Another round of dangerous heat arrives during a dangerously hot summer

An excessively hot summer across the U.S. rolls on this week as another stretch of dangerously hot temperatures spreads from I-5 to I-95.

Temperatures could easily crest the century mark for many locations over the next couple of days as broad ridges of high pressure rule the roost to start the week.

Tuesday will feature the worst temperatures, with 100-degree readings expected in Oklahoma City, Nashville, Washington, and Philadelphia. The high heat will stretch all the way into New England, where daytime highs in the 90s will reach into Maine.

Humidity will make the heat exceptionally dangerous in many of these locations, with heat indices exceeding 105 for many communities. 

The National Weather Service's new HeatRisk product shows widespread "major" to "extreme" impacts from this latest bout of high heat.

Folks living with chronic health conditions, working outside, or living without air conditioning will feel the greatest impacts from this latest heat wave. Heat exhaustion or heat stroke can develop in under an hour with these conditions.

Very warm and humid nights won't provide much relief from the blazing daytime temperatures. The compounding effects of hot days and steamy nights will make this especially tough for folks without adequate cooling at home. Fans alone won't be enough to combat this heat.

This is the latest volley of blistering temperatures during an already-hot summer across almost all of the United States.

Source: IEM

Data collected by the Iowa Environmental Mesonet shows that just about everyone save for the northern Plains and Upper Midwest has dealt with above-average temperatures so far this season. Much of the excessive heat has been driven by warmer-than-normal nighttime low temperatures, a side effect of the increased humidity we've seen this season.

Source: IEM

Folks across the Southwest have taken the brunt of the extreme heat this year, with week after week of brutally hot temperatures roasting the region. Phoenix has seen above-average temperatures on 163 of the 195 days we've trudged through so far in 2024. The last time they saw a below-average day was at the beginning of May.

Source: IEM

It's not just the Southwest dealing with the heat. It's a similar story back east. Washington, D.C., has seen above-average temperatures for 78 percent of the year through Saturday, July 13, and we're about to add another week of excessive heat to those grim statistics.

The relentless heat we've seen so far this year is exactly what you'd expect to see in a changing climate. Temperatures have steadily risen each decade across just about the entire United States—and we're even outpacing the new climate standards that run from 1991 to 2020. 

Source: Climate Central

Climate change sets a new baseline for extreme heat throughout the United States and around the world. As the entire frame of reference moves toward a hotter climate, warm temperature extremes are far more likely that cold temperature extremes. Excessive heat will come in hotter than what we grew used to just one or two generations ago.

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NWS Unveils New 'HeatRisk' Forecasts to Highlight Dangers of Extreme Heat

Heat kills more Americans every year than tornadoes, hurricanes, lightning, and flash floods combined. A new product released by the National Weather Service aims to cut down the tremendous number of heat-related illnesses and fatalities we see across the country every year.

HeatRisk is the culmination of a joint effort by the NWS and health experts with the Centers for Disease Control to increase awareness of the dangers of extreme heat throughout the U.S.

The new product is similar to the Winter Storm Severity Index in that it uses a five-category scale to relay the dangers posed by conditions on a particular day.

According to the NWS, forecasters will consider factors like:

• The time of year
• How far above normal temperatures are for that time of year
• How long the unusual heat will stick around
• Temperatures reaching thresholds known to cause heat-related illnesses

Those are important considerations when it comes to heat safety. A heat wave in May might warrant a higher rating as it may deliver a greater "shock to the system" (so to speak) than similar heat in August. Time, place, and duration all play a major role as well. A week of 90°F readings in Florida might not affect residents as much as a week of 90°F readings in Vermont.

HeatRisk will show up in products and on maps using a five-category scale ranging from 0 to 4, with "little to no risk" on the low end to "extreme impacts" on the high end. These categories are designed to quickly convey the severity of heat in the forecast for any particular area.

If you're in the red for "major impacts," for instance, it's going to be hot enough that fans won't effectively cool down indoor spaces, and anyone outdoors in the heat of the day—even healthy individuals—are at risk for heat-related illnesses.

Why this standardized scale matters

Excessive heat is a silent killer. A widespread heat wave can kill dozens and even hundreds of people without ever making the news. Elderly people, folks living with chronic conditions, and people working and living outdoors routinely succumb to heat exhaustion and life-threatening illnesses like heat stroke.

We're bombarded by alerts for tornadoes, hailstorms, hurricanes, and even wildfires. But heat—a deadlier threat than them all—routinely gets shrugged off. It's just summer, after all, what's the big deal?

This kind of a product is a long time coming. For years, we've relied on the heat index and the wet bulb globe temperature (WBGT) to measure the impacts of heat on the human body.

Meteorologists and medical scientists worked together to create the heat index to measure the combined impact of temperature and humidity on the human body. The NWS uses the heat index as a metric to issue heat advisories and excessive heat warnings, the thresholds for which are higher in areas like Phoenix or Miami than they are in cooler climates like Boston or Seattle. 

The wet bulb globe temperature (WBGT) goes a step further by factoring in more than just the temperature and humidity, including parameters like cloud cover, winds, and the sun angle to more accurately judge heat stress on the human body. The WBGT is especially useful for keeping athletes safe on hot days. 

Those two metrics often lead to confusion for the general public. Making matters worse is the widely believed lie that the heat index is "made up" for ratings (oh brother).

HeatRisk seeks to cast aside those ignorable digits in favor of an easy-to-understand scale that can help everyday folks use the forecast to stay safe, with the added benefit of alerting local officials and hospitals that they may need to provide relief and assistance to those who need it the most.

Right now, HeatRisk forecasts are still in the "experimental" stage. The NWS often introduces new products like this as experimental services that aren't used in everyday forecasting and decision-making.

You can provide feedback on the HeatRisk program directly to the NWS, and they'll use all the comments, concerns, and suggestions they receive to tweak the product before making it an official service.

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Records shattered as historic heat continues gripping desert southwest

The southwestern United States is firmly in the grips of a historic heat wave that's bringing the region not only some of their most extreme temperatures on record, but some of the highest heat the weather is capable of producing.

The National Weather Service office in Phoenix, Arizona, didn't mince words last Friday.

We are still anticipating this current heat wave to continue through next week and likely beyond with it rivaling some of the worst heat waves this area has ever seen.

[...] this should go down as one of the longest, if not the longest duration heat wave this area has ever seen.

That's no small feat coming from a part of the country where the average high temperature in July soars above the century mark.

Unfortunately, the dire predictions are panning out as we head into the weekend.

A near-record ridge of high pressure parked over the region is cranking the heat far above normal levels. Ridges of high pressure foster sinking air, which warms up and dries out as it sinks toward the ground. The result is a terrible streak of very high temperatures that's approaching the upper bounds of what we've ever measured in the desert southwest.

El Paso, Texas, is in the midst of its longest streak of triple-digit days on record. As of Friday, July 13, the city has seen 29 consecutive days with a high temperature of 100°F or hotter, shattering the previous record of 23 days set back in July 1994. The record will keep on growing through next week.

Phoenix, Arizona, tied its second-hottest low temperature on record on Thursday when the city's temperature bottomed out at 95°F early in the morning. Friday marked their 15th consecutive day with a high temperature of 110°F or hotter, and they'll easily beat their all-time record of 18 days with supercentenarian highs by early next week.  

The low temperature in Death Valley, California, will only dip to a cool 100°F on Sunday morning.

The world's deadliest weather disaster isn't hurricanes, or tornadoes, or floods—it's extreme heat. Brushing off excessively high temperatures is easy and tempting from an air conditioned office, but consider how many people here at home and around the world lack simple things like air conditioning or clean water.

I wrote about the phenomenon of folks brushing off extreme heat a few years ago:

It's called survivorship bias. Lots and lots of people died before air conditioning as a direct result of not having air conditioning.

It's sort of like the folks who scream "why do we need to coddle kids with all these safety features, I grew up just fine!" Sure, you may have turned out okay! But cemeteries are too full of too many little kids who, it turns out, couldn't get by without car seats or vaccines or wall-fastened dressers or unleaded paint on the windowsill. 

Even with air conditioning all over the place today, heat is still the leading weather-related cause of death in the United States. One bad heat wave can kill hundreds of people, a higher toll than years and years of tragic tornadoes combined. Last year's awful heat wave in the Pacific Northwest was Washington's deadliest weather disaster on record, and it killed nearly 600 people up in British Columbia.

Lots of people died before air conditioning. Lots of people still die without air conditioning. Consider yourself fortunate if you don't have to worry about that.

People are acclimated to high heat in this part of the country, of course, but no humans are capable of withstanding day after day, week after week, of extreme heat of this caliber. Without access to air conditioning, shade, and proper hydration, the human body starts shutting down after such prolonged exposure to extreme heat. Similar heat waves have killed dozens or hundreds of people in past decades.

[Satellite image via NOAA]

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