Extreme Heat Searing the Southwest This Week;
All-Time Record-High Temperatures
May Threaten Las Vegas and Phoenix

By Chris Dolce | Weather.com | June 20, 2017

Dangerously hot temperatures are gripping the Southwest this week, potentially threatening the all-time record-high temperature in both Las Vegas and Phoenix.

A large dome of high pressure in the upper atmosphere has developed over the Southwest. Beneath the dome, sinking air is causing temperatures to soar well over 110 degrees in many areas.

This is a classic pre-monsoon heat event for the Southwest region. Some of the highest temperatures of the year can be recorded before the onset of the summer monsoon. Humidity levels are low, so the sun's energy can be used to heat the air rather than being absorbed by water vapor or used for evaporation.

Various heat alerts have been issued by the National Weather Service across Arizona, western New Mexico, southern Utah, southern Nevada and portions of California.

Southwest heat alerts

Several daily record highs were set Monday, including Death Valley, California (125 degrees), Phoenix (118 degrees tied), Tucson, Arizona (115 degrees), Palm Springs, California (119 degrees), Fresno, California (108 degrees) and Reno, Nevada (103 degrees).

The 115-degree high in Tucson, Arizona, on Monday was the third hottest temperature ever recorded in that city and only the sixth time it has been 115 degrees or hotter since the late 1800s.

Dangerous Heat This Week

Here's what can be expected from this excessive heat event.

Arizona

Southwest forecast highs, Tuesday June 20 to Thursday June 22

California

Nevada

The hot conditions this week will be particularly dangerous for vulnerable groups, such as the sick and the elderly. The NWS offered useful heat safety tips that can be incorporated into a daily routine when extreme heat sets in.

Also, remember that flights planes may be delayed or canceled in temperatures of 115 or more due to the lost of lift that planes need to fly.

Summer's Peak Heat Arrives Early in Southwest

The North American monsoon typically begins to take shape in the Southwest as we head through July. This seasonal shift in wind direction brings increased moisture, fueling more frequent showers and thunderstorms.

Average warmest day of the year across the Lower 48 states. (NOAA/NCEI)As a result, parts of the region see their hottest readings, on average, from the latter half of June into early July when the air mass is still very dry. For example, average highs in Phoenix and Tucson reach a maximum of 107 and 102 degrees, respectively, during this time.

The hottest temperatures on record in Phoenix (122 degrees, June 26, 1990) and Tucson (117 degrees, June 26, 1990) also occurred in early summer.

Record-High Temperature Recap

A number of daily record highs were set on Sunday, including Death Valley, California (124 degrees), Fresno, California (107 degrees), San Jose, California (103 degrees), and San Francisco (97 degrees airport).