Late-September heat wave shatters century old records in midwestern, northeastern US

From the Midwest to the northeastern United States, the first week of fall has not only produced higher temperatures than those seen during astronomical summer, but also some of the latest 90-degree Fahrenheit temperatures on record in many locations.

A large, sprawling area of high pressure centered over the Great Lakes and Ohio Valley has been the culprit of the unusual heat and humidity.

A high pressure system is a clockwise flow of sinking air that can produce long periods of hot and dry weather.

In many locations, record highs that have stood since the late 1800s and early 1900s have been shattered this past week.

"Many nighttime lows in the Midwest have been near or even above late-September average highs," AccuWeather Meteorologist Bill Deger said. This includes Chicago, where overnight temperatures only dropped to the lower 70s during the middle of the week.

"This stretch of 90-degree heat in Chicago is now the longest this year, surpassing the streak from June 10-12," AccuWeather Meteorologist Matt Greene said.

Tuesday was the last and seventh day in a row of 90-degree temperatures in Chicago.

The September 2017 heat wave in Chicago was the second latest streak of three or more consecutive 90-degree days on record. The stretch that spanned Sept. 30 to Oct. 2 in 1971 sits at the top spot.

Saturday's high of 95 F in Chicago also tied June 12 as the hottest day of 2017 and marks the latest day in recorded history that the mercury reached that level. The previous record for temperatures reaching or exceeding 95 was on Sept. 15, 1939, when the mercury reached 99 degrees.

It is not just Chicago that is sizzling in the year's hottest weather.

In St. Louis, there have been 121 cooling degree days spanning Sept. 20-26 compared to an average of 31 cooling degree days for the seven-day period. A cooling degree day is the number of degrees that a day's average temperature is above 65 and people start to use air conditioning to cool their homes or business.

Pittsburgh recorded 13 consecutive days of highs in the 80s through Thursday. The previous longest stretch so far this year was the 10 days spanning late June to early July. On Sunday, Pittsburgh reached 90 F for the first time since June 13.

New York City recorded its first 90-degree day since Aug. 1 on Sunday.

As of Sept. 23, temperatures in Traverse City, Michigan, reached or exceeded 93 F for four straight days. Previously, Sept. 23 had been the latest 90-degree day on record in Traverse City. That record was broken for three consecutive days as of Sept. 26.

Traverse City and Chicago are not the only locations to see temperatures this high this late in the year.

Niagara Falls, New York; Cleveland; Burlington, Vermont; and Madison, Wisconsin, have all experienced the highest temperatures on record for this late in the year.

Cleveland reached 94 F on Sept. 25 and set new record highs for five consecutive days dating to Sept. 21. 90-degree heat is expected again on Tuesday, making this the longest stretch of such heat this year.

Record highs were smashed in Burlington, Vermont, on Sept. 24-27, when the mercury topped out at 90 or above each day. Previously, the latest Burlington had exceeded 90 degrees was on Sept. 16, 1939.

"Syracuse failed to reached the 90-degree mark once in July or August, but reached the milestone on Sept. 25 and 26," Deger added.

Although temperatures in the major cities of the Interstate-95 corridor have fallen short of breaking previous record highs, 90-degree heat has still baked Philadelphia, Baltimore, Allentown and Reading, Pennsylvania; Buffalo and Rochester, New York; and Washington, D.C., on multiple days since Sept. 20.

Dry conditions have accompanied the heat and, in part, have contributed to it. Dry ground heats up more quickly and efficiently than moist ground. Many locations have not had rain in the past one to two weeks.

Unusual warmth continued to smash century old record highs across interior regions of the mid-Atlantic and Ohio Valley through Wednesday.

Those who are eager for fall’s return do not have to wait much longer.

Autumn weather is due to arrive by week’s end.

By Friday, high temperatures across the eastern U.S. will be slashed by at least 15-25 degrees when compared to the highest temperatures during the past seven days.

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