A strong autumn cold front is sweeping across the Lower 48, bringing a sharp drop in temperatures and a few strong to severe storms. It’s one of several fronts that will gradually cause the stair-step drop in warmth associated with the changing seasons, replacing summerlike mildness in the East with a dry, crisp autumnal air mass by Sunday.

The front’s parent storm system already dropped up to 28 inches of snow on parts of the Rockies as it unleashed a surge of cold air in the mountains before bringing severe weather to the Great Plains. Nearly two dozen tornadoes were reported, concentrated heavily in northwest Kansas and Oklahoma on Tuesday and in northeastern South Dakota and western Minnesota on Wednesday.

Now in a weakened state, the front is still surging east and will present at least some severe weather risk, albeit less widespread. The National Weather Service Storm Prediction Center issued a “slight” risk of severe weather, or Level 2 out of 5, for parts of the Tennessee and Ohio valleys Friday, with a “marginal” risk, or Level 1 out of 5, in the Mid-Atlantic and Northeast on Saturday.

As of midday Friday, the front stretched from near Buffalo to Little Rock, Ark., and into southeastern Texas, where it was slowly approaching Houston and Galveston.

Summerlike warmth ahead of the front

Low-level southerly winds will strengthen as the low deepens, or intensifies, dragging a strip of warmth over the East Coast on Friday and Saturday. Highs could climb well into the 80s as parts of the Mid-Atlantic and Northeast enjoy a late-season taste of summer.

Highs could hit 84 on Friday in Washington and Philadelphia and 82 in New York. That’s about 15 degrees above average.

Boston, farther to the east, will see its warmest day Saturday, when highs climb into the upper 70s. It has yet to dip into the 40s this fall, the latest on record it has remained so mild.

The warmth is far from record-breaking, but its magnitude is impressive nonetheless. It also fits into a pattern of a mild fall that has yet to see much in the way of cool temperatures east of the Appalachians. Washington hasn’t dropped below 54 degrees so far this season, making Saturday night’s nippy forecast the latest on record for the nation’s capital to hit that mark.

Syracuse, N.Y., was expecting highs in the mid- to upper 70s on Friday before falling into the upper 50s for highs Sunday. The city has hit 70 degrees a record 153 times this year, beating out 1991, which held the previous record of 150 days.

A pronounced trend exists as Syracuse logs an increase of about two more 70-degree days per year every decade, with an average of 13 more such days now than in 1950. The uptick in mild days is commensurate with what’s to be expected from human-induced climate change in a warming world.

Caribou, Maine, will remain stuck in the lower 60s on Friday, but the city hit 77 degrees Tuesday — two degrees warmer than Phoenix. Nowadays, Caribou sees an average of 16 more 70-degree days than was typical in the 1930s.

Caribou dropped to 33 degrees on Oct. 5 but has yet to record its first freeze; current forecasts indicate it probably won’t over the next week. That would mark its latest first freeze on record.

Strong storms possible

The warm, moist air ahead of the front will destabilize the atmosphere and provide fuel for storms. Showers and thunderstorms will develop Friday during the afternoon hours from the Ozarks to the eastern Great Lakes, with clusters and a few squall lines and line segments possible.

Cities such as Nashville, Louisville, Cincinnati and Cleveland all have a Level 2 out of 5 risk of severe weather.

The storms will be scattered. Instances of strong to isolated damaging wind are likely, and an isolated tornado can’t be ruled out. A dip in the jet stream is contributing to wind shear, or a change of wind speed and/or direction with height; that’s sufficient to cause storms to rotate.

The threat of strong storms will shift east Saturday, but the setup will become a “high shear/low CAPE” environment — notoriously challenging for forecasters. Severe weather requires two basic ingredients: instability, or storm fuel (CAPE), and spin (shear). Saturday will feature copious spin, but fuel will be lacking. That means a “fineline,” or shallow squall line of low-topped downpours, will probably form.

Finelines aren’t tall, meaning they don’t tap into the full wind dynamics of the atmosphere. That means any severe weather will be isolated at most; gusty winds are the primary hazard in Saturday’s marginal-risk area, which includes Washington, Baltimore, Philadelphia, New York and Boston. There is a very low risk of an isolated tornado.

Cooling temperatures behind front

Temperatures and humidity will fall quickly behind the front, the mercury plummeting 15 to 20 degrees in most places.

Morning lows started in the 20s and 30s from Montana and Idaho south into Utah, Wyoming and Colorado; a few places west of Denver in the higher elevations were starting in the single digits and teens.

Gates Park in northwest Montana reached minus-1 degree overnight Tuesday into Wednesday morning, confirmed to be the lowest official reading anywhere in the Lower 48. Light snow was falling in parts of the Treasure State on Friday morning.

Temperatures on Wednesday morning fell into the 40s as far south as Phoenix, making the ordinarily scorching city chillier than Chicago. The Windy City hasn’t made it into the 40s yet this season, only the second time on record this late in the year.

Even Las Vegas made it into the 40s, as did Seattle and Portland, Ore. The cold over the Intermountain West and Rockies is amplified by a blanket of snowfall from an early-week storm system that dropped more than two feet in spots.

Cool air will surge south and east behind the front Sunday morning, with upper 40s reaching all the way to the Louisiana Gulf Coast. Much of the Interstate 95 corridor on the East Coast will dip into the 40s as well. By Sunday afternoon, highs from Atlanta to Boston will be only in the 60s, compared with the 70s and 80s in previous days.

Jason Samenow contributed to this report.