June 14, 2021

Phoenix Could See One Of Its Hottest Streaks On Record As Extreme Heat Builds Across West

A significant heat wave will build across much of the western United States this week, bringing record-threatening temperatures to a region that could stand a break from the relentless warmth and dryness. Excessive heat warnings in place for communities like Phoenix, Arizona, which could be on the cusp of one of their hottest stretches ever recorded. 

The Setup

Source: Tropical Tidbits

A steep upper-level ridge will build over the western United States and Canada, allowing strong high pressure to dominate the region's weather through next week. Air sinks beneath a ridge, warming up and drying out as it falls toward the surface. This pattern will allow the hot summer sun to push afternoon temperatures toward the top of the records, especially in the Southwest. 

The ridge will grow strongest by mid-week—potentially leading to triple-digit high temperatures as far north as the Canadian Prairies—before beginning to weaken later in the week.

The animation above (from Tropical Tidbits) shows the upper-level ridge on the Monday morning run of the GFS model. The map depicts the 500 millibar level of the atmosphere, which is usually around 20,000 to 25,000 feet above sea level, giving us a great view of the ridge over the west.

The Heat

Beginning on Monday, forecasters expect the high temperature in Phoenix, Arizona, to push 115°F and hit or exceed that mark every day through Friday. Triple-digit readings are likely in and around Las Vegas, Nevada, during the same period, while the interior suburbs of Los Angeles and San Diego could push 100°F for several days during the first half of next week. Highs could peak near 110°F in California's Central Valley toward the end of the week. Record-breaking heat will even stretch as far north as Utah and western Colorado.

An excessive heat warning is already in effect for a significant portion of the Southwest for the next week, while a slate of heat alerts will progressively cover more communities across the western United States in the coming days.

It's a "dry heat," of course, but that doesn't matter much when temperatures are this darn hot. The low temperature in Phoenix will sit close to 90°F for a couple of days this week. When it's in the upper 110s during the day and near 90 at night, you're not cooling off that much!

The heat index tells you what the outdoor temperature feels like to your body when you factor in the humidity. (Humid air prevents sweat from evaporating, which limits your ability to cool off efficiently.) If it's 90°F with a heat index of 105°F, the heat is hitting your body as hard as an actual air temperature of 105°F even though it's much cooler.

The actual air temperature in much of the west is going to be hotter than the highest heat index you'll ever encounter in the southeast or central states.

This is a brutal, uncompromising heat that's tough for anyone to handle. The healthiest, most summer-hardy person is liable to fall out if they're not careful in these temperatures. This is going to be an awful stretch of weather for anyone who can't get relief from the elements, especially folks who are low-income, elderly, work outdoors, or those who live with illnesses that make heat tough to handle.

The Records

Extreme heat itself isn't rare in the Southwest. Phoenix, Arizona, has recorded a high temperature of 115°F at least once in 15 of the last 16 years, while temperatures of 110°F or greater are a yearly occurrence in Las Vegas, Nevada. 

What is unusual, however, is the timing and duration of this heat wave. It's early in the season for such hot temperatures in the Southwest. Phoenix doesn't typically record its first 115°F reading until the first week of July, while the end of June marks the average first appearance of a high temperature of 110°F in Las Vegas.

The duration is also what's really going to take a toll. Phoenix's longest streak of days with a high temperature at or above 115°F was 4 days. The NWS's current forecast calls for five days of afternoons hitting that mark, every day from Monday through Friday, which would make this the hottest stretch the city's recorded in living memory in terms of afternoon highs.

The Drought

Extreme heat baking areas experiencing an extreme drought is terrible news for folks who live in fire-prone areas. Last week's update of the U.S. Drought Monitor found that week-over-week drought conditions stayed the same or worsened west of the Rockies, and that's probably going to be the story over the next few months as hot temperatures and little rain exacerbate damage to the parched land.

The combination of widespread drought and persistent above-average temperatures has experts terribly worried about this year's wildfire season in the western states. Fire activity is already above-average for this point in the year and the region still has a long, hot stretch to get through before hoping for beneficial rainfall from the Southwest's midsummer monsoon and California's wet season in the fall.

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I contribute to The Weather Network as a digital writer, and I've written for Forbes, the Washington Post's Capital Weather Gang, Popular Science, Mental Floss, and Gawker's The Vane. My latest book, The Skies Above, is now available. My first book, The Extreme Weather Survival Manual, arrived in October 2015.