacific ridge.