From the ABC 7 Weather team

Dangerous Heat: Heat Advisories & Excessive Heat Warnings

June 18, 2012 - 09:07 PM
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You might be aware by now that we’re expecting some potentially dangerous heat in the D.C. region by the middle of this week. But what exactly is dangerous heat, or more specifically, how hot does it need to get in order to be considered “dangerous”?

You might be aware by now that we’re expecting some potentially dangerous heat in the D.C. region by the middle of this week. But what exactly is dangerous heat, or more specifically, how hot does it need to get in order to be considered “dangerous”? 

The answer to that question is: it all depends upon where you live.

Initially, that might make perfect sense to you. Where it’s generally hot for more of the year in cities like Phoenix, Las Vegas, and Dallas we have slightly different dangerous heat criteria than for cities where it’s generally cooler for more of the year like Detroit, Boston, and Buffalo.

So let’s start at the beginning. There are two types of official heat alerts issued by the National Weather Service: A heat advisory, and an excessive heat warning. The only difference between the heat advisory and warning is the heat index (H.I.) value. The heat index is the combination of heat and humidity that makes it feel even warmer than the ambient air temperature. 

Courtesy: NOAA/NWS


For most of the country, a heat advisory is issued when the heat index is expected to rise at or above 105°. Again, for a majority of the country an excessive heat warning is issued when the heat index is expected to be above 110°. These are the general guidelines, but for particular regions each local NWS office has its own specific criteria. Some local regions can meet heat advisory/excessive heat warning criteria too if low temperatures don’t dip below certain thresholds. Here are two examples of regional differences in heat advisory criteria.

Courtesy: NOAA/NWS


Note how you have distinct differences between the Upper-Midwest/Great Lakes/Northern Plains forecast areas, the Midwest/Central Plains regions, and the Rocky Mountain/Front Range region. For the mountains, a heat advisory is issued when the H.I. is forecast to reach above 95°! For the plains areas leading up to the mountains (known as the Front Range) the H.I. needs to reach over 100° before a heat advisory is issued. For the yellow shaded areas (Northern Plains/Upper-Midwest/Great Lakes) the heat index needs to reach 100° for a heat advisory, and for the red shaded areas (Central Plains/Midwest) a H.I. of 105° or greater needs to occur.

Courtesy: NOAA/NWS


Here’s a look at the Mid-Atlantic/Northeast region heat advisory criteria. Note the Philadelphia metro area. We’ll get to that. For the areas shaded in green, a H.I. of 100° is needed before a heat advisory will be issued. For the D.C. Metro and much of the Mid-Atlantic, a H.I. of 105° is required before our NWS office in Sterling, VA will issue a heat advisory. And for one little area of South Carolina and Georgia, the H.I. can get as high as 114° to only warrant a heat advisory! 

I’m sure you’ve noticed the Philly area denoted by the shading that refers to “Kalkstein Procedures”. Dr. Laurence Kalkstein is a professor that has developed specific procedures for certain cities all over the world that are particularly susceptible to heat related fatalities. Philadelphia is one major metro that has its own heat procedures/alerts/warnings researched by Dr. Kalkstein, that’s why you see more alerts issued for Philadelphia in relation to the D.C. region and the Northeast. 

That covers some of the regional variations for heat advisory criteria. Take the same maps and change the heat index values, and you’ve got the same areas defined yet for excessive heat warnings. Here’s the map of our region with the respective heat index values needed for a heat warning: 

Courtesy: NOAA/NWS

 

As you can see, we need a H.I. value of 110° in order for an excessive heat warning to be issued.
Our local NWS office has a webpage that defines certain severe criteria for our specific region:

Regional D.C. Severe Definitions 

Remember to be careful when we’re expecting a bout of extreme heat. If working outdoors, take frequent breaks, try to stay in the shade as much as possible, wear breathable hats that block the sun but let you expel heat from your head, wear light colored/loose fitting clothes, and drink plenty of water.

We’ll keep you up to date on the expected heat right here on WJLA.com, along with our team of ABC7 meteorologists on WTOP radio, News Channel 8, and of course ABC7 News!

 

 

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