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Central states and Southwest to swelter as new heat dome builds up

More than 60 million people are under heat alerts Tuesday

The American GFS model simulates the heat dome shifting west. (WeatherBell)
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The weather pattern from late May to late June was dominated by a relentless heat dome that brought record temperatures everywhere except the northwest and northeast United States. After a bit of a pause, the heat dome is back. It’s settling over the Midwest and Plains and is forecast to gradually shift westward in coming days.

The heat dome could linger in the Southwest for well over a week, meaning a prolonged period of above-normal midsummer temperatures from Texas, which experienced widespread record heat in June, through Phoenix and Las Vegas.

Summer in America is becoming hotter, longer and more dangerous

On Tuesday, heat advisories and excessive-heat warnings covered a sprawling zone from northern Louisiana to southern Minnesota and Wisconsin, affecting over 60 million people. Temperatures 10 to 15 degrees above normal are affecting much of this zone, with highs in the 90s and triple digits.

The core of the heat — with high temperatures from 100 to 105 degrees — is affecting the zone from northern Texas and Arkansas through Kansas and Missouri, including Kansas City, St. Louis, Oklahoma City, Dallas and Little Rock.

Temperatures won’t hit records in most of these cities, but the combination of elevated temperatures and tropical humidity in the air poses a danger to vulnerable populations, including older adults, outdoor workers and anyone without access to air conditioning.

“Heat related illnesses increase significantly during extreme heat and high humidity events,” wrote the National Weather Service office in St. Louis, where it could feel as hot as 110 degrees factoring in the humidity.

Daytime highs in the St. Louis metro area will crest at around 100 degrees each day through Thursday, but the nighttime lows are equally problematic. Temperatures aren’t expected to dip below 80 degrees Tuesday night and 79 degrees on Wednesday night. That lack of a real nocturnal cool-down prevents the human body from “resetting” after a day of hot weather, compounding heat stress.

Lower-income households — those that often have other vulnerabilities at play during heat events — are disproportionately likely to be without access to adequate cooling.

It will be the same story in Little Rock, where a heat advisory is in effect Tuesday and where every day through Friday should see highs within a degree either side of 100. Overnight lows will bottom out around 80.

“If you can, check up on those who may be more vulnerable or are without some form of [air conditioning],” the Weather Service in Little Rock wrote in an online discussion. “The other hazard of note [will] be overnight low temps, as we [will likely] see a few consecutive nights with temperatures only falling into the upper 70s, or even low 80s, and not allowing for any overnight relief.”

By late in the workweek, dew points — a measure of how much moisture is in the air — will lurch into the mid-70s, combining with the blistering temperatures to lead to stifling heat indexes of 110 to 115 degrees during the day.

In Dallas, every day, essentially until further notice, temperatures will feature highs in the 100-to-105- degree range and nights in the 80-to-84-degree range.

“A Heat Advisory will likely be needed across North & Central Texas Wednesday,” wrote the Weather Service office in Fort Worth.

The toasty temperatures in Texas come after the hottest May-June period on record in Houston, Austin and San Antonio — not to mention cities outside the Lone Star State, including New Orleans, Shreveport, Atlanta, Tampa and Orlando. That figure largely stems from extreme readings associated with the previous heat dome.

San Antonio had 17 days of triple-digit heat in June. The norm is two.

Heat to scorch Southwest

The heat broiling the central states is just the start, however. Over the coming week, the heat dome will move west, backtracking before putting it in park over the Four Corners region and potentially remaining anchored until at least mid- to late July.

Heat domes are composed of high-pressure systems. That translates to sinking, drying air and hot temperatures in the summertime. The high acts as a force field of sorts too, diverting jet stream energy and disturbances to the north and fending off widespread inclement weather.

While the heat may not challenge many records, it will be notable for its persistence. Phoenix will be as hot as ever, with highs above 110 degrees this weekend as the heat dome arrives. Las Vegas will see highs climb from the upper 90s to around 107.

Unlike the heat dome in June, this one will arrive at the wettest time of year across the Southwest. The monsoon looks to continue in full force, unimpeded by the stagnant ridge of high pressure that will remain parked over the region.

The addition of humidity from the monsoon has already been noteworthy over the Southwest. In Albuquerque, dew points have been running in the lower- to mid-50s, compared with an average in the lower- to mid-40s. As the clockwise-spinning heat dome continues to work west, it will entrain a tongue of additional moisture and continue to draw it north over the Southwest and Four Corners.

Most of the week will feature isolated-to-scattered heavy monsoonal downpours over the Southwest, and since the heat dome has diverted any upper-level winds around it, odds are the downpours will be especially slow-moving. That could cause isolated flood concerns, particularly over any burn scars left by wildfires in recent years.

Fortunately, most places in the Southwest desperately need the rainfall. A whopping 58 percent of New Mexico is under “severe” or top-tier “exceptional” drought, according to the U.S. Drought Monitor, and the monsoon presents a glimmer of hope in finally making a dent in the deficit.

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