The Washington PostDemocracy Dies in Darkness

Autumn is quickly approaching, but the Lower 48 remains entranced by summer heat

A late-summer heat dome is causing record warm temperatures and holding off fall coolness.

A pedestrian crosses a shimmering Pennsylvania Avenue as temperatures climb to the 90s on Aug. 12, in Washington, D.C. (Bill O'Leary/The Washington Post)

By the last days of August, thoughts are turning to the beautiful blue-sky days of fall, cooler temperatures, changing leaves and pumpkin spice everything. But don’t plan on autumnal bliss across the Lower 48 just yet.

Another large heat dome of cloud-squashing high pressure has brought late summer heat from coast to coast once again. About a quarter of the population in the contiguous United States was under a heat advisory or excessive heat warning on Wednesday.

Numerous heat records have already been set in this late-summer blast, which began in earnest earlier this week. Many more are likely on the way, adding to the roughly 1,300 high maximum and 3,000 high minimum records observed in the United States so far this August.

On Wednesday, temperatures running 10 to 20 degrees above normal stretched from the rolling plains of Kansas, through the bustling Chicago region, and across the urban and rural Northeast to the Mid-Atlantic. Similar levels of high heat began to expand across the intermountain West and into the Southwest.

Washington has surpassed its annual average of 90-degree days, with more hot ones ahead

Heat indexes of 105 to 110 should be common again in the central United States, with values approaching 100 from Florida to New Jersey in the east.

Records falling across a wide swath

Record highs fell in recent days across the central United States. More record highs are anticipated Wednesday in Arizona, New Mexico, Texas and from Florida to New York on the East Coast. Record temperatures may continue into Friday in the East and Southwest.

A sampling of record highs from Tuesday includes:

  • Garden City, Kan.: 106 degrees, tying 106 in 1964.
  • Dalhart, Tex.: 103 degrees, besting 101 in 2011.
  • Vicksburg, Miss.: 101 degrees, besting 98 in 1983.
  • Sidney, Neb.: 99 degrees, besting 98 in 1948.
  • Colorado Springs: 94 degrees, tying 94 in 1964.

This bout of heat is also delivering plentiful warm morning lows. Record low maximums were set Tuesday in Miami (84-tied); Monroe, La. (78); Atlanta (77); Des Moines (76-tied); and Houston (80-tied).

Another slew of record low maximums are on the way for Wednesday. Then Thursday may have the most warm lows of this stretch, with up to three dozen forecast.

Canada sets new all-time heat record of 121 degrees amid unprecedented heat wave

“Nationwide, unusually hot summer days (highs) have become more common over the last few decades. The occurrence of unusually hot summer nights (lows) has increased at an even faster rate,” notes the Environmental Prediction Center.

Eastern U.S. heat

The heat wave is currently peaking on the East Coast. The National Weather Service in New York indicated heat indexes may reach 95 to near 100 degrees. Heat advisories were issued Wednesday and Thursday for New York City area, extending west into parts of New Jersey and southern New York, and then across much of southern New England, including Boston.

Heat advisories are also in effect for the Buffalo region, as well as parts of West Virginia. In both cases, heat indexes values are expected to rise to near 100 on Wednesday and Thursday. In addition to heat concerns, air quality is poor along much of the urban Northeast Corridor. with code orange alerts up from near Washington, D.C., to around Boston.

By the weekend, some relief is a good bet as the heat dome gets squashed south. However, above-average temperatures are generally expected through the first week of September.

Southwest and Intermountain West heat

Temperatures will approach 110 degrees Wednesday in Phoenix, but it gets worse before it gets better.

The Weather Service said temperatures will rise to “between 110-113 degrees in the Phoenix area to as high as 115 degrees over the western deserts for Thursday and Friday.”

The Phoenix office calls Wednesday and Thursday a “major” threat from heat, and Friday is expected to rise to “extreme” levels of dangerous weather. A record high of 113 on Friday seems likely to be challenged or topped.

Phoenix is under an excessive heat warning from Wednesday through Friday.

Temperatures in the Southwest are typically on the way down this time of year, in part thanks to the monsoon season that delivers higher moisture and more clouds. A pause in the monsoon season is heavily responsible for this heat outburst.

The 2021 Southwest monsoon is in full force in Arizona

Higher chances of rain are expected to return to the Southwest by early next week, likely knocking readings back to a normal kind of hot.

Fall appears to be delayed

Climatological fall begins in one week. This version of the fall season covers the months of September, October and November. For the calendar purists, the autumnal equinox is on Sept. 22.

The latest three-month outlook from the NWS Climate Prediction Center indicates a high chance of unseasonable warmth continuing into and perhaps through the fall season. Their outlook, for September through November, is below.

As with much of the summer season, the greatest likelihood of above-average temperatures sits over the Southwest, the Northeast, and northern Alaska, where some of the starkest warming in climate change has happened thus far. Although other areas of the country have lower chances of above-average readings, the majority will likely be warmer than typical.

Extension of summertime warmth into fall is an expected response to climate change, one that has been more common in recent times.

According to Climate Central, in an analysis of warming fall in the United States, “Of the 242 cities analyzed, 95% (230) experienced an increase in their average fall temperatures since 1970.” They also noted that half of cities saw an increase of two or more degrees in fall temperature.

In addition to lengthening the growing season and pushing back the onset of pumpkin spice lattes, a longer warm season can mean lengthier allergy suffering, increased energy demand, and pests like ticks or mosquitoes lingering longer into the year.

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