When might summer weather return to the northeastern US?

Much cooler and less humid air swept across the Northeast this past weekend and left many wondering where summer went. AccuWeather long range meteorologists explain when summer weather will likely return to the region.

After it got rolling in June, it seemed like heat and humidity would never take a break this summer in the northeastern United States. However, now that much cooler and less humid weather has arrived, it may stay settled over the region for a while, AccuWeather meteorologists say.

Much cooler and less humid air swept across the Northeast this past weekend in the wake of Tropical Rainstorm Debby.

"A southward dip in the jet stream will persist from the Midwest to the Northeast for the next one to two weeks," AccuWeather Lead Long-Range Meteorologist Paul Pastelok said.

The jet stream configuration tends to lock out hot air from building northward or developing in place.

"There can be brief warmups and surges of humidity, but that is usually just ahead of a new push of cool air that will soon overtake the region," Pastelok said.

Some other factors are helping the cooler pattern linger.

"First, we don't have the intense sun angle that was present from late June through July and the start of August," Pastelok said, adding, "We also have a wet ground now, and what sun's energy is out there is being used up evaporating the moisture from the ground instead of heating up the environment."

This is a time of the year when daily historical average temperatures start to tick downward every few days as the nights gradually lengthen and the period of daytime heating slowly shrinks. The end result is that it tends to get progressively more challenging for long-duration extreme heat.

So, when a long-lasting jet stream dip develops in the region, it can easily result in temperatures no higher than the historical average and occasionally below the historical average, as has been occurring.

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"There is an excellent chance this same jet stream setup with its cool conditions will last up to around the last week of August," Pastelok said, "We are expecting a typhoon to curve away from eastern Asia in a couple of days and that translates to cool weather in the Northeast from six to 10 days later."

For the middle of August, high temperatures typically range from the mid-70s in northern Maine to the upper 80s around the Chesapeake Bay. Nighttime lows generally range from the mid-50s in northern Maine to the low 70s in the major Interstate 95 cities and coastal shore communities of the mid-Atlantic region and coastal shore communities. These highs and lows are already down a few degrees from the mid-July historical averages.

Actual temperatures during July were 2-4 degrees above the historical average, making the current pattern feel even cooler.

The pattern can bring periods when highs and lows can dip 5-10 degrees below the historical average, especially across the interior Northeast. Some people who mind the cool air may reach for jackets and long sleeves rather than shorts and short sleeves.

Throw in occasional bouts of clouds and rainy conditions and temperatures can take a bigger hit.

It may feel a bit warmer near the Atlantic Seaboard, in part due to more humidity. However, there will still be some frequent intrusions of cooler and less humid air to the coast into late August.

The same pattern will create some slight window for a low-grade tropical feature to form near the Atlantic coast due to the warm waters of the Gulf Stream. However, prevailing west-to-northwest winds would tend to steer any feature out to sea. Only if the jet stream were to bend around enough might there be room for such a system to approach the coast. This sort of setup may be enough to allow Ernesto to approach Newfoundland or perhaps Nova Scotia early next week.

In order for a system, such as Ernesto, to back into the mid-Atlantic region, the jet stream dip would have to back westward, say to the Midwest rather than the Northeast, Pastelok explained.

"Toward the end of the month, the jet stream pattern may flatten out across the U.S. and that should allow less intrusions of cool air into the Northeast from central Canada," Pastelok said, "In turn, some warmer air from the Plains should be able to mix eastward."

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