All-Time Extreme Heat Expected in Southwest U.S.

A dangerously intense heat wave will grip the Southwest U.S. this weekend and may persist through next week. The NWS has already plastered much of the region with excessive heat warnings. A strong ridge of high pressure, expected to rank among the Southwest's hottest on record at upper levels, will pave the way for this prolonged heat wave. The all-time hottest surface temperature records for Las Vegas, Phoenix, Tucson and Needles may be challenged, as temperatures soar to 115° - 125° Sunday through Thursday next week. The most intense heat is expected Monday through Wednesday, with 120° predicted by Weather Underground for Phoenix on Tuesday. Extreme heat will also extend northwest across the highly populated Central Valley of California.

According to wunderground weather historian Christopher C. Burt, Phoenix has only reached 120° three times at its official NWS site (Sky Harbor Airport):

122° (June 26, 1990)
121° (July 28, 1995)
120° (June 25, 1990)

The city's next-highest temperature was 119° on June 29, 2013. Phoenix has hit 118° eleven times, most recently on June 19, 2016, which stands as the hottest temperature ever measured so early in the summer. That record for early heat may be broken next Monday, June 19, as Weather Underground is calling for a high of 119°F.

The multi-day nature of this heat wave will add to its dangerous nature: prolonged heat waves are especially hazardous because there is less chance for people to cool down by night, thus increasing the stress on those who do not have access to air conditioning. Relatively high levels of moisture in central California will help keep nights especially warm, noted Daniel Swain in a California Weather Blog post on Friday.

The longest streak of days with temperatures of 115° or hotter in Phoenix is four days, set in 1990, and the city has a chance at matching or beating that record over the six-day period Monday through Saturday. Another city that may set a multi-day heat streak record is Needles, CA. The record 120°+ consecutive day stretch there is three days: June 28 - 30, 2013. The latest Weather Underground forecast calls for a solid week of 120°+ temperatures, Sunday through Saturday.

Here are the highest temperatures predicted by WU for the peak of next week's heat wave, compared to the hottest temperatures ever recorded at various Desert Southwest locations. The WU forecasts were retrieved at 5:00 pm EDT Friday and are valid at each city's official observing site.

Death Valley (Furnace Creek) is predicted to top out at 123°F. That's well short of its all-time high of 134°F--but Chris Burt explains that this record from 1913 is likely inaccurate and that 129°F is the highest reliably-measured temperature at Death Valley.

Longer and more intense heat waves are a key indicator of our planet's warming climate, as noted in a website from Climate Signals that puts the evolving Southwest heat wave in the context of recent observations and research.

Vehicles are no place to leave kids or pets!

Whether or not you are in a heat wave, the sunshine of summer can bring temperatures inside vehicles to deadly levels in as little as 10 to 20 minutes, even if the windows are cracked and even if temperatures outside are in the 70s. Children and pets should never be left inside an unattended vehicle--even for a few minutes!--at this time of year. Jan Null (Golden Gate Weather Services) has a comprehensive website on the heatstroke risk to children left in vehicles. More than 700 U.S. children are known to have died in this manner since 1998. As reported by Capital Weather Gang, the U.S. House of Representatives is now considering the HOT CARS Act of 2017, which would mandate that all new vehicles include a system to notify drivers if they're about to leave a child in the back seat.

See our Friday morning post for more on the two Atlantic systems that may become tropical cyclones in the next several days. We'll have a full update on these twin threats on Saturday.

Bob Henson contributed to this post.