Excessive Heat Warning
An NWS bulletin issued when forecast heat index will reach 105 to 110 F (regional threshold) for 2+ days, signaling severe risk.
Excessive Heat Warning is the NWS's highest heat alert level. It is issued when forecast heat is severe enough to threaten health for the general population, not just vulnerable groups. Thresholds vary regionally: HI 105 F sustained 2+ hours per day for 2+ days in much of the US, HI 110 F in the desert Southwest where populations are more acclimatized.
The watch-warning hierarchy:
- Excessive Heat Outlook: a forecast 3-7 days out indicating potential heat risk.
- Excessive Heat Watch: heat is possible in 24-72 hours, conditions favorable.
- Excessive Heat Warning: heat is imminent or occurring in next 12-36 hours, life-threatening.
The recommended actions intensify: avoid outdoor activity entirely between 11 AM and 6 PM, seek air-conditioned space (not just shade), shower or bathe in cool water multiple times per day, and check on neighbors more frequently. Most US municipalities open additional cooling centers, extend hours of existing ones, and activate emergency public messaging.
Excessive Heat Warning triggers OSHA's most stringent mandatory work-rest cycles for outdoor labor, school district closures in some states, and elective surgery rescheduling in hospitals without robust HVAC.