A pair of autumn storm systems will sweep across the Western and central United States over the coming week, bringing heavy snow in the mountain West and dangerous severe weather in the central states, including possible tornadoes.

Already, the National Weather Service Storm Prediction Center has declared a Level 3 out of 5 “enhanced risk” for severe weather Sunday in the Central Plains, noting that “initial storms will quickly become supercells [or rotating thunderstorms], with large hail and locally damaging winds expected.”

“A couple of tornadoes may also occur,” forecasters noted. More rounds of severe thunderstorms are likely Monday and Tuesday as a second system comes through.

Meanwhile, as the back-to-back storm systems cut across the West, snow is expected to spread from the high elevations of the Sierra Nevada to the central and northern Rockies.

While the West is cold and the central United States is stormy, abnormally warm, mild weather will dominate in the Eastern states.

Appetizer flakes through Saturday preview fall’s first full-fledged mountain snowstorm

On Friday, the first of the two systems was just beginning to enter the picture as it approached the Pacific coastline. Cold air from the north was nestled within a dip in the jet stream that was spilling south.

The counterclockwise-swirling system will push a ribbon of moisture northward ahead of it, which will be forced up over the mountains of Southern California into the frigid air at high altitudes. Up to a foot of snow may fall in the central Sierra Nevada. A few wet flakes are also possible in the higher elevations surrounding the Great Basin through late Saturday.

A pair of autumn storm systems produced snow in California's Sierra Nevada on Oct. 8. (Storyful)

Snow is already falling in Yosemite National Park outside the valley, where a winter weather advisory is in effect. The highest peaks of northeast Utah and west-central Wyoming near the Wind River Reservation, north of the Wyoming Basin, could see significant accumulations through Saturday. Between two and four inches are likely above 7,000 feet, with a half-foot possible upward of 9,000 feet. A few 10-inch totals are probable on mountain peaks, according to the Weather Service in Riverton, Wyo.

The Western Uinta Mountains in Utah could see up to a foot with 45 mph winds above 9,000 feet through Saturday.

That initial round of mountain snow will pave the way for the next storm that will swing in from the Pacific Northwest early next week. Widespread and substantial mountain snows, in some places measured in feet, are likely Monday and Tuesday, particularly across Wyoming and Montana.

Billings, Mont., could see snowfall Tuesday, just a week after it hit a record high of 86 degrees.

Heavy snow is also a good bet in the mountains of Colorado, and even Denver and Boulder could see their first flakes of the fall, although it’s not clear whether they will amount to much.

Rounds of dangerous severe weather likely

As the first system ejects from the Rockies on Sunday, it will release two disturbances — one lobe of spin will kick through the Texas Panhandle and ride along the Red River on the southern border of Oklahoma, while another will swing across the northern Plains.

The southern system will tap into a supply of warm, humid air brimming with moisture from the south, all while dragging east an insurgence of arid desert air. The clash of the air masses will result in severe thunderstorms late Sunday.

Most at risk will be the Interstate 35 corridor from Dallas to Oklahoma City and eastward, where a change in wind speed and/or direction with height, known as wind shear, may cause a few storms to rotate. An isolated tornado is possible with initial isolated storms during the evening before they congeal into a wind-driven squall line or clusters by nightfall.

A couple of isolated strong to severe thunderstorms are possible across the Mississippi Valley on Monday, though they will probably be tamer than on Sunday.

By Tuesday, when the second storm enters the Plains, the atmosphere will reload, with additional rotating thunderstorms likely in central and western Oklahoma and parts of the Texas Panhandle and central Lowlands. Oklahoma City, Lawton and Woodward, Okla., as well as Wichita Falls, Tex., and the Interstate 44 and Highway 287 corridors, could be in line for hazardous weather.

Heavy rainfall possible

The pair of systems will bring multiple rounds of heavy rainfall to the central United States next week, beginning this weekend, when a widespread 1.5 to 2.5 inches will fall across North Dakota. Dry conditions leading up to the rainfall, however, may mitigate the risk of flooding.

“Perhaps some heavy rain is possible at times, although the overall threat of flooding remains uncertain at this time given the long duration of the rain and the antecedent dry conditions,” wrote the National Weather Service in Bismarck.

There could also be some heavy rainfall from central Oklahoma and Kansas northeast into Missouri and parts of the Corn Belt as the second storm comes in. Predicting exactly where is a challenge, as it depends on the exact evolution of thunderstorm activity early next week.

The storminess will largely avoid the Eastern United States as the pair of systems follow the jet stream bulging into Canada over eastern North America. Beneath this bulge in the jet stream, temperatures will be abnormally warm east of the Plains, in some cases more than 20 degrees above normal.