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(CNN) — Heat alerts are in effect from California to Alabama as high temperatures will be 10-15 degrees above average on Sunday.

Las Vegas, Phoenix, and Tucson will all see high temperatures of at least 110 degrees, and all three are likely to tie or break their daily record high temperatures. In Texas, cities including Dallas, San Antonio, and Lubbock will all exceed 100 degrees.

Cities in the New England are also under a heat advisory including Boston and Providence where a heat index of up to 96 degrees is anticipated.

Other hot locations include Orlando, Montgomery, Alabama, and Columbia, South Carolina, which will at hit a high temperature of at least 95 degrees Sunday. But those locations will not be under a heat advisory because the National Weather Service (NWS) uses different criteria for heat advisories in different parts of the country.

If the forecast calls for a heat index of 105 degrees in Dallas or Atlanta, the NWS will issue a heat advisory, but that’s not high enough for San Antonio, which needs a heat index of at least 108 degrees or Columbia, South Carolina, which needs at least 110 degrees before heat advisories are issued.

Desert heat is different

The criteria changes again in the Southwestern desert.

Since it’s so hot for much of the year in the Southwest, NWS offices there do not issue heat advisories for desert locations, only excessive heat warnings.

These heat warnings have a more flexible criteria than in other regions of the country. It’s called HeatRisk, and is a level-based system.

The Las Vegas office is special in that it forecasts for the highest and lowest elevation points in the contiguous US: Mount Whitney and Badwater Basin in Death Valley, respectively.

“Death Valley regularly exceeds 115 degrees in the summer but areas like Mount Whitney do not,” says Jenn Varian, meteorologist at NWS in Las Vegas. “So elevation, the type of terrain out West and even the time of year play a major role in how we issue these excessive heat warnings.”

A few southwestern NWS offices also take into account the number of tourists that come to the city and the transient population that’s unaccustomed to the extreme desert heat.

“Many tend to seek higher terrain in an attempt to escape the heat,” Varian explains. “While you’ll find relief from triple-digit temperatures, the mountains of southern Nevada are still expected to be well-above their seasonal averages as well. As a result, extra heat precautions must be taken even if you are in higher elevations or mountainous regions.”

Nighttime temperatures are also important

Saturday morning Phoenix recorded a low temperature — yes, a LOW temperature — of 94. This is a problem, because it doesn’t allow the body to successfully cool down at night. The temperature needs to drop to at least 80 degrees for recovery to begin. In fact, a person can lose up to 2 liters of fluid overnight through sweating if the temperature never drops below 85 degrees.

Over 20 locations could break record hot low temperatures Sunday through Tuesday.

Safety is key

The purpose of the advisories and warnings is to protect the public and provide guidance about the likelihood of heat-related ailments, including heat cramps, heat exhaustion, stroke and possibly death.

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