Much of the area from the Great Lakes to New England and mid-Atlantic can expect a break from August heat and humidity late this week and this weekend.
The cooler, less humid air will advance southeastward in stages as two surges of Canadian air move through. The pattern will last at least a few days.
"Many people will be able to open their windows, turn off the air conditioners and let in some fresh air," according to AccuWeather Senior Meteorologist Brett Anderson.
"There should be a substantial drop in energy demand in what is typically a hot and humid part of the year," Anderson said.
The most notable change will be at night, where lows in the 60s to near 80 Fahrenheit will be replaced with lows in the 40s to the middle 60s.
As is often the case, the coolest conditions will settle over northern New England and the central Appalachians, but even areas along the Atlantic coast and the Ohio Valley can expect temperatures and humidity levels to be trimmed.
The leading batch of cool air will settle into the eastern Great Lakes and central Appalachians into Thursday morning. At the same time, the reinforcing batch of cool air will expand from the northern Plains to the western Great Lakes region.
That second push of cool air is likely to be marked by a batch of heavy, gusty and locally severe thunderstorms.
It may take until Thursday night for the cooler air to become noticeable by most people in the coastal Northeast. However, some more sensitive individuals may notice a slight reduction in humidity levels during Thursday midday and afternoon in the Interstate-95 corridor.
By Friday, less humid air will have infiltrated the entire Northeast and will retain its autumnlike feel across the Great Lakes region.
However, some showers are likely to riddle areas near the Canada border, while drenching thunderstorms fire near the boundary between the cool air settling in and hot, humid conditions farther south over the Plains and Tennessee Valley.
Since the sun is still strong during much of August, it will allow temperatures to climb to within a few degrees of average during the afternoon hours.
Typical highs during the middle of August range from the middle to upper 70s over the northern tier of the Midwest and Northeast to near 90 over the lower Ohio Valley and the Chesapeake Bay region.
The weather will still be warm enough to swim in the ocean, most lakes and pools as water temperatures tend to change much more slowly than the air temperature. However, some upwelling caused by area breezes may produce pockets of chilly water for a day or two.
People camping this weekend may need to bring along a blanket and a jacket for the cool nights and mornings.
However, the Canadian air mass will retain its identity at night in the form of a quick cooldown during the evening and a mid-September-like temperatures during the nighttime and start of the day.
During the pit of the cool spell, lows are forecast to range from the middle 40s over the mountains of northern New England to the middle 60s over the Ohio Valley and the upper 60s near the mouth of the Chesapeake Bay.
"Temperatures are forecast to trend upward next week over the Midwest and Northeast as a west-to-east jet stream configuration develops," according to AccuWeather Long-Range Meteorologist Max Vido.
This pattern will keep cool air pent up over Canada and will allow the sun to warm the air over much of the nation.
Likewise, extreme heat building up over the southern Plains may not have an easy ride to the Midwest and Northeast, at least not right away.
"Even so, widespread 90-degree temperatures may return to the Ohio Valley and Interstate 95 mid-Atlantic corridor," Vido said.
There may be one more brief cooldown early next week, before the warmth builds.
"As the pattern evolves, temperatures are likely to trend to and above average near and after the middle of the month with the point that normal temperatures begin their downward trend," Vido added.