Following a surge of heat, high humidity and thunderstorms, bursts of cooler and less humid air will sweep across the midwestern and northeastern United States this week.
High temperatures in the 80s to near 90 F and AccuWeather RealFeel® Temperatures well into the 90s will be replaced by highs in the 60s and 70s across the Great Lakes region and the 70s to lower 80s in the Ohio Valley and coastal Northeast by the middle of this week.
The cooler air will waste no time plunging southward from Canada over the Upper Midwest.
A stiff breeze will help to sweep the cool air from the Dakotas, Minnesota, Wisconsin and the Upper Peninsula of Michigan during Monday night to the Lower Peninsula of Michigan, Iowa and the northern portions of Illinois, Indiana and Ohio on Tuesday.
During Tuesday night and Wednesday, cooler and less humid air will ease into the Ohio Valley and coastal mid-Atlantic but will be more noticeable across the central Appalachians and New England.
"Additional waves of cool air are likely to sweep into the Midwest and Northeast later this week and perhaps into the end of August," according to AccuWeather Meteorologist Max Vido.
The reason for the retreat of the heat and a return of cool waves will be due to the position of the jet stream.
The jet stream is a high speed river of air at the level where jets cruise. This river tends to separate cool air to its north from warm air to its south. So when the jet stream plunges southward, cool air is able to invade areas well to the south.
"At the start of this week, the jet stream was generally west to east across the northern tier of the U.S.," Vido said. "The pattern allowed the most recent surge of heat."
To the disappointment of summer weather enthusiasts, the heat is being truncated.
"The latest indications are that a similar jet stream block will return and may be in no hurry to leave," Vido said.
The most persistent cool air will be centered from the Great Lakes to the Appalachians.
Some spikes of warmth can occur ahead of each push of cool air. The greatest warming during these episodes is likely to be along the Atlantic Seaboard.
"Following a double dose of cool air this week, the cool push next week may not be as extreme with more Pacific air and less Canadian air to work with," Vido said.
"Since it will already be cool to start with next week, there may not be much difference in temperature across much of the Midwest and interior Northeast from later this week through next week."
It is still possible the blocking pattern may break down during September.
If so, the new jet stream pattern may allow some episodes of long-lasting warmth, relative to average for the time of the year, especially along the Atlantic Seaboard.
If the blocking pattern lingers or is replaced by something similar during September, then the heat from early this week could represent the highest temperature mark until next spring or summer in some locations, especially in the Midwest.