While much of the US has been sweltering under above-average temperatures, a major shift in the weather is on the way, set to bring significant relief from the heat.
Forecasters predict temperatures could drop by nearly 20 degrees later this week, thanks to an expansive area of high pressure moving in from Canada.
Cooler conditions will first reach the north-central United States on Wednesday, with a gradual cooldown spreading across much of the eastern half of the country in the days that follow.
By Thursday, highs in the 70s are expected across wide portions of the Plains, Midwest, Northeast, and parts of the Ohio Valley.
Friday is shaping up to be the coolest day of the week for many in the Midwest, Ohio Valley, and Northeast.
In New York City, highs will only reach the mid-to-upper 70s, a sharp contrast to the mid-90s earlier in the week and more typical of early June than early August.
The same pattern bringing cooler air is also expected to trigger rounds of heavy rain and thunderstorms, raising the risk of flash flooding in areas already saturated from earlier summer storms.
More than 150 million people from the central US to the East Coast are currently under a Level 3 of 4 or Level 4 of 4 heat risk, according to the National Weather Service (NWS).
'Be sure to limit outdoor activity, stay hydrated, and ensure access to A/C or cooling centers. Additionally, be sure to check on vulnerable family members, friends and neighbors,' the weather service said.
By tomorrow, temperatures are expected to dip in the north-central US to parts of the Northeast.
In Chicago, for instance, highs will climb into the 90s on Tuesday but are expected to struggle to reach 80 degrees by Wednesday.
On Thursday, the high pressure system will reach parts of the Midwest, such as St Louis, Missouri that is facing above 90 temperatures.
The city is set barely reach 80 degrees on Thursday.
States in the South, such as Arkansas, could hit 100 degrees Tuesday and Wednesday, but see a dip to the mid-90s by Thursday.
Following highs near 97 degrees on Tuesday and Wednesday, Washington, DC is forecast to see a significant cooldown, with temperatures expected to reach only the upper 70s on Friday, an uncommon occurrence for August.
Typically, high temperatures in the nation's capital during this time of year remain in the 80s.
Tropical moisture is expected to surge northward, fueling persistent downpours from parts of Tennessee into southern New England.
Rainfall totals of 2 to 4 inches are likely in areas where the heaviest and most sustained rain develops later this week.
The NWS has also warned that high rainfall rates associated with widespread thunderstorms could lead to an increased risk of flash flooding.
AccuWeather meteorologist Elizabeth Danco said: 'Flash flooding will be the primary hazard.
'Drenching thunderstorms could also bring reduced visibility, ponding on roads and slowed travel. There will also be a risk of locally damaging wind gusts and lightning strikes.'
Heading into the weekend, temperatures in the East are forecast to run 5 to 10 degrees below normal, while much of the Plains will see highs 10 to 15 degrees below average, according to the NWS.
Highs in the 70s are expected to extend as far south as Kansas and the Oklahoma Panhandle.
Weather.com meteorologist Caitlin Kaiser said: 'Taking a peek even further into the first week of August, cooler-than-average temperatures are expected to stick around across much of the Great Lakes, Midwest and Northeast.
