Relief from a major heat wave impacting nearly half of the U.S. is on the way, but the transition from extreme heat to cooler, more comfortable temperatures won't be easy. 

"A dramatic cooldown like this always comes at a cost, and in this case it's rain and thunderstorms with a renewed risk of flash flooding for areas that have already been hit hard during a summer full of it," warns CNN Meteorologist Mary Gilbert.

The start of meteorological summer was among the warmest in U.S. history. The country experienced its seventh-warmest June on record, boosted by the first major heat wave of the summer that hit in late June. 

The extreme heat surged northward into New England, breaking records from June 21 to 25. Boston reached 102 degrees Fahrenheit, Newark 103 degrees, and JFK saw back-to-back 102s, its first triple-digit June days. Over 107 million people were under heat alerts in 26 states during the peak of the heat.

Another major summer heat wave rolled in over the weekend and sent temperatures soaring to record levels again for many locations. Sunday's high in Tampa soared to 100 degrees, the first time the city's temperature has ever hit the triple digits in 135 years of record. The next day brought even more intensity as Tampa's heat index, or what it felt like, spiked to 120 degrees. Because that value didn't occur during an official hourly observation, it narrowly missed becoming an official record, according to WFLA's chief meteorologist Jeff Berardelli.

By some measures, this week's heat wave is worse than June's stretch of scorching heat. The heat this time around is more expansive, impacting more states. 

The same day Tampa's temperature set an all-time record for the city, there were over 162 million people across portions of at least 31 states under heat alerts. By Wednesday, the number of states enduring sweltering heat had jumped to 33, but the number of people under heat alerts dropped slightly, with almost 144 million people under heat alerts during the middle of the week.

The experimental HeatRisk Tool developed by the National Weather Service and the Centers for Disease Control and Prevention gives another perspective on how heat is impacting the country. HeatRisk is supplementary to National Weather Service heat alerts, providing "a forecast of the potential level of risk for heat-related impacts to occur over a 24-hour period." 

Over 120 million people face major to extreme HeatRisk, levels three and four out of four, respectively. Extreme HeatRisk means "health systems [are] likely to see increased demand with significant increases in ER visits," according to the NWS. 

The forecast for Friday projects that nearly 30 million people will be at the two highest levels of HeatRisk. That is still a significant number, but a huge reduction in the number of people exposed to extreme heat compared to Wednesday.

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The transition to a cooler pattern won't be easy. More than 77 million people across parts of at least 29 states are at a marginal risk, level one out of five, for severe thunderstorms Wednesday into early Thursday morning, according to the Storm Prediction Center. 

A cold front settling south out of the Northern Plains and Great Lakes will spark strong to severe thunderstorms fueled by the high levels of heat and humidity in the central and eastern U.S. Damaging winds and large hail are the most likely hazards with any severe storms that develop.

The Weather Prediction Center has outlined portions of over 30 states on Wednesday for the potential of excessive rainfall capable of producing flash flooding. The eastern slopes of the Rockies and Upper Midwest have the highest probability of torrential rainfall, facing a slight risk — level two out of four — for possible flooding.

Much of the country suffering in scorching heat will see a dramatic drop in temperatures by Friday. Computer models are suggesting temperatures will drop to as much as 10 to 20 degrees below average from North Dakota southward to Oklahoma and eastward to the Mid-Atlantic and Northeast. 

That will mean highs slipping into the 70s for a large chunk of the U.S., including major cities like Chicago, St. Louis, Detroit, Philadelphia, and New York City, which is currently enduring a major heat wave.

The relief will be welcome, but meteorologists will keep a wary eye on the forecasts for the rest of this summer. 

Our overheating planet is supercharging extreme weather events, like heat waves. The World Meteorological Organization noted an Intergovernmental Panel on Climate Change report confirmed "human-caused climate change has increased the frequency and intensity of heatwaves since the 1950s and additional warming will further increase their frequency and intensity."

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