Scattered showers and isolated thunderstorms into this evening with periods of heavy snow over the Sierra. A second weather system will bring periods of moderate to very heavy snow over the mountains with widespread moderate to heavy rain in the Valley with gusty south winds.
Discussion
Showers are ongoing in the Valley with chances of isolated thunderstorms across the Central Valley this afternoon and evening. Best convective potential will be in the foothills to Redding. Brief heavy showers and small hail are the primary threats with any thunderstorm development.
Additionally, light to moderate snowfall will continue through tonight in the Sierra generally above 3500 feet with the potential of periods of heavy snow as lingering snow showers drift over. Please exercise caution if traveling today and check the latest road conditions. Overall, precipitation trails off on Saturday outside of lingering light snow in the mountains.
Our attention turns to early Sunday morning through Tuesday morning as the next atmospheric river impacts Northern CA. A Winter Storm Warning has been issued for Western Plumas County/Lassen Park and West Slope Northern Sierra Nevada from Sunday morning to Tuesday morning for heavy to very heavy snow above 4500 feet.
Total snow accumulations of 2 to 4 feet with 5 to 6 feet at mountain peaks expected. Travel with this system to be very difficult to impossible from travel delays, snow covered roads, and reduced visibility, with possible chain controls and road closures.
Snow probabilities match the higher snow total nature of this system with a 60-100% chance of 24 inches or greater for elevations greater than 6000 feet. Between 4500 and 6000 feet, there is a 40-90% chance of 12 inches or more. Heaviest snow accumulations will occur on Sunday, particularly Sunday morning.
Snow levels will be 3000 to 4000 feet initially, increasing to 4000 to 6000 feet by Sunday afternoon and settling at 4000 to 5000 feet on Monday.
Additionally, a Winter Storm Warning has been issued for the Shasta Lake Area, Northern Shasta County, and across the Coastal Range from Sunday morning to Monday late morning. Heavy snow is expected above 3500 feet with 2 to 5 inches below 3500 feet, including the higher elevation of I-5. Travel will be difficult from snow covered roads with possible chain controls and road closures. Snow probabilities show a 40-80% chance of 12 inches or greater above 3500 feet with only a 25-50% chance of 4 inches or more below 3500 feet. Similarly, heaviest snowfall will be on Sunday.
Snow levels are 2000-3000 feet in the Coastal Range and Northern Sacramento Valley increasing to 3000 to 4000 feet Monday morning.
Rain moves into the Valley late Saturday night from the southwest and spreads northward throughout the Valley. Forecast totals from Saturday night through Tuesday morning is 1.75-3.00″ in the Valley with highest totals in the Central Sacramento Valley.
The foothills will see 2.00-4.00″ and highest near Chico. Sunday morning will be wettest period with periods of moderate to heavy rainfall Sunday and Monday. There is a 30-60% chance of 2 inches or more with 65-90% chance of seeing one inch or more.
Gusty winds will move through the Valley late Saturday night through Sunday. South to east winds move through the Valley around 25 to 35 mph with gusts 40 to 50 mph. Strongest winds will be Sunday morning and taper off through the afternoon and early evening. Gusty winds could blow around unsecured objects and cause difficult travel conditions for high profile vehicles.
Sunday brings another round of thunderstorm chances (10-15%) from I-80 to Chico and then Monday (10-20%) from Southern Sacramento County through Northern San Joaquin. Brief heavy rain showers and small hail are the primary threats with this and be sure to check the latest forecast for any changes to the forecast.
EXTENDED DISCUSSION (Tuesday THROUGH Friday)
Ensembles and cluster analysis support the eastward propagation of the main low center responsible for the high impact weather over Sunday-Tuesday, resulting in lingering moisture on the back end of the low. This will result in a few lingering Valley/foothill showers and mountain snow Tuesday afternoon.
The National Blend of Models has a less than 20% chance of 0.50″ or more of rainfall in the Valley and a 30-50% chance in the foothills.
Additionally, there is a 40 to 60% chance of 2.00″ or more of snowfall above 5000 feet in the Sierra and Southern Cascades. By Wednesday, afternoon conditions will be generally dry in the Valley with lingering showers in the foothills and mountains. Rainfall amounts will be less than a 0.25 inch, with snow amounts less than an inch in the mountains. Impacts are expected to be minimal, if any.
A weaker shortwave trough will move through the area Thursday through Friday, bringing with it the potential for light to moderate Valley rain and a few inches of mountain snow. The NBM has a less than 20% chance of 0.50″ or more of rainfall in the Valley and a 30 to 50% chance of 2.00″ or more of snowfall in the mountains.
Exact details are uncertain at this point, given the bust potential with this system. However, it will be significantly weaker compared to the upcoming atmospheric river forecast to impact the region Sun-Tues.
