Methodology · Updated October 22, 2026

How our data works

Everything on HeatStorms.org comes from public, freely available sources. This page explains where each number comes from, how we calculate what we show, and what each one can and can’t tell you.

“Hotter than normal” map

The map’s default layer shows how much each day’s forecast high differs from normal for that date, across the United States, Canada and Mexico. Red means hotter than normal and blue means colder; areas within about 2°F of normal are left clear.

• Grid: 529 points spaced 2.5° apart (roughly 250 km) over land in the three countries. Colors between points are smoothly interpolated, so the map shows the big picture rather than local detail.
• Forecast: 7-day daily high temperatures from Open-Meteo, refreshed every 8 hours.
• Normals: for each point and date, the average daily high within three days either side of that date, using 10 years spread evenly across 1991–2020 (1991, 1994, 1997 and so on to 2018), from Open-Meteo’s historical reanalysis archive. Sampling 10 of the 30 years keeps the job within the free data limits while staying centered on the standard 1991–2020 climate period.
• Updates: a small job on our server runs hourly, refreshing the forecast or adding normals for upcoming dates, and publishes the result for the map to read.

Because forecasts and normals both come from model data on a coarse grid, individual cities can differ from these values by several degrees, especially in mountains and along coasts. For a specific place, use the “How hot is it where you are?” check on the homepage.

Heat risk map (US)

The map shows HeatRisk, an experimental index from the National Weather Service (NWS). It rates the risk of heat-related health impacts over each 24-hour period on a scale from 0 to 4:

• 0, Little or no risk from the expected heat.
• 1, Minor: mainly affects people who are extremely sensitive to heat, especially outdoors without cooling or enough water.
• 2, Moderate: affects most people who are sensitive to heat.
• 3, Major: affects anyone without effective cooling or hydration, with likely impacts on health systems and heat-sensitive industries.
• 4, Extreme: rare and/or long-lasting extreme heat with little or no relief overnight, affecting anyone without effective cooling or hydration.

HeatRisk combines how unusual the forecast heat is for that place and time of year, how long it lasts, and whether those temperatures have been linked to higher rates of heat illness in data from the Centers for Disease Control and Prevention. NWS updates it hourly with forecasts up to seven days ahead, which is what the day buttons above the map switch between. We draw it directly from the NWS map service; we don’t modify the values.

HeatRisk is experimental and supplements, rather than replaces, official NWS heat alerts. The colors in our legend closely match NWS’s, but the map itself is the authority.

Heat alerts

The alerts section lists every active Heat Advisory, Extreme Heat Watch and Extreme Heat Warning issued by the National Weather Service, pulled live from the NWS alerts service each time the page loads. Warnings are listed first, then watches, then advisories. These are the official alerts to act on.

For Canada, we show every active heat alert from Environment and Climate Change Canada, such as Heat Warnings and Extreme Heat Warnings, pulled live from its public MSC GeoMet alerts service. Canada issues alerts for individual forecast regions, so we group them by alert type and province, and outline the affected regions on the map. Heat warning criteria in Canada vary by region, based on local daytime highs, overnight lows and humidity.

The “states and provinces affected” count combines the US states covered by NWS alert zones with the Canadian provinces and territories under an alert.

“How hot is it where you are?”

When you search for a city or ZIP code, we look up its location and fetch the 7-day forecast of daily high temperatures from Open-Meteo. We then compare each day with its normal high.

To calculate the normal, we take the daily high temperatures for that location from 1991 through 2020, the standard 30-year climate period, and average the highs within three days either side of each date. That smooths out quirks of individual days while staying true to the season. The historical temperatures come from Open-Meteo’s archive, which is built on weather reanalysis data, a reconstruction of past weather that blends observations with computer models.

Each day is then labeled by how far it sits from normal:

Reanalysis data describes an area about 10 to 25 kilometers across, not a single thermometer, so our numbers can differ from your local weather station by a few degrees, especially near coasts, mountains and cities. Your browser remembers your last search and the normals for that place, so repeat visits load faster.

Named heat storms (coming soon)

We’re building a tracker that will detect, name and follow heat storms over North America. It will be based on the same idea Reuters used when it introduced the term: find places where daily highs reach the hottest 10% for the date compared with a fixed historical period, require that heat to persist for about five days, merge neighboring hot areas into a single event, and track events that grow past a size threshold. We’ll publish our exact criteria here before the tracker launches.

Limitations

• Not an official warning service. For decisions about your safety, follow the National Weather Service, Environment and Climate Change Canada, Mexico’s Servicio Meteorológico Nacional, and local officials.
• Forecasts change. Days further out on the map and in the 7-day comparison are less certain than today.
• Coverage varies by feature. The hotter-than-normal map and the local comparison cover the US, Canada and Mexico; HeatRisk covers the contiguous United States; heat alerts cover the United States and Canada.
• Outages happen. If a data source is down, that section will say so rather than show stale numbers.

Data sources and credits

• NOAA / National Weather Service, HeatRisk (experimental).
• National Weather Service, API alerts service.
• Environment and Climate Change Canada, MSC GeoMet weather alerts (Open Government Licence – Canada).
• Weather data by Open-Meteo.com, licensed under CC BY 4.0.
• Boundaries: US Census Bureau (via us-atlas) and Natural Earth countries, Canadian provinces and Mexican states, public domain.
• Map display: Leaflet.

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