This story, first published late Wednesday afternoon, was updated Thursday morning.

An active jet stream is slicing across the center of the country and bringing a wild clash of seasons, with unseasonable warmth in the East and a frigid taste of winter in the West. Parts of the Rockies and western Plains are digging out from feet of snow that fell Tuesday and Wednesday, while a renewed dose of summer spreads over the Mid-Atlantic, Southeast and New England through Saturday.

In between, the seasons are waging war, brewing strong thunderstorms and heavy rainfall. Both Sunday and Tuesday featured rounds of damaging winds, hail and tornadoes on the Great Plains. Now, parts of Texas are dealing with up to 10 inches of rain and flooding, while strong thunderstorms may flare up across parts of the Plains and Midwest on Thursday afternoon.

The cold, snowy side

On Thursday, the main feature dominating the weather map was a cold front stretching from the Texas to Canada. Out ahead of it warm, moist air was flowing northward with a blast of cool, dry air on its backside.

The chill could be felt from Montana and the Dakotas to the Desert Southwest on Thursday morning with widespread lows in the 30s and 40s, with 20s in the mountains.

In Montana, temperatures dipped below zero for what could be the first time this season on Wednesday, while dropping into the 40s as far south as Phoenix. Emblematic of the divided nation, it was colder in Phoenix than it was in Chicago, which has yet to fall into the 40s this season for only the second time so late in the year.

Las Vegas also had lows in the upper 40s at night the past several days, and 40s were observed in Seattle and Portland, Ore., too. Boston, meanwhile, hasn’t fallen below 51 degrees yet this season, beating the record for the latest sub-50 reading observed. Washington is in a similar boat and hasn’t made it below 54 degrees since the spring, the furthest into the fall it has ever remained this warm.

The snowstorm responsible for drawing the cold air so far south dumped one to two feet or more on parts of Wyoming, Montana, Idaho, Utah and the western Dakotas. Up to 28 inches fell in Montana, according to AccuWeather, while up to 27 inches fell in western South Dakota.

The snow produced blizzard conditions for a time Wednesday in parts of eastern Wyoming and western South Dakota, where wind gusts topped 70 mph.

“Extremely dangerous or impossible travel conditions due to icy, snow-packed roads and whiteout conditions in heavy snow and blowing snow,” wrote the National Weather Service in Cheyenne, Wyo. “Avoid unnecessary travel!”

Snow even fell along the northern rim of the Grand Canyon and in Flagstaff, Ariz. More than a foot of snow fell in Park City, Utah.

A few mountain snow showers will continue as cold air blows east in the high terrain of Utah and Colorado on Thursday.

The warm side

In the eastern United States, temperatures and humidity levels were projected to be more characteristic of late August or early September than the middle of October. Highs on Thursday and Friday will climb some 15 to 20 degrees above average from the Mississippi River to the East Coast. Friday will be the hottest, with the highest temperatures concentrated in the Mid-Atlantic and Southeast.

Washington, D.C., could climb to 84 degrees on Friday, about a dozen degrees above average. Philadelphia could reach the lower to mid-80s as well, and even New York City will edge close to 80.

Syracuse, N.Y., has hit at least 70 degrees for eight straight days and 152 times in 2021, the most instances on record in a calendar year. Highs are forecast to remain in the 70s until Sunday, when a cold front will lower temperatures about 20 degrees.

Caribou, Maine, near the border with Canada, has yet to record a freeze this fall and is likely to “at least tie the latest first fall freeze on record,” according to the Weather Service. On Wednesday, Caribou set a record high of 77 degrees and was two degrees warmer than Phoenix.

Much of eastern North America has seen abnormally mild weather this month. The most exceptional warmth has focused over the Upper Midwest and Ontario.

Warmth of such unusual intensity and duration is consistent with what scientists expect to see with greater frequency in a warming world as the summer season lengthens.

This week, the front responsible for the cold in the West will finally sweep through the East, returning more autumn-like temperatures by Sunday.

The clash in between

The storm that formed on the interface of the air masses is dragging east a strong cold front that will reverse the seasons and conjure up some active weather. That’s especially true in the southern and central Plains, where a multiday severe weather outbreak brought widespread reports of hail and damaging wind, along with a smattering of tornadoes.

One rotating supercell thunderstorm dropped a picturesque tornado near Clinton in west-central Oklahoma after dark on Tuesday night, the churning vortex flanked by a slender satellite funnel. Other tornadoes touched down in western Kansas. It was the second time in three days that the National Weather Service’s Storm Prediction Center had declared a level 4 out of 5 “moderate risk” of severe weather in Oklahoma and the only two times this year.

The concern has since shifted to flash flooding, with the parent system’s trailing cold front set interacting with remnant moisture from former Hurricane Pamela. It is depositing a strip of heavy downpours that has produced up to 8 to 10 inches of rain in South Texas to the east of San Antonio, where flash flood warnings were in effect Thursday morning.

The heaviest rainfall, centered near Victoria, Tex., was streaking north toward the west side of Houston, where several inches of rain are possible through Thursday afternoon before tapering off.

In addition to the heavy rain in Texas, there is a low-end risk of severe storms in parts of the Midwest on Thursday as the cold front blasts east. The Weather Service’s Storm Prediction Center has highlighted zones in central Oklahoma and from northeastern Arkansas to central Indiana as having an elevated storm threat. These zones include Indianapolis and Oklahoma City.

On Friday, the threat of severe storms will shift east into the Tennessee Valley and lower Ohio Valley, running through Louisville and Cincinnati as the cold front heads toward the East Coast.

correction

A previous version of this story incorrectly stated Syracuse set record highs of 79 degrees on four instances over the past 8 days. Those represented the highest temperatures this month but were not records.