Tens of millions of people are slated to travel this upcoming holiday week, taking to the roads and skies in droves and hoping for smooth sailing. But a powerful storm system is set to make a cross-country trip as well, already bringing heavy rain and mountain snow to parts of the Pacific Northwest as it plots its coast-to-coast journey. Its energy will spawn a new storm that could bring a swath of heavy snow to the nation’s center while posing a severe weather risk for others.

The Pacific Northwest, which — after days of relentless cloudiness, coastal rain and mountain snow — is finally beginning to dry out. A persistent atmospheric river — a channel of moisture-rich air transporting water vapor long distances — that was aimed at Washington and Oregon for days is at last shifting to the south, bringing rainy weather to central and southern California. For example, Los Angeles was placed under a flood advisory for a time this morning.

The energetic upper-level pocket of energy directing that moisture north will slowly consolidate and meander east in the coming days, bringing a period of snow to Nevada’s Great Basin Monday night and rain showers over the Desert Southwest on Tuesday. By Wednesday, some mountain snow is likely for the Four Corners region, adding a touch of meteorological ambiance to Christmas day.

At the same time, a second hose of atmospheric moisture will again eye the Pacific Northwest, but quickly sweep down the coast Tuesday night into Wednesday. A new round of showers is possible for many along the Pacific Coast on Wednesday.

Simultaneously, a more powerful low will develop, colliding with southern California on Wednesday. Moderate to perhaps heavy rain is possible in some areas close to the coast, with a healthy dose of mountain snow for the southern Sierra Nevada. One to 2 inches of rain is possible in many areas in and around Los Angeles, but it’s a high stakes/low confidence forecast. Even the National Weather Service notes that “as little as [a] 60 mile deviation in the storm track could greatly alter the amount of rainfall.”

Depending on how the system evolves, significant delays are possible at major air travel hubs, including Los Angeles International Airport. The Interstates 5 and 15 corridors may also be impacted.

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More precipitation is possible over the Southwest, particularly Arizona and New Mexico, on Thursday and Friday. Significant snow accumulations could be in the offing in the higher elevations.

Then things become even more interesting.

As the storm emerges in the Plains, it will intensify near or over the Oklahoma and Texas Panhandles. From there, it looks to trek northeast, making a run towards Lake Michigan into Sunday.

North of the storm’s center, moderate to heavy snow is possible. While specifics of the system cannot be yet determined, the ingredients could come together to produce a foot-plus of snowfall in some spots – with the greatest likelihood in the southern High Plains and eastern Colorado.

Depending on the exact track and intensity of the low, additional snow is possible from Kansas to the Upper Midwest. A scenario like this could also pose issues at Chicago-area airports this weekend.

That target area could shift in the coming days, as computer models are not yet in agreement over this storm’s track and intensity.

After sweeping north through the Great Lakes region, the storm’s center could track along/north of the Canadian border into southeastern Ontario and Quebec. Snow is possible in some parts of northern New England if it takes that track. However, significant track error exists at 6 to 7 days out; we can only describe broad patterns or general storm tracks.

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Meanwhile, more than an inch of rain is possible in a few locales south of the storm center, within its main precipitation shield.

This system could also help spark severe weather along the storm’s trailing cold front as it moves east, colliding with an unusually mild air mass across the eastern U.S., with portions of the southern Plains and Southeast most at risk.

Specifics are unclear since this is an extended forecast, but it’s a weather pattern that is in many ways similar to the same that brought a deadly round of severe weather to Mississippi, Alabama, and Louisiana last week.

Behind this weather system, cooler weather will bring at least a temporary quelling of the uncharacteristic warmth that’s overspread much of the Lower 48 for the holidays.