First day of fall to feel more like summer in northeastern US

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By Alex Sosnowski for AccuWeather.com

Autumn officially arrives on Thursday, Sept. 22, at 10:21 a.m. EDT, but autumn conditions will wait until this weekend to invade the northeastern United States.

High temperatures much of this week will run about 10 degrees Fahrenheit above average. In some cases, highs will be 15 degrees above average. In many cases, nighttime lows will be 5-10 degrees above average through Friday.

For example, in New York City, temperatures typically range from a low near 60 to a high in the middle 70s during the third week of September. Highs most days this week will be the lower to middle 80s with lows well into the 60s.

People with vacation plans at most of the beaches will have warm weather and warm water from Delaware to Cape Cod. However, there will be potential problems as rough surf is stirred by tropical systems Julia and Karl.

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The only other deterrent to summery conditions will be areas of fog that can linger into the midday hours, according to AccuWeather Chief Meteorologist Elliot Abrams.

"Unlike July, the long nights during September, combined with a clear sky, light winds and high humidity in this pattern will favor late-night fog formation," Abrams said.

Thanks in part to Typhoon Malakas curving away from mainland Asia, one or more waves of cooler, less humid air will invade the Northeast this weekend into early next week.

The transition to the more typical late-September conditions will begin Friday night in the eastern Great Lakes and northern New England and will progress to the Delaware and Chesapeake Bay regions by Sunday.

Temperatures will be trimmed back to normal or slightly below-normal levels. By Sunday, highs will range from the upper 50s to the middle 60s over much of New England and the central Appalachians. In the coastal areas of the mid-Atlantic, highs will be mainly in the 70s.

Accompanying the arrival of the lower temperatures will be breezy, if not windy, conditions for a time in most places, making it feel even cooler.

"The arrival of the cool air may occur with little or no rain in much of the mid-Atlantic to southern New England this weekend," Abrams said.

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The air will get cold enough to bring the risk of scattered frost to the normally colder locations from the central Appalachians to northern New England during Saturday and Sunday nights.

While the cooler and less air will sweep into the Northeast, it will also ooze southward across the Carolinas, Georgia, Alabama and northern Florida early next week. For many in the Southeastern states, this could be the most substantial break from heat and high humidity since the spring.

"Despite a couple of pushes of cool air, this month has the potential to be one of the warmest Septembers on record in some areas," AccuWeather Senior Meteorologist Brett Anderson said.

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"Not only have most days been very warm, but we have not had many cool nights typical of September," Anderson said.

Beyond this weekend, a see-sawlike pattern is in store into early October, which will favor some warm and cool swings.

"However, temperatures are likely to still average near to above normal along the Atlantic Seaboard," AccuWeather Lead Long-Range Meteorologist Paul Pastelok said.

Temperatures will trend to normal to below normal from the Appalachians to the Midwest.

Normal temperatures trend downward by a degree every two to three days during October.

Factoring in the anticipated weather pattern with normal temperature trend, frequent highs in the middle to upper 80s are less likely, while highs in the 70s to lower 80s are likely to be more common along the Interstate 95 corridor during the first part of October.

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