A pair of back-to-back storms were forecast to dump snow and heavy rain over parts of the Pacific Northwest, northern California and the Rockies, triggering winter storm and flood warnings.

Storms pushing in from the Pacific are the result an atmospheric rivera corridor of moisture-filled air that forms when warmer air from the tropics moves toward the poles.

As the first of the two storms pushes inland on Monday, it will bring heavy coastal rain and snow to mountainous areas of the Pacific Northwest and northern California, according to the National Meteorological Service. By evening and into Tuesday, the wintry mix is ​​expected to spread across the northern Rockies and Great Basin, bringing up to 8 to 12 inches of snow to the Cascades.

The next storm is expected to approach the Pacific Northwest Tuesday night, unleashing showers and thunderstorms along the coast and portions of the Coast Range that have the potential to trigger flooding. As the storm moves east, it could bring more snow to the Cascades.

“Precipitation totals could reach 3-4 inches at the Olympics and 1-2 inches for Seattle and Portland,” said Elizabeth Danco, a meteorologist with AccuWeather.

The Seattle weather service office warned residents about potential river flooding this week.

“There is a small break between each system. As a result, several rivers are expected to reach action stage, with minor flooding possible, particularly by Wednesday or Thursday,” the outlook reads. “The extent of flooding will depend on precipitation rates, temperatures, snowpack and total precipitation with each of these weather systems next week.”

Winter storm warnings were active for several western states Monday morning. In central Oregon, weather officials urged residents to remain cautious as they expect 8 to 16 inches of snow and wind gusts up to 50 mph.

“Travel could be very difficult. Hazardous conditions could affect the Monday morning and evening commute. Strong winds could bring down tree limbs,” the weather service in Medford, Ore., wrote in a winter weather advisory that could be extended Tuesday. Similar warnings were issued in parts of Washington, northern California, Utah and Nevada.

In the Northern Hemisphere, atmospheric rivers usually occur in winter and are often cited for strong westerly storms. According to NASA, these are the largest freshwater transport mechanisms on Earth.

Storms loom after record snowfall in Las Vegas, other cities

The atmospheric river approaching the North West Coast is set to arrive as residents in some areas are still digging out of record snowfall brought on by a winter storm last week.

In Colorado, Lincoln and Elbert counties received historic snowfall between Tuesday and Saturday, with areas receiving 35.5 and 41.5 inches of snow, respectively.

From Wednesday through Friday, New Mexico’s Colfax County was blanketed with 40 inches of snow, while parts of Albuquerque received 7.5 inches of snow. New Mexico Governor Michelle Lujan Grisham issued a state of emergencyunlocking $1.5 million in state funding to support agencies responding to the winter storm.

Parts of Las Vegas saw more than 12 inches of snow last week, breaking a decades-old record and causing road closures and power outages. The Las Vegas weather service office warned Monday of strong gusts of wind that could cause “tree and power line damage” and create “hazardous travel conditions.”

Contributed by: John Bacon, Dinah Voyles Pulver.

This article originally appeared on USA TODAY: Storms threaten Seattle with heavy rain, snow in northwest