By Monday morning, the National Weather Service had a Heat Advisory posted for the eastern San Fernando Valley and downtown Los Angeles, another covering the high valleys around Idaho’s Bear Lake Range and Caribou Range, and a stack of similar alerts stretching toward the Northeast. The wording on all of them looks almost identical — until you notice that some say “Advisory” and others say “Warning,” and that single word is doing more work than most people give it credit for.
The National Weather Service issues three tiers of heat alerts, not one, and each tier carries a different temperature threshold and a different set of instructions. Treating an advisory the same as a warning, or shrugging off either because “it’s just a hot day,” is exactly the mistake heat-safety officials say drives emergency room visits every summer. The Centers for Disease Control and Prevention recorded 1,714 heat-related deaths in 2022, the highest total in more than two decades, according to a report on the alert system from Tampa public radio station WMNF.
The Heat Index Is the Real Trigger, Not the Thermometer
None of these alerts are keyed to the plain air temperature on its own. The National Weather Service works off the heat index — the “feels like” number that combines temperature and humidity — because that combination, not the thermometer reading, is what actually strains the human body. In the New York City area, CBS News New York reports the local forecast office issues a Heat Advisory when the heat index is expected to hit 95 to 99 degrees, or 100 to 104 degrees, for at least two days running. Push past that into the 105-degree range and the office moves to an excessive heat watch or warning instead.
A heat wave, by the same NWS definition CBS cites, is simply three or more consecutive days with highs at or above 90 degrees. New York sees about two of those a summer, averaging four days each. What changes the danger level isn’t the wave itself but how humid it gets inside it.
Thresholds Change Depending on Where You Live
Here’s the part that trips people up: there is no single number that means “advisory” everywhere in the country. Each of the National Weather Service’s roughly 120 local forecast offices sets its own thresholds based on what’s normal — and dangerous — for that climate.
| Region | Heat Advisory threshold | Next tier up |
|---|---|---|
| New York City area | Heat index 95–99°F, or 100–104°F for 2+ days | Excessive heat watch at 105°F+ |
| Tampa & Orlando, Fla. | Heat index above 108°F | Excessive heat warning at 113°F+ |
| Miami-Dade & Broward, Fla. | Heat index above 105°F for 2 hours | Excessive heat warning at 110°F+ |
| Los Angeles basin | Highs of 85–98°F | Extreme heat watch at 90–108°F |
| Eastern Idaho highlands | Upper 90s to around 100°F | Extreme heat warning issued separately at higher elevations |
That regional variation is why a Los Angeles County resident and a Miami-Dade resident can both be told “Heat Advisory” on the same afternoon under very different numbers. A body acclimated to Southern California’s dry 90s reacts differently than one used to South Florida’s tropical humidity, and the local NWS office calibrates for that. Earlier this month, a Heat Advisory covering Orlando and central Florida reflected exactly that Florida-specific threshold, while a separate advisory tracked a heat wave building over the Rockies that threatened to break all-time record highs under a completely different set of criteria.
Advisory Now, Warning Next: How Fast the Alerts Escalate
The Los Angeles alert issued Sunday shows how quickly the tier can shift. The Heat Advisory covers Sunday morning through Tuesday morning across downtown L.A., the San Fernando and San Gabriel valleys, and Santa Clarita. Right behind it, the forecast office has already posted an Extreme Heat Watch for Tuesday morning through Thursday evening, with monsoonal moisture expected to push the discomfort higher as the week goes on.
“There is a high risk for dangerous heat illness for anyone, especially for the very young, the very old, those without air conditioning, and those active outdoors.”
National Weather Service, Los Angeles/Oxnard forecast office
Meanwhile in eastern Idaho, the NWS Pocatello office posted a shorter-fuse advisory: upper 90s to near 100 degrees across the Challis-Pahsimeroi valleys, Lost River Valley and Bear River Range, running out at 9 p.m. Monday. No two of these alerts share an exact timeline, because no two heat events unfold the same way.
What You’re Actually Supposed to Do When One Is Posted
The instructions attached to both tiers are blunt, and they’re the same core list whether the header says “advisory” or “warning”: drink fluids before you’re thirsty, stay in air conditioning when you can, avoid strenuous outdoor work in the hottest part of the day, and check on relatives or neighbors who don’t have reliable AC. The Los Angeles bulletin adds one line that shows up in nearly every NWS heat product for a reason: never leave children or pets in a parked car, because interior temperatures reach lethal levels within minutes, even with the windows cracked.
In New York City, heat is blamed for roughly 370 deaths a year, and the toll skews heavily toward neighborhoods with less tree cover and less air conditioning — the South Bronx, upper Manhattan and central Brooklyn among them, per CBS’s reporting. Age matters more than geography, though: older adults remain the highest-risk group in every jurisdiction that tracks the numbers, advisory or warning alike.
For the high valleys around Island Park, Idaho, the current advisory is due to lift at 9 p.m. Monday. The one covering greater Los Angeles doesn’t ease until Tuesday morning — and by then, an Extreme Heat Watch is scheduled to take its place through Thursday evening. The label on the bulletin will keep changing through the week. The instructions underneath it, mostly, will not.
