In hot weather, National Weather Service issues heat-related announcements based on the forecasted conditions in an area.
There are 3 types of events that are of note:
- Heat Advisory
- Excessive Heat Watch
- Excessive Heat Warning
Here’s a simple explanation of each:
Heat Advisory
A Heat Advisory is typically issued within 12 hours of what NWS determines to be dangerous heat conditions.
These advisories are not often issued in areas that typically receive very hot weather. For instance, you’re less likely to get a Heat Advisory in Florida than you are in Pennsylvania.
Heat advisories are issued when the Heat Index is expected to be at least 100 on consecutive days and night air temperatures aren’t going to drop below 75 degrees.
That’s not a hard rule, however, and NWS can issue a Heat Advisory on varying conditions based on geography.
Excessive Heat Watch
A Heat Watch is issued when what NWS determines to be excessive heat is expected within the next 24-72 hours.
This will be issued by NWS when a heat wave is believed to be happening but its timing is uncertain.
Excessive Heat Warning
A Heat Warning will be issued by NWS when the Heat Index will be 105 or greater for at least 2 days and nighttime temps will not drop below 75 degrees.
Heat warnings are typically issued within 12 hours of these conditions. But again, these rules are not set in stone and can vary based on location.
Heat Index
This is the chart National Weather Service uses to determine the Heat Index, the basis for issuing either a Heat Advisory, a Heat Watch, or a Heat Warning.
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