Hot Nights Will Make This Weekend's Excessive Heat And Humidity Even More Dangerous

Parts of 19 states are under an excessive heat warning over the next couple of days as the combined effect of high heat and stifling humidity climbs to dangerous levels. The heat baking the Midwest this week will grow more intense and spread east toward the Mid-Atlantic and Northeast through the weekend. It's not the worst heat wave we've ever seen, but it'll get hot enough—and stay warm enough at night—that it could take a serious toll on vulnerable populations.

The Heat Index

The above map shows the National Weather Service's heat alerts as of about 9:30 PM on Thursday night. Excessive heat warnings cover a vast area—the widest-reaching heat wave we've seen in a couple of years—including the entire state of Ohio and several of the smaller states on the East Coast, as well as just about every major city between Kansas City and Boston.

These alerts are in place because of the heat index.

Humidity affects us on a hot day because the excess moisture in the air hampers your body's ability to cool itself off. Meteorologists and other scientists worked together to create the heat index to measure the impact that the combination of heat and humidity has on your body.

The heat index isn't made-up for clicks or ratings.

The heat index isn't a scare tactic to make people think the heat is worse than it is.

People who say stuff like that are trying to act cool and hip when they're really just waving their ignorance around for everyone to laugh at.

The heat index is a real metric that tells you what the combined heat and humidity feels like to your body. If it's 93°F with a heat index of 105°F, the moisture in the air is making your body work as hard as it would if the actual air temperature were 105°F. That means heat-related illnesses will set in even faster than you would expect given the air temperature.

Heat alerts are relative to where you live—it's harder to get an excessive heat warning in Wilmington, North Carolina, than it is in Syracuse, New York, because people in the south are used to and equipped for sustained hot and muggy weather. I wrote a post for Outside last summer explaining how each NWS office issues these alerts, complete with a map showing the heat alert criteria for almost every office east of the Rockies.

Here's the National Weather Service's forecast (as of Thursday evening) for the heat index at 5:00 PM EDT on Friday, July 19:

 ...and here's the same forecast for the same time on Saturday, July 20:

...and the maximum heat index forecast for Sunday, July 21, from the Weather Prediction Center:

(Note: The WPC issues maximum heat index forecasts between 3 and 7 days out. I tried to make my own map of their forecasts, but it...uh...didn't turn out well, so I used their map here.)

It's going to feel like the 110s again across much of the Midwest on Friday, with those heat indices moving into the I-95 corridor by Saturday and Sunday. Some models are trying to put Friday's heat index readings into the 120s across the corn fields in places like Iowa. That may be extreme—but not unheard of!—but it underscores just how darn hot and muggy it's going to get.

It won't take long to come down with a heat-related illness in this kind of weather. Folks who work outside or work-out outside are most susceptible to feeling the effects of the daytime grossness. Keep drinking water even when you're not thirsty. You lose liquid faster than you realize when it's hot out.

It's Going To Stay Unbearably Warm At Night

It's bad enough that it'll be dangerously hot and humid during the day, but the temperature at night is extremely concerning in a situation like this. The effect is most pronounced in cities and communities up north where home air conditioning isn't as common as it is down south.

I know folks in the Midwest get salty when we focus on the East Coast when a heat wave affects such a large area, but there are hundreds of thousands of elderly people and folks with health problems in cities like Washington and Baltimore and Philadelphia who will need air conditioning over the next few days but won't have access to it or simply can't afford it.

Those vulnerable populations who can't access air conditioning will be at great risk for illness over the next couple of days if they can't find relief from the heat and humidity. Heat doesn't necessarily kill people because it gets ridiculously gross during the day. A prolonged heat wave kills people because it stays unbearably warm at night, preventing folks who suffer during the day from feeling any relief at night. The stress from the heat compounds day after day until it's too much to take.

Here are the predicted low temperatures on Friday morning:

And Saturday morning:

And Sunday morning:

Some of those temperatures, especially in the Mid-Atlantic and Northeast, are hardly low temperatures at all. 82°F for the low in Washington D.C. and 81°F in New York City on Sunday morning? That's horrendous, and even worse when you account for the humidity that doesn't go away when the sun goes down.

The Heat Will Break Next Week

We're dealing with this heat wave because the jet stream is firmly wedged north of the Canadian border, creating a large ridge of high pressure over the eastern two-thirds of the United States. Ridges of high pressure during the summer are often called "heat domes" because they foster heat waves like the one we're seeing right now.


The ridge will begin to break on Sunday in the middle of the country and by Monday on the East Coast, bringing relief to those who need it the most. It's not going to get cool—hey, it's the middle of July—but we'll go from "dangerously warm" to just "miserably warm," and the latter is at least bearable.

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