Summer and fall to play tug-of-war in northeastern US into late September

By Alex Sosnowski, AccuWeather senior meteorologist
September 20, 2018, 2:53:53 PM EDT

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The northeastern United States will be in the middle of an atmospheric battle zone into the end of September as waves of cool air begin to push out of Canada and tropical air fights back.

Cool air has been building across central Canada over the past week and is forecast to bulge southward at times in the coming days.

The pattern for the next couple of weeks or so will translate to long stretches of dry weather, but there is still the potential for some days with drenching rain ahead.

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It is the type of pattern that may require shorts and short sleeves in the afternoons, long sleeves and jackets on certain nights and even rain gear once in a while.

Shots of cooler, less humid air to follow Florence

Following Florence's flooding rainfall, dry air has settled into part of the Northeast.

The vast majority of the region can expect rain-free weather through Friday morning with lower humidity and noticeably cooler nights, when compared to much of the month so far.

However, the tug-of-war between warm air and cool air will just be getting started.

Warmth and humidity are forecast to surge on Friday ahead of another push of cool air into the first part of the weekend.

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The leading edge of the cool air may be accompanied by showers and locally heavy thunderstorms centered on late Friday and Friday night.

Temperatures will climb well into the 70s to the middle 80s on Friday, before temperatures are then slashed by Saturday.

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"The most pronounced cooling this weekend will be from New England to the upper part of the mid-Atlantic, central Appalachians and eastern Great Lakes," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.

"Nighttime temperatures will dip to the chilly middle 30s to lower 40s across the northern tier [of this region] during Friday and Saturday nights," Pastelok said.

Along with the occasional cool days will come the risk of widespread late-night and early-morning fog. The fog is likely to be common in the river valleys, but it can occur anywhere the sky is mostly clear and winds are calm at night.

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The boundary separating the cool air across the north and steamy air to the south is likely to stall over part of the Ohio Valley and southern Virginia this weekend.

"As a storm moves eastward along that boundary, rain may spoil the second half of the weekend in the central Appalachians and mid-Atlantic," Pastelok said.

Another cycle of warmth followed by a major cooldown is likely to follow next week with rain in the offing during the transition.

Additional tropical threats, temperatures during early autumn may depend on strength of Bermuda high

"Later this month, the cooldowns are likely to be more robust over the North Central states with more of a tendency to push to the Atlantic coast," Pastelok said.

There is a chance that a persistent Bermuda high pressure area may flex its muscle once again and delay or mitigate that major widespread Atlantic coast cooldown.

It was a very strong Bermuda high that created tropical conditions during early September and allowed Florence to reach the Carolina coast this past Friday.

Such a pattern may allow another tropical feature to move up from the Caribbean and drift near the Atlantic coast around the start of October before much cooler air punches through.

Even with the waves of cool air en route to the Northeast, as temperatures are tallied for the entire month, September is likely to be one of the warmest on record for the Northeast, according to AccuWeather Senior Meteorologist Brett Anderson.

Warm and muggy nights during the first two weeks have probably tilted the scale so much that the cool shots forecast during the second half of the month are not likely to erase the overall warmth.

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