Northern California to be hit by freezing temperatures, atmospheric river this week
The next few days will bring a dramatic weather shift. From bitter cold to widespread rain and significant Sierra snow, impacts peak Wednesday through Saturday as our first major storm system of the season arrives.
Expect rounds of heavy rain and Sierra snow, flooding concerns and travel impacts through the weekend before Thanksgiving.
Freeze Warning Tuesday
The week begins with a Freeze Warning across the Valley on Monday night and goes on until Tuesday morning. Areas below 2,000 feet in elevation can expect overnight lows to drop below 36 degrees by Tuesday morning.
Across the Sierra, a Cold Weather Advisory has been issued for elevations above 6,000 feet as temperatures drop below 20 degrees. Some areas in the Sierra can reach lows ranging from 5 to 11 degrees.
With bitter cold temperatures on the way, make sure to protect outdoor animals and plants sensitive to the cold. Areas of frost will be possible. Make sure to dress warmly Monday night through Tuesday morning as the cold air settles in.
By Tuesday afternoon, temperatures warm to the upper 50s in the Valley with 30s and 40s over the foothills and Sierra.
A brief break through the afternoon before our attention turns to more active weather through the rest of the week. We'll watch for the first round of valley rain and Sierra snow to start as early as Tuesday evening.
Atmospheric river impacts through the weekend
From Tuesday night through the weekend, a strong atmospheric river is expected to deliver significant rainfall and snowfall leading to potential flooding concerns and travel disruptions.
The action starts as early as Tuesday night across the Sacramento Valley as the atmospheric river makes landfall across the North Coast. Rain and snow will begin to pick up in intensity through early Wednesday.
Snow levels will be between 3,000-4,000 feet Tuesday night into early Wednesday. Expect winter travel conditions and possible chain controls over the Sierra. A Winter Storm Watch will go into effect starting Tuesday evening through Thursday night for the potential of heavy snow for elevations above 3,500 feet.
Snowfall intensifies through Wednesday evening, especially north of I-80.
Four to eight inches of snow are expected from 3,500–4,500 feet, north of I-80. Expect 8–15 inches for locations above 4,500 feet, north of I-80. For Sierra peaks, up to 2+ feet of snow will be possible.
Across the Valley, rain spreads through Wednesday afternoon with the heaviest rainfall north of I-80.
Sacramento and its surrounding suburbs fall in the fringe of seeing pockets of heavy rainfall throughout the day. Rain spreads south toward the San Joaquin Valley and Stanislaus County through the afternoon before retreating further north by Wednesday night.
A Flood Watch has been issued for the northern Sacramento Valley as prolonged periods of moderate to heavy rain will cause flooding and flash flooding risks.
Flooding Concerns:
- Burn scars face a high risk of debris flows.
- Sharp rises are expected on rivers and creeks.
- Poor drainage and urban street flooding
In areas not under the Flood Watch, urban flooding will still be possible around poor drainage areas and leaf-clogged storm drains.
By Thursday, rain tapers in the Valley south of I-80 but remains steady in the northern Sacramento Valley. Snow levels begin to rise to 7,000–8,500 feet Thursday afternoon.
Rounds of rain and snow will continue through the upcoming weekend, yet details on timing are still being worked on. Expect this weekend to be a wet one with more valley rain impacting the weekend before Thanksgiving.
Travel across the Sierra will also be impacted with possible chain controls in effect through the weekend.
Multi-day rain and snow totals
Forecast details beyond Wednesday are a bit fuzzy with the potential of the atmospheric river stalling. This occurs when the large, plume of subtropical moisture parks itself over an area for multiple hours or days. Forecasting this stalling is tricky to pinpoint and we'll get a better idea as it is closer to happening.
Rainfall totals will largely depend on whether the atmospheric river stalls, but forecast amounts are substantial especially north of I-80.
- North Sacramento Valley: 3-6 inches
- South Sacramento Valley: 2-4 inches
- San Joaquin Valley: 1-3 inches north of Stockton, 0.5-1.5" Modesto southward
- Foothills South of HWY 50: 2-4 inches
- Foothills North of HWY 50: 3-6 inches, locally 7-10 inches in western Plumas, Sierra, and northern Yuba County
Over the next 7 days, for elevations above 6,000 feet, we are expecting 1-3 feet of snow with more especially across mountain peaks.
Amounts will be dependent on snow levels, as atmospheric rivers are known for higher snow levels because of the warmer moisture they transport.
Snow levels (Tuesday - Thursday)
- Tuesday: 3,000 to 4,000 feet
- Wednesday: 4,500 to 5,500 feet
- Thursday: 7,000 to 8,000 feet
With a lot of heavy snow on the way, it's important to check road conditions often and be prepared for winter travel across the Sierra. Chain controls and delays are expected.
What is an atmospheric river?
As our first atmospheric river event of the season arrives, many question: What is an atmospheric river?
Some may know it as the "Pineapple Express" -- and, yes, atmospheric rivers are the same.
Atmospheric rivers are relatively long, narrow regions in the atmosphere, like rivers in the sky that transport most of the water vapor outside of the tropics. Along the West Coast, our moisture comes from the tropics near Hawaii and gets directed to the coastline.
Atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River, according to the National Weather Service.
There are five categories of these rivers with each varying in strength and impacts.
Not all atmospheric rivers cause damage. Most atmospheric rivers often provide beneficial rain or snow that is crucial to the state's water supply. But, if an atmospheric river stalls in one location for multiple days, the impacts can be hazardous.
The atmospheric river to make landfall this week is between a 3 and a 4 on the scale. This means it will be mostly beneficial, thanks to our dry soils, but can provide some hazards including flooding.
Make sure to stay with the CBS Sacramento First Alert Weather team this week as we keep you updated on the impacts we could see and how long the rain and snow stay around.