When a heat wave gripped the southwest in 2011, people crowded California's Santa Monica Beach to get some relief from the blistering temperatures. Image credit: Jonathan Alcorn/AFP/Getty Images

It’s not exactly news that it gets hot in the desert during the summer, but some heat waves are so extreme that they make the people who proudly proclaim “it’s a dry heat!” head indoors for some sweet air conditioning. A dangerous heat wave will grip the southwestern United States between today, June 17, and Tuesday, June 21, with high temperatures on a couple of the days easily breaking into the top five hottest days ever recorded in many cities.

Afternoon temperatures on Monday, June 20, as forecast by the GFS weather model. Image credit: Pivotal Weather

The U.S. National Weather Service’s (NWS) official forecast on Thursday, June 16, is so miserable to look at it makes you want to eat a bowl of ice cream even if you’re thousands of miles away. The NWS in Phoenix, Arizona, predicts a high temperature there of 116°F on Sunday and 117°F on Monday. A few dozen miles to Phoenix’s southeast in Tucson, the NWS predicts a high temperature of 115°F on Sunday and 113°F on Monday. Excessive heat will also stretch into southern California and the Central Valley on Monday, with temperatures climbing above 100°F in Los Angeles, with even hotter temperatures in L.A.’s eastern suburbs. Death Valley, California, could come close to 120°F on Monday and Tuesday.

Temperatures near or above 100°F are also possible east of the Rockies in parts of the Plains states.

Phoenix probably won’t break its all-time heat record (122°F on June 26, 1990), but it could tie for fifth place if the predicted Monday high of 118°F occurs as expected. A high of 115°F in Tucson on Monday would be the third-hottest ever recorded, tying with the toasty afternoons of June 25 and June 28 back in 1994. Similar top-five or top-10 afternoons are possible across the desert and into the Los Angeles area. The periods of record for most of these cities stretch back into the early 1900s or late 1800s.

A weather model image showing the atmosphere around 18,000 feet above the ground, depicting the intense ridge of high pressure over the central and southwestern United States. Image credit: Tropical Tidbits

Why is it going to be so unusually hot? The jet stream will retreat to the border between the U.S. and Canada over the weekend, allowing an intense ridge of high pressure to build up across much of the United States for the next couple of days. The weather under a ridge is typically calm and dry due to sinking air, so not only are we talking about the desert in the middle of June, but the weather pattern is adding insult to injury in this situation. Temperatures will be the hottest when the ridge is at its most intense.

The hottest time of the year for the desert southwest is usually the middle of June through the middle of July, when the intense sunshine and calm weather conditions join forces to let the air get just about as hot as it can get. Starting in about mid-July, however, the monsoon season begins when prevailing winds begin to blow from the south and allow tropical moisture from the Pacific Ocean (and sometimes the Gulf of Mexico) to help create daily showers and thunderstorms over the desert. Temperatures still routinely crack 100°F during monsoon season, but the regular storms serve to moderate them a little bit.

This kind of heat is still dangerous even if it feels cooler because the humidity is very low. Your body cools off through evaporative cooling—the surface of your skin cools off as your sweat evaporates. This is why you have such a hard time cooling off when it’s humid outside. When it’s both hot and dry out, though, your sweat can evaporate too efficiently and you can quickly become dehydrated and fall ill from heat exhaustion or heat stroke. Make sure you take care of yourself during the worst of the heat; even if you think the heat isn’t too bad, it’s likely taking a toll on your body that you won’t realize until it’s too late.