A massive heat wave that has set scores of temperature records from Texas to California is swelling into the eastern United States. Over 100 million Americans from the Gulf Coast to the Great Lakes are under heat alerts through the middle of the week as temperatures soar toward the triple digits.
Oppressive humidity levels will make it feel 5 to 15 degrees hotter, producing heat index values from 100 to 115 degrees over a large swath of the central and eastern Lower 48.
Heat advisories or excessive-heat watches and warnings cover the entirety of Arkansas, Mississippi, Kentucky, Missouri, Iowa, Illinois and Indiana and parts of more than a dozen other states.
The National Weather Service forecasts that temperatures could challenge records in more than 100 cities through Wednesday, from Denver to Charleston, S.C.
The heat is projected to be most prolonged and intense in the middle of the country.
Relentless heat is forecast in St. Louis, where the mercury is predicted to hit at least 100 each of these days — with heat index values of up to 113. It is under an excessive-heat warning for “dangerously hot conditions,” according to the Weather Service.
The sultry air is simultaneously fueling the risk for severe thunderstorms along the northern periphery of the heat wave. The Weather Service is carefully monitoring the potential for the development of a violent complex of storms that could sweep from the Upper Midwest into the Ohio Valley and the Mid-Atlantic on Monday afternoon into early Tuesday.
Forecasts into next week call for the punishing heat wave to persist over the central states. Heat waves like this are typical staples of summer, but their impacts are made more severe and prolonged by human-caused climate change.
Where the heat is now, where it’s headed and how long it could last
The excessive heat is the result of an intense and sprawling zone of high pressure, sometimes referred to as a heat dome.
The dome, centered over the Southwest on Saturday, has shifted east. On Monday, it was hovering over the lower Mississippi Valley, placing much of the eastern half of the country, aside from the Northeast, in its crosshairs. By Wednesday, it will shuffle toward Nashville before oozing west again.
On Monday, readings above 100 degrees are forecast for most of Texas, with upper 90s from the Corn Belt all the way east to the Carolinas. Record-challenging highs near 100 are forecast in Denver, Dallas, Omaha, Memphis and Charlotte, among many other locations.
Temperatures in the upper 90s to near 100 could make it as far north as Minneapolis on Tuesday, with 98 in Atlanta, 97 in Chicago and 101 degrees in Raleigh, N.C. Columbia, S.C., could hit a whopping 102 degrees.
The combination of heat and humidity in Charlotte on Tuesday and Wednesday — producing heat index values near 110 — may be the most intense there since 2010.
On Wednesday, temperatures in the upper 90s will be ubiquitous from the central Plains and Texas through the eastern Great Lakes, Midwest and interior Mid-Atlantic and Appalachians, as well as the Southeast. Records could threaten the zone from roughly Flint, Mich., through Columbus, Ohio, and Knoxville, Tenn., to Atlanta.
The heat isn’t going anywhere anytime soon. It may shift west a bit and consolidate over the central states late this week into the weekend. The Weather Service’s Climate Prediction Center projects a continuation of above-normal temperatures in the central states over the next two weeks.
Heat fuels storm threat
Along the northern periphery of the heat dome, where the sweltering heat meets cooler air, the resulting temperature contrast is anticipated to brew severe thunderstorms. Storms were already apparent midday Monday in the Upper Midwest and Great Lakes.
Concern was growing that a bow echo, or curved squall line capable of producing damaging straight-line winds, would organize and propagate south and east Monday afternoon and night through the Upper Midwest and Ohio Valley. The Weather Service’s Storm Prediction Center placed the zone from Wisconsin to northern West Virginia at the greatest risk from this possible thunderstorm complex, or mesoscale convective system (MCS).
“It appears possible that a long-lived bowing MCS could result in a swath of considerable wind damage along this corridor,” wrote the Storm Prediction Center, which also cautioned that large hail and a few tornadoes are possible. Cities in the elevated-risk corridor include Chicago, Milwaukee, Detroit, Indianapolis, Columbus, Cleveland and Pittsburgh.
There’s an outside chance the MCS meets the criteria of a derecho, which is a fast-moving, extensive, long-lived and violent thunderstorm complex.
It’s unclear if the storm complex will survive its trip over the Appalachians on Monday night, and what the repercussions would be for Washington and Baltimore, but it’s worth watching the area in the late overnight into the early morning Tuesday.
Afterward, another such complex could develop over the Upper Midwest or Great Lakes and shift into the interior Northeast late Wednesday into Thursday.
Records set so far
As the heat spread over the zone from Texas to California’s Central Valley late last week into the weekend, it set a slew of records.
Phoenix nabbed a trio of daily high-temperature records in a row — 113 degrees on Friday, 114 on Saturday and 112 on Sunday. The average high there this time of year is about 105 degrees.
On Friday, the morning low in Phoenix didn’t dip below 87 degrees, meaning that, when factored in with the afternoon high of 113, the day’s average temperature was 100 degrees. That’s the earliest triple-digit daily average temperature on record in Phoenix.
Las Vegas hit 109 on Friday and Saturday, tying or breaking records, and Salt Lake City made it to 102 on Saturday and 103 on Sunday. Those also broke records.
Denver soared to 100 on Saturday, a tie for its earliest instance of a record touching the century mark.
In Texas, sweltering days and sultry nights set dozens of records. Dallas saw a morning low of 80 degrees on Sunday, a record-warm minimum temperature. Elevated overnight temperatures often play an even greater role than daytime highs in amplifying heat stress on the body and contributing to heat-related illnesses and fatalities in vulnerable populations. Dallas then hit a record high of 103 degrees Sunday afternoon.
Abilene, Tex., saw three daily records in a row — 102 degrees on Friday, 108 on Saturday and 109 by Sunday. San Antonio also tied or broke records those days, at 101, 104 and 105 degrees, respectively, as did Austin, at 101 on Thursday, 103 on Friday, 104 on Saturday and 105 by Sunday.
In Houston and Galveston, Tex., the heat has been overlapping with oppressive humidity, contributing to heat indexes in the 105-to-110-degree range.
It could be worse, however — southeast Oklahoma saw heat index values in the incredible 120-degree range Saturday. They were due to air temperatures around 105 combined with dew points, a measure of humidity, in the upper 70s to near 80.