Wednesday marks the autumnal equinox, and, right on cue, the atmosphere is set to deliver a swift change of seasons. Temperatures are sliding 10 to 20 degrees along a strong fall front sweeping across the nation, bringing about severe thunderstorms and heavy rainfall. In parts of the Mountain West, the front produced the first blanket of snow since the spring.

Flood watches are in effect for the Central Appalachians and the Carolina Coastal Plain, as well as parts of the eastern Great Lakes, where several inches of rain could fall in a short time.

Behind the front, a surge of cool, dry air will spill eastward, causing temperatures to drop below freezing near the international border and within the high terrain of the Intermountain West. Frost advisories and freeze warnings are in effect for portions of Wyoming, Colorado, northeast Minnesota and adjacent northern Wisconsin.

The setup

On Tuesday morning, the cold front stretched from Chicago to St. Louis to Dallas. Temperatures were approaching 70 degrees even at the southern tip of the Hudson Bay, while parts of North Dakota at a more southern latitude were sitting in the 30s and 40s with fog. The front will lose some of its potency as it shuffles eastward but will still induce a marked change in seasons.

In addition to falling temperatures, the humidity will plummet. Dew points, a measure of how much moisture is in the atmosphere, will fall from near-70 into the 40s, slicing the amount of water in the air by 60 percent or more.

The front is being dragged east by a lobe of low pressure over the Great Lakes; that low will slide northeast and weaken while a new low takes shape over the northern Appalachians. That same parent system helped tug ashore a ribbon of moisture into the Pacific Northwest last week, the atmospheric river bringing heavy rainfall to western Washington and Oregon and northwestern California.

On Wednesday, the front will stretch from the Deep South up the Appalachians and into Quebec; by Friday, it will have exited eastern New England.

Chicago hit 82 on Monday but will sit in the mid-60s on Tuesday; Minneapolis was in the 80s on Sunday and, like the Windy City, is in the 60s now.

Even Duluth, Minn., may fall to around 40 on Tuesday night. The temperature drop will be more subtle on the East Coast, falling 5 to 10 degrees from current levels by the end of the week.

Heavy rain and flood threat

The influx of cool, dry air and high-altitude jet stream energy will force the humid air ahead of the front upward, causing bands of heavy rain that may bring a localized flood threat. Flooding won’t be widespread, but a few places could see training, or repeating, thunderstorms.

The National Weather Service Weather Prediction Center highlights Tuesday and Wednesday as featuring a Level 2 out of 4 risk of excessive rainfall and flooding advancing from the Ohio Valley toward the Central Appalachians.

Weather models predict up to four inches of rain in parts of northwestern Ohio, northeastern Indiana and southern Michigan through Tuesday. Where the rainfall breaks out first in Michigan, rainfall rates could top an inch per hour. The rain will remain west of Detroit through evening, but they’ll get soaked overnight into Wednesday.

A second bull’s eye will be found in eastern South Carolina and southeastern North Carolina, where tropical moisture streaming ashore will fuel torrential downpours. That feature is located ahead of the front and is the result of convergence, or air moving toward the same regions.

Much of that moisture is left over from the remnants of Tropical Storm Nicholas, its skeletal atmospheric swirl a nearly stationary feature that lingered for days over the Deep South before sauntering northeastward.

Precipitation will also be enhanced by air forced up the Appalachians on Wednesday, which is why there is a flash flood watch up along them and to their east, including areas just west of D.C. and Richmond.

“Given the local enhancement of the higher terrain and a very moist air mass, widespread rainfall amounts of two to four inches are expected by Thursday morning,” wrote the Weather Service office serving the Washington region. “However, localized amounts could exceed that, especially along the ridges.”

Both the nation’s capital and Baltimore may see an initial round of showers Wednesday afternoon before a round of heavier downpours and thunderstorms overnight into early Thursday as the front passes. Localized flooding cannot be ruled out.

The Northeast and New England will see downpours Thursday into Friday but should miss out on the heaviest totals. The Weather Service has placed the zone that includes Maryland, Delaware, eastern Pennsylvania, New Jersey and southern New York at a Level 1 of out 4 risk for excessive rainfall.

Severe weather potential

As is the case with many transition season fronts, the atmosphere has plenty of shear, or change of wind speed and/or direction with height, but largely lacks the instability, or juice, needed to bubble up intense thunderstorms. That’s why each of the next three days features only a Level 1 out of 5 “marginal” risk of severe weather.

Tuesday’s risk focuses over northern Louisiana and much of Arkansas, where a sporadic instance of gusty winds or hail is possible. A greater chance could manifest Wednesday from roughly Lake Erie south to the tail of the Appalachians. There’s a nonzero chance of an isolated, short-lived tornado.

Areas from Virginia Beach to Albany, N.Y., could see some severe weather potential Thursday, depending on when the front comes through. If it stalls and coincides with peak daytime heating in the afternoon, more fuel will be available for storms. Richmond, Washington, Philadelphia, Baltimore, New York City and Binghamton, N.Y., are all encapsulated in that risk.

Thunderstorm chances flatline as the front pushes offshore Friday.

The front’s snowy side

The storm front first swept ashore in the Pacific Northwest over the weekend, bringing the first snowfall since the spring to the high elevations. Mount Hood and Mount Rainier were blanketed in fresh snow. Even Mount Shasta in Northern California, which lost much of its snow cover this summer, received a coating.

The insurgence of crisp air flipped remnant rain to snow near Yellowstone National Park on Sunday, bringing up to a half-foot of snow in the mountains. Even the southern reaches of the Columbia River Basin saw a few flakes, an unmistakable harbinger of winter’s fast approach.

Looking ahead

Farther ahead, there are signs that reinforcing shots of cool air will dominate in the next week to 10 days, meaning the feel of autumn won’t go anywhere anytime soon. While there are signs that most of the Lower 48 should end up near or slightly above normal temperatures in the coming 30 days, it’s probable that seasonally cool air will stick around until at least the end of September.

Jason Samenow contributed to this report.