Published September 27, 2017
Waves of cool air will continue to drop in from Canada and will keep prolonged heat waves at bay across the northeastern United States into August.
This is not the type of weather pattern would negatively affect a vacation at the beach or spoils summer sports activities.
However, it may help lower the electric bill a bit, since fans and air conditioners may not need to be on full blast every day and all night.
"A southward dip in the jet stream is likely to be a semi-permanent feature over the Great Lakes and Northeast through at least the middle of the summer," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.
The jet stream is a high-speed river of air at the level in the atmosphere where jets cruise at. This river often separates cool air to the north from warm air to the south.
The southward dip in the jet stream will allow bouts of cool air to routinely sweep through.
"There will still be some brief switches back to heat during the period," Pastelok said.
Ahead of each cool push, temperatures can spike to average or above-average levels. When the spikes of heat occur, they will be the longest-lasting over the lower Ohio Valley and coastal mid-Atlantic regions, where a heat wave or two can occur.
A heat wave in the Northern states is loosely defined as three or more days in a row with highs of 90 degrees Fahrenheit.
When and where the sun is out, the strong mid-summer sunshine can negate the cooling effects to a great extent. However, temperatures at night are expected to fall to noticeably cool levels for the time of the year.
The greatest swings in temperature and humidity levels will be across the upper Great Lakes, upstate New York and northern New England. Temperatures in these areas may fluctuate by 10 to 20 degrees Fahrenheit or more from one day to the next.
Temperatures in parts of the northern tier and the central Appalachians will occasionally dip into the lower 60s and even the 50s in some cases at night in the coming weeks.
Warm nights have been largely responsible for average temperatures of 1 to 3 degrees above normal for New England, the mid-Atlantic and Great Lakes during the first half of July.
Farther south, day-to-day changes in temperature, and especially humidity levels, are likely to be less extreme.
"It is tough to cut temperatures and humidity in the Tennessee Valley, the mid-Atlantic coast and the Southeast states in general this time of the year," Pastelok said. "However, it may not be as sticky as often as it can be, even in these locations."
For much of the nation, the next three weeks represent part of the hottest time of the year on average and have been referred to as the "dog days of summer."
In the eastern half of the nation, high temperatures are typically well into the 80s across the north to the lower to middle 90s across the south during late July and early August.
However, at times, the anticipated pattern may result in high temperatures 5 to 15 degrees lower than those levels.
Each wave of cool air will crash into moisture from the Gulf of Mexico. Each collision will likely be accompanied by a crop of showers and thunderstorms.
Any thunderstorm in the middle of the summer can become heavy and gusty at the local level. Compared to average, this type of setup can lead to a greater amount of flash flooding and severe weather incidents over large tracts of the region.
"It is possible that toward the end of July and at the start of August an even more pronounced and longer-lasting dip in the jet stream occurs in the Northeastern states," Pastelok said.
Such a pattern could result in greater cooling and possibly even stormier and wetter conditions for some areas than what is anticipated through most of July.