This is the second part of our look at heat warnings, how they work, and why they matter. Last week, I broke down the heat index. If you missed that, you can catch up [here].
Today, we’re focusing on a relatively new tool from the National Weather Service: HeatRisk. We all know extreme heat can be uncomfortable, but for certain heat-sensitive groups, it can be dangerous. That’s exactly who this new alert is designed to protect.
HeatRisk is still new to many of us. In fact, we just added these maps to our weather system for the first time last year. So why do we need another way to measure extreme heat? Simple, it’s about adding another layer of protection. The heat index tells us how hot it feels, but HeatRisk considers how that heat affects public health.
I remember when I was working in Los Angeles at NBC, a brutal heat wave swept through Southern California. Temperatures soared, 105° in Los Angeles and 114° in Burbank, where I lived. You’d think those numbers would trigger a heat alert. But at the time, the National Weather Service (NWS) did not issue heat advisories or warnings for Southern California. There was no system in place for this.
That heat wave changed things. It became a turning point. The NWS turned to its experimental HeatRisk system to help communities make better decisions and issue more effective warnings. It officially launched in 2017 and has been expanding ever since.
What is HeatRisk?
As I mentioned last week, the heat index is based on just two ingredients: temperature and humidity. It gives us that familiar “feels like” number. But it doesn’t tell the whole story, especially in places where humidity isn’t the main concern.
That’s where HeatRisk comes in. Instead of focusing on humidity, it uses minimum temperatures (how cool it gets at night) as a stand-in for moisture. This makes a lot more sense for areas like the West Coast or the desert Southwest, where humidity is usually low but temperatures can soar dangerously high.
Take this week, for example. Phoenix is forecast to hit 117° on Thursday, not only would that shatter the daily record by 5°, it would also tie the hottest August day ever recorded. Meanwhile, Tucson is expected to reach 111°, just one degree shy of its all-time August record. For context, the average high in Tucson this time of year is around 100°.
In this case, the risk isn’t about humidity; it’s about heat extremes and how far outside the norm these temperatures are. That’s what HeatRisk is designed to capture. When you see deep purple on the map, that’s a signal: we’re in extreme territory.
How HeatRisk is calculated:
HeatRisk starts with the big picture: climate. Specifically, it asks how the forecasted temperatures compare to the hottest 5% of days for a given location and time of year. In other words, how far above normal is it? That’s why the alert in Arizona this week is a great example. These temperatures aren’t just hot, they’re unusually hot for early August.
Seasonality also plays a role. Is this an early-season heat wave, before your body has had a chance to adjust? If so, the risk is higher.
Then there’s duration. Is the heat sticking around for a while, or is it brief? And importantly, are temperatures cooling off at night? If they’re not, the body doesn’t get a break, and heat stress can build day after day.
HeatRisk looks at both high and low temperatures, evaluating how extreme and persistent they are. It’s not just about how hot it gets during the day; it’s about how long that stress lasts over several days.
For example, when you look at Thursday’s forecast in southeast Texas, you’ll see a small area flagged under a major heat risk. But what exactly does “major risk” mean? Keep reading to find out.
What makes HeatRisk maps stand out is that they don’t just show how hot it is; they highlight who is most vulnerable in different kinds of heat.
So, who’s at risk?
- The elderly and young children – These groups are more sensitive to heat and less able to regulate body temperature.
- The homeless – Without consistent shelter or access to cooling, they’re especially exposed.
- Those with medical conditions or on certain medications – Some medications and underlying health issues can make it harder for the body to handle heat. (If you’re unsure, check with your doctor.)
- Outdoor workers – Especially those new to the job, returning after time off, or working in high-heat settings like construction or agriculture.
- Indoor workers without air conditioning – Think warehouses or industrial settings with little ventilation.
- Athletes and outdoor enthusiasts – If you’re exercising during the heat of the day, especially if you’re not used to the conditions, haven’t hydrated properly, or are new to that level of activity, you’re more at risk.
- Anyone without reliable access to cooling or hydration – Even healthy people attending outdoor events can be at risk if they don’t recognize the early signs of heat stress.
- Newcomers to a hotter climate – Acclimatization takes time. If you’re not used to this level of heat, it can hit harder.
What the HeatRisk Levels Mean:
- Level 1 (Low) – Heat may affect those who are extremely sensitive, like the elderly or very young.
- Level 2 (Moderate) – Higher risk for people exposed to heat without good cooling or hydration—like outdoor workers or those without AC.
- Level 3 (Major) – Widespread risk for anyone without proper cooling or hydration.
- Level 4 (Extreme) – Serious impacts possible across healthcare, infrastructure, and heat-sensitive industries.
How to Use HeatRisk in Real Life:
If you’re starting a new activity, like jogging during your lunch break in August, it’s smart to watch for orange-level HeatRisk days. Early on, your body isn’t used to the heat or the activity, so consider running in the morning instead. Once you’re acclimated, you might only need to adjust plans on red-level days.
Or maybe you’re visiting the desert Southwest in April, coming from a cooler climate. Temps in the 80s or 90s might feel intense even though they’re normal for the region. Watching for orange-level days can help you plan hikes early in the day, stay hydrated, and avoid heat illness.
The beauty of the NWS HeatRisk forecast is that it’s flexible. You can tailor it to your own heat sensitivity, activity level, and situation.
We show these maps when heat waves hit Houston, but you can explore them anytime here: NWS HeatRisk
