Suddenly, It Will Be Autumn for Much of the U.S. This Weekend

Oct. 1 should have been a cool and crisp Sunday in the Twin Cities, the kind in which a temperature reading around the daily average high of 56.7 degrees would have been perfect for finding the right pumpkin to carve or getting lost in a corn maze.

Instead, the region sweltered in summerlike temperatures that reached a meteorological record for October of 92 degrees. Only a week later, there is frost in the region’s forecast for Saturday.

These contrasts won’t be found just in Minnesota. A surge of cold air late this week and into the weekend will have an impact just about everywhere east of the Rockies, Alex Lamars, a meteorologist with the Weather Prediction Center, said. It will go from sweaty to sweater weather in a few days.

High temperatures were 15 to 25 degrees above average across the Great Lakes, the Ohio Valley and the Northeast on Wednesday afternoon. But that warm outlook will change on Friday and through the weekend across the east-central United States and toward the Eastern Seaboard, where highs falling 10 to 20 degrees below normal are likely.

As the cold air sweeps in, rainfall that is in the forecast for portions of the northeast could be enhanced by Tropical Storm Philippe, threatening to wash out anyone’s fall festivities.

Although temperatures are forecast to be below average, only a few places in the East may experience record daily cold temperatures. By contrast, above-average temperatures will continue to be felt out West, where over a dozen daily records for warm temperatures could be tied or broken.

The cold air will be an abrupt arrival of the fall season for some, especially in the central and northeastern United States, Mr. Lamars said. These areas have experienced among the warmest-ever starts to fall.

This milder weather pattern comes after what was the third-warmest summer on record for the United States. The coming burst of fall air will be a refreshing break from the relentless heat along the Gulf Coast, especially in Louisiana, which had its warmest summer on record. The last time New Orleans was below 65 degrees was May 2, and temperatures there are forecast to drop into the low 60s, Mr. Lamars said.

Texas and Florida might also feel the relief after a punishing summer season that ranked as their second warmest. “The last time Houston had a low temperature, even below 65 degrees, was on May 8th,” Mr. Lamars said, adding that they may finally dip into the upper 50s this weekend.

Houston, New Orleans and many other locations in the Gulf Coast region had extremely sultry summers and set heat-index records for hours at or above the 110-degree mark. The cooler air mass expected this weekend will also be drier, with humidity levels dropping to some of the lowest those areas have experienced since April or early May.

Sudden temperature swings are expected as the country transitions to more autumnal weather. Even in Minnesota, the cold swing is not entirely unusual for fall’s arrival, Tyler Hasenstein, a meteorologist with the National Weather Service in the Twin Cities, said.

Below-normal high temperatures in the low 50s are recorded every three to five years in the Twin Cities in early October, he added. What was unusual this season was the extreme heat at the start of the week. Temperatures rarely reach the 90s in Minneapolis in October, Mr. Hasenstein said.

The conditions expected this weekend in Minnesota are forecast to bring relief from the heat and humidity, Mr. Hasenstein, said. Even with frost expected on the ground in some places, most areas in the region will remain above freezing.

The first frosts and freezes of the year could occur in other parts of the Northern and Central Plains, the Upper Midwest, the Great Lakes and the interior Northeast, Mr. Lamars said, which “is on par with historical expectations.” Most of those areas expect to experience the first freeze in late September or early October.

After this weekend, things will shift again, and above-average temperatures will build in the central United States for next week. However, people on the West Coast might get their turn to have a taste of fall, and maybe sip a pumpkin-spice latte, by the middle of next week.