Near-record warmth has spread over much of the eastern half of the country, giving a taste of spring that has many people eagerly looking forward to summer. Highs some 15 to 20 degrees above average for mid-March span the entire Eastern Seaboard and Gulf of Mexico coastline, with unseasonable mildness as far north as Montana and the Upper Midwest.

In the days ahead, that dome of warmth will be reduced to a sliver — vanishing as winter attempts a final stand.

The clashing air masses will bring heavy, flooding rains and strong to severe thunderstorms for some, with a return to below-average temperatures and the chance of snow for others.

Already, the seasons are waging war over the Rockies, where the High Plains and Front Range are bracing for a top-tier snowstorm. All-time snowfall records could come toppling down in Cheyenne, Wyo., where 30 to 36 inches is predicted. Denver could see a two-foot snowstorm, while Boulder, Colo., could flirt with 30 inches.

Warmth in the East

In the Mid-Atlantic, highs Thursday are expected to peak in the upper 70s to near 80. That could claim a daily record for Washington, which hit 78 on March 11, 1967. Baltimore is looking at mid to upper 70s, too, and Richmond is set to hit 80. All three are under red flag warnings, the dry air in place contributing to high fire danger while also allowing temperatures to overachieve.

Boston is expected to hit 70 degrees Thursday, beating out the daily record of 67 set in 1990. The average high for the date is only 45 degrees, the month of March being notoriously cruel in Boston. It would be the city’s first 70-degree reading of the year, a few weeks ahead of the average date of April 5. Last year, Boston hit 70 degrees on Jan. 11.

New York probably won’t approach record territory Friday. Southerly winds will bring air ashore off the Upper Bay, where water temperatures are still only in the 40s. The high Thursday in Central Park is expected to be in the lower to mid 60s. It’s not typical for Boston to be that much warmer than New York City, but a southerly wind in Boston trucks in air that’s spent less time over water. It also induces downsloping from Great Blue Hill, or the process by which air that travels downhill warms. That can tack on an extra degree or two.

Atlanta and Birmingham, Ala., were also in the mid-70s on Thursday and could be a degree or two higher Friday; averages this time of year are around 60. Jackson, Miss., could spend the rest of the week in the 80s.

A less-than-graceful cool-down

The mildness has an expiration date, though. Already, a cool-down was in the works as a frigid upper-level low pressure system eyed the Four Corners region. After triggering potential record snow in Colorado and Wyoming, it will shift east, bringing a chance of strong to severe thunderstorms over the Great Plains this weekend. Areas between north central Texas and Kansas, including the Interstate 35 corridor, Oklahoma City and the DFW Metroplex, could be impacted.

Farther east, a weak cold front marked the boundary between exceptionally mild air to the south and less impressive warmth farther north. That front was draped along Interstate 40 from eastern Oklahoma through Arkansas and Tennessee. The system ejecting from the Rockies will tighten that front in the days ahead.

Moisture pooling along it will build, the front focusing repeated rounds of showers and downpours. Flood watches stretch north of the front from southeast Kansas through Missouri and into southern Illinois and southwest Kentucky. That’s where a broad two to four inches of rainfall is likely through the weekend, with additional heavy rain possible at the start of next week. In the Ozarks of southern Missouri, rain totals could be even higher.

“Widespread rainfall amounts through this weekend [are] expected to range from three to five inches,” wrote the National Weather Service in Springfield, Mo.

Flooding is also possible along the southern stretches of the Mississippi River, where upstream snow melt combined with sodden soils and additional runoff from heavy rain could result in pockets of inundation.

When the cooler air reaches the East Coast

Meanwhile, the South will continue to remain mild through the weekend, but temperatures in the East will tumble as a cold front approaches. Boston could go from a high of 70 degrees Thursday to lower to mid-40s Saturday, with highs around freezing Monday. There’s even a chance New England could see some snow mid-to-late next week.

Cincinnati, predicted to peak near 70 on Thursday, will crash slide gradually to around 60 for the high on Friday, 55 on Saturday, and into the upper 40s with rain on Sunday.

Washington will split the difference, settling in the 50s beginning Saturday behind the front. Forties are possible Monday before a rebound. Temperatures will plateau throughout much of the upcoming week in the 50s.