The next few days will see a dramatic change in weather. From bitter cold to widespread rain and significant snow in the Sierra, the impact will be Wednesday through Saturday as our first major storm system of the season arrives.

Expect heavy rain and Sierra snow, flooding concerns and travel impacts the weekend before Thanksgiving.

Freeze Warning Tuesday

The week starts with a freeze warning in the valley Monday evening through Tuesday morning. Areas below 600 meters elevation can expect minimum temperatures to drop below 36 degrees on Tuesday morning.

A cold weather advisory has been issued throughout the Sierra for elevations above 6,000 feet when temperatures drop below 65 degrees. Some areas in the Sierra can reach lows of 5 to 11 degrees.

If bitterly cold temperatures are on the way, make sure you protect outdoor animals and plants that are sensitive to the cold. Frost areas are possible. Be sure to dress warmly Monday evening through Tuesday morning as the cold air moves in.

Tuesday afternoon temperatures will warm into the upper 50s in the Valley, with 30s and 40s over the foothills and Sierra.

A short break in the afternoon before our attention turns to more active weather for the rest of the week. We expect the first round of Valley rain and Sierra snow to begin as early as Tuesday evening.

Atmospheric river effects throughout the weekend

A strong atmospheric river is expected to bring significant rainfall and snowfall Tuesday night through the weekend, leading to potential flooding issues and travel disruptions.

The action starts across the Sacramento Valley on Tuesday evening, as the atmospheric river makes landfall on the North Coast. The rain and snow will start to increase in intensity from Wednesday.

Snow levels will be between 3,000 and 4,000 feet Tuesday night through early Wednesday. Expect winter travel conditions and possible chain checks across the Sierra. A Winter Storm Watch will be in effect Tuesday evening through Thursday night due to the potential for heavy snow at elevations above 11,500 feet.

Snowfall will increase through Wednesday evening, especially north of I-80.

Four to eight inches of snow is expected at elevations of 3,500 to 4,500 feet, north of I-80. Expect 8-15 inches for locations above 4,500 feet, north of I-80. Up to 2 feet of snow is possible for Sierra peaks.

Rain will spread across the valley Wednesday afternoon with the heaviest rainfall north of I-80.

Sacramento and surrounding suburbs are in the vicinity of heavy rainfall throughout the day. The rain will spread southward toward the San Joaquin Valley and Stanislaus County during the afternoon before retreating further north Wednesday evening.

A Flood Watch has been issued for the northern Sacramento Valley as extended periods of moderate to heavy rainfall could cause flooding and flash flooding.

Flood problems:

• Burn scars are at high risk from debris flows.
• Sharp increases are expected on rivers and creeks.
• Poor drainage and flooding of urban streets

In areas not under Flood Watch, urban flooding will still be possible around poor drainage areas and leaf-clogged storm drains.

Rain will taper off Thursday in the valley south of I-80, but remain steady in the northern Sacramento Valley. Snow levels will begin to increase to 7,000 to 8,500 feet Thursday afternoon.

Periods of rain and snow will continue to fall over the coming weekend, but details on timing are still being worked out. Expect this weekend to be a wet one with more rain in the valley the weekend before Thanksgiving.

Travel through the Sierra will also be affected by possible chain controls in effect this weekend.

Multi-day rain and snow totals

The forecast details beyond Wednesday are a bit vague due to the potential of the atmospheric river stalling. This happens when the large plume of subtropical moisture parks itself over an area for several hours or days. Predicting this stagnation is difficult to determine and we will get a better idea as it gets closer.

Precipitation totals will depend largely on whether the atmospheric river stalls, but forecast amounts are significant, especially north of I-80.

• North Sacramento Valley: 3-6 inches
• South Sacramento Valley: 2-4 inches
• San Joaquin Valley: 1-3″ north of Stockton, 0.5-1.5″ Modesto south
• 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

We expect 1 to 3 meters of snow at altitudes above 1800 meters over the next seven days. Especially about mountain peaks.

Amounts will be highly dependent on snow levels, as atmospheric rivers are known for higher snow levels due to 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 plenty of heavy snow on the way, it’s important to check road conditions often and be prepared for winter travel through the Sierra. Chain checks and delays are expected.

What is an atmospheric river?

As our first atmospheric river event of the season arrives, many are wondering: 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 water vapor outside the tropics. Along the west coast, our moisture comes from the tropics near Hawaii and is directed to the coastline.

Atmospheric rivers can vary greatly in size and strength; the average atmospheric river transports an amount of water vapor approximately equal to the average water flow at the mouth of the Mississippi River, according to the National Weather Service.

There are five categories of these rivers, each varying in strength and impact.

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 remains stationary in one location for several days, the consequences can be dangerous.

The atmospheric river that makes landfall this week is between a 2 and a 3 on the scale. This means that it will be mostly beneficial, thanks to our dry soils, but it can also bring some dangers, including flooding.

Make sure to stay with the CBS Sacramento First Alert Weather team this week as we keep you updated on what impacts we could see and how long the rain and snow will stick around.