January 15, 2019 – Wet and windy weather is expected across the region through Thursday with periods of heavy snowfall in the mountains. A brief period of dry weather is expected before precipitation chances return on the weekend.
Water vapor imagery this afternoon distinctly shows the pair of systems impacting Northern California. The lead shortwave is currently moving through the Bay Area driving a broad axis of light rainfall, generally on the order of 0.10″ per hour.
Mountain impacts are already underway with accumulating snows over the heavily traveled passes on I-80 and Highway 50. The heaviest burst of snow is likely to occur early this evening into the overnight hours with over a foot possible during this timeframe. In addition to the increased coverage of precipitation, an uptick in the wind speeds will be evident by later tonight. Winds are not expected to slacken given the enhanced pressure gradient in advance of the more vigorous system.
Lurking out around 150W longitude is the mentioned powerful Pacific system expected to impact much of the state Wednesday evening through Friday morning. Forecast anomalies with this system with regards to typical atmospheric parameters (wind, heights, etc.) are around 3 to 4 standard deviations from climatology. The major hazards will include locally heavy rainfall, several feet of mountain snows, high winds, as well as a chance for some scattered thunderstorms.
The biggest hazard to travel will be the heavy mountain snows, generally 5,000 feet and above. Snow levels will rise ahead of the next storm which suggests the heaviest accumulations should sit above 6,500 to 7,000 feet. The stronger winter storm is forecast to spread several feet of snow to the Sierra and Southern Cascades. Travel impacts will be high and travel is not advised given intense snowfall rates coupled with high winds leading to blizzard/whiteout conditions. Such hazards should persist through Thursday night with scattered snow showers lingering into the following morning.
While this will be a long duration rainfall event, the potential threat for thunderstorms may ignite some short-term flash flood concerns. This is particularly the case over any recent burns scars so those will bear watching. Difficult to say this far out where any such storms would develop, but instability appears decent enough to support scattered convection Wednesday evening into the overnight hours. Locally gusty winds would be possible within any thunderstorms, but a more widespread high wind threat remains given the strength of the approaching storm. 50 to 60 mph southerly wind gusts cannot be ruled out across the Valley with slightly higher numbers over mountain locales.
Once the system clears out on Friday morning, a transient upper ridge moves across the western U.S. supporting a brief period of improved weather. Global model agreement is rather good suggesting high confidence in this pattern change ahead of the weekend system.
Extended Discussion (Saturday through Tuesday)
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 on Sunday but rain and snow amounts will be relatively light. Snow levels look to be around 7000 feet Sunday falling to 4500 to 5500 feet Sunday night. Some mountain travel impacts will be possible but should be limited to mainly high Sierra passes.
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.