UPDATE: Halloween Storm Forecast

The storm continues progressing towards the Sierra and will arrive tomorrow evening. The storm arrives a little later than initially thought, but the heaviest snow and precipitation will still fall Friday evening through Saturday morning. It will continue into Saturday afternoon before tapering off Saturday night. Snow levels will begin around 8000 ft. and will drop down to below 5000 ft. Saturday morning. The heaviest precipitation will fall with the snow levels between 5500 ft. and 7500 ft.

• Around Tahoe: 4-8" of snow with highest elevations totaling 1'+ of fresh snow
• Further south: 8-12" of snow with highest elevations totaling 18"+ of fresh snow
• Heaviest snowfall timing: Friday evening through Saturday morning
• More spillover precipitation than previously thought

Snow accumulation for the next 72-hours. 95% probability of receiving up to but not exceeding the contoured values. So Truckee has a 95% chance of receiving up to 4-6" of snow. Image via WPC.

The strong upper level low pressure system (trough) shifts its overall tilt as it impacts the Sierra tomorrow. The trough becomes more negatively tilted (think of this symbol: \ ), the flow wraps back around on the lee side (east side in this case) inducing more upslope flow for the east side of the Sierra increasing snow and rainfall totals. 

Storm total precipitation forecast. Via NWS WFO Reno. 

Additionally, the cold air in the trough will help produce convection, which will pump moisture up and over the Sierra. A broad area of upper level divergence (supports upward motion) will aid this process and stay intact as it moves over the Sierrra, which does not always happen. And finally, the lack of downslope winds due to the trough's trajectory and vertical temperature and wind profile will not disrupt upward motion on the east side of the Sierra. 

Snow accumulation will be the toughest to gauge with the warmer ground. We could see snowfall totals diminished due to the first brave snowflakes melting. However, as they melt and evaporate, the ground cools and with the near to below freezing temperatures forecast, will set the stage for snowfall accumulation eventually. 

Storm total snowfall forecast. If snow levels drop quickly, snow totals could increase by 10-25%. Via NWS WFO Reno. 

In summary, this system isn't the coldest, it isn't the wettest, but it has plenty of both to give us a nice early season snowfall. It's trajectory and intrinsic vertical temperature and wind profiles will also help give the east side more snow and rain than it would normally receive. 

Enjoy a rainy/snowy Halloween and be safe on the roads this weekend!