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Fall on hold: Forecasters predict long-lasting warm temperatures in eastern U.S.

(National Weather Service)
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This story, first published Tuesday, was updated Wednesday.

For about a week after the fall equinox, much of the eastern two-thirds of the Lower 48 states enjoyed crisp, refreshing autumn weather. But now Mother Nature has changed course. Warm, humid conditions more typical of late summer have returned and show little sign of retreating.

Forecasts now call for above-average temperatures lasting at least 10 days, with high temperatures in some areas nearly 30 degrees above normal at times. The core of the anomalous warmth is predicted to focus in the north-central United States, but above normal temperatures are anticipated to prevail in most places east of the Rockies.

The weather pattern may trigger heavy rainfall and severe storms in the central United States and could eventually support new tropical storm activity near the Gulf of Mexico.

It’s not clear when this warm pattern will break down, and October is almost certain to end up warmer than normal over a large part of the nation.

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Unusual heat in the Northern Plains

The heat is already baking parts of the Northern Plains, where temperatures topped 90 on Tuesday in western North Dakota compared to average highs in the 60s. Bismarck and Minot both soared to 91 degrees, breaking records for Oct. 5.

Adjacent parts of Manitoba and Saskatchewan saw temperatures climb higher than in the heart of Texas. Winnipeg reached 85 degrees Tuesday, its 5th hottest October day on record.

The heat expanded as far west as eastern and central Montana; Glasgow and Billings set record highs of 90 and 86.

This latest bout of heat follows an even more intense round last week when it hit 100 degrees in Hazen, N.D., the highest temperature ever recorded so far north so late in the calendar year.

The heat is combining with bone-dry conditions to bring critical fire weather concerns across the northern part of the central United States, with relative humidity values falling as low as 10 percent.

In the coming days, periodic fall cold fronts may attempt to transport cooler air to the Northern Plains and Midwest, but the fronts will largely fizzle before they reach the Eastern Seaboard and are unlikely to displace milder-than-average temperatures in most areas.

Expansive and enduring warmth

The pattern of enduring warmth is the product of a bulge in the jet stream over the middle and eastern parts of the Lower 48. To the west, a dipping jet stream is set to bring cool conditions and even mountain snow in the Rockies. This pattern appears to be locked in for at least the next 10 days or so.

Temperatures in the central and eastern United States will remain in the 70s and 80s, delaying the traditional stairstep decline that typically occurs during October. The European modeling system shows temperatures at least 5 to 15 degrees above average east of the Rockies.

Chicago could see highs approaching 80 degrees this weekend, with sunny conditions and a slight uptick in humidity.

Columbus Day in the nation’s capital could feature highs in the upper 70s, and none of the predicted highs over the next 10 days are below the normal of 72 degrees.

The actual departures from average aren’t terribly remarkable, but the longevity of the warmth as well as the lingering summertime moisture is noteworthy.

The warm air is delaying fall foliage, which is rather faint even as far north as the Maine-Canadian border region.

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The rain and severe storm threat

Where the warm and cold air masses clash, severe thunderstorms and heavy rainfall will result. Already, meteorologists at the National Weather Service Storm Prediction Center are monitoring a risk of dangerous thunderstorms across the south-central Plains and Ozark Plateau from Sunday into Tuesday of next week. Cold air arriving from the west will lift warm, moist and unstable air to the east, brewing thunderstorms with the potential for large hail and damaging winds.

An insurgence of jet stream energy aloft, meanwhile, will induce wind shear, or a change of wind speed and/or direction with height. That could manifest in a tornado risk as well.

In the southeastern United States, a “cutoff low,” or zone of low pressure and cool air aloft, will linger for days, holding back temperatures but tapping into moisture from the Gulf of Mexico and leading to heavy rainfall. Since it’s pinched off from the jet stream, it has nothing to move it along.

Flash flood watches blanket most of northwestern Florida, Alabama and northwestern Georgia where several inches of rain had fallen and another three inches or more may fall by Friday afternoon.

Tracking the tropics

The weather pattern is also one to watch for potential storminess in the Gulf of Mexico, especially in the middle and latter part of the month.

The upcoming pattern will feature several decaying cold fronts that will shift east with time, and pockets of spin on the tail end of them may eventually wind up in the gulf. Those eddies occasionally prove conducive to tropical development there and in the Caribbean, where water temperatures are unusually warm.

There’s no specific gulf threat at this time, but given the pattern in place, forecasters will closely monitor the situation.

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Climate change connection

The record-challenging heat in unusual locations and duration of warm conditions over such a broad area are weather features consistent with a warming world.

Every season except summer is getting shorter, a sign of trouble for people and the environment

Climate research has shown an expanding summer season and a trend toward shorter autumns, a pattern that this year exemplifies.