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September ties for second-warmest on record for Lower 48

The month ended with a blistering heat wave as the calendar flipped to October.

Five states saw their warmest September on record: Colorado, New Mexico, Texas, Louisiana, and Ohio. (NCEI/NOAA) (NCEI/NOAA)
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In the latest episode of toasty weather as our planet continues to heat up, NOAA’s National Centers for Environmental Information (NCEI) announced that September 2019 tied for the second-warmest September on record in the Lower 48. Records toppled as a stubborn heat dome baked the Southeast, bringing unrelenting scorchers that climbed into the triple digits in some spots.

And we’re not done yet. The Climate Prediction Center’s three-month temperature outlook favors anomalous warmth across the contiguous United States through the remainder of autumn.

September 2019 by the books

The average temperature across the Lower 48 was 68.5 degrees, which is 3.5 degrees above the 20th century average. According to the NCEI, September 2019 ties for second place for the warmest in a 125-year stretch of records. In terms of precipitation, the month was on par with average.

The most abnormally hot weather was in the eastern two-thirds of the nation, beneath a high pressure heat dome that delivered blistering temperatures and high humidity. Five out of 48 states recorded their warmest September on record: Ohio, Colorado, New Mexico, Texas and Louisiana. Thirty-three others were above-average. Only eight states — on the West Coast and in northern New England — remained near average during September.

In the Southeast, an exceptional dry spell beneath the dominant high pressure led to scant rainfall and drought.

Florida, Georgia, Alabama and Mississippi saw their driest Septembers on record — as did Kentucky and West Virginia. Huntsville, Ala., which averages 3.72 inches of rain in September, saw barely a quarter of an inch. Lexington, Ky., only saw a trace of rain, or raindrops that were not enough to be measured.

The dry air made it easier for the atmosphere to heat up, with the mercury hitting the century mark in a number of places. Of Alabama’s 142 weather stations, 108 of them measured their warmest September on record. Huntsville hit 100 degrees twice — a mark the city hasn’t reached so late in the year since the 1990s.

The September heat is nothing new. It’s another month in a supercharged summer that featured brutal conditions across a broad swath of the country. Atlanta hasn’t seen a below-average temperature day since Aug. 29. So far for October, aptly-nicknamed “Hotlanta” is running more than 12 degrees above normal. In fact, since the start of June, Atlanta has only recorded 20 days below average — compared to 110 above.

The setup

Some interesting trends can be gleaned from breaking down the temperature data. Notice the bull’s eye of record-setting drought over the Southeast beneath where the persistent heat dome set up. High pressure systems swirl clockwise, meaning it would likely bring a moist southerly flow over the Plains.

A big part of the story is the warm low temperature rankings, and we see that nightly lows were their highest on record for most of the central U.S. and southern Corn Belt. Nine states saw their warmest September nightly lows, but only two of those same states took the cake for their high temperatures.

Warming nightly lows are a staple of climate change, more so than daytime highs. That’s in part because of the way the atmosphere cools and “decouples” at night these days. In other words, the layer of air that gets heated by the ground is thinner. This makes warming in that layer more significant, since the heat isn’t distributed as widely.

Ongoing heat wave illustrates a hallmark of our changing climate: Warmer nights

There’s also an increase in moisture — potentially stemming from both a larger climate signal and from the persistent high pressure off to the east. High moisture content in the air makes it tougher for nighttime lows to fall as low as they would in drier air, since moisture is effective at trapping heat and acts as a baseline to how far temperatures can fall.

A map of how each state placed in terms of its precipitation for the month shows the pattern that dominated in September. North Dakota saw its wettest September on record, while Montana — a state that was largely buried by feet of snow at the end of the month — came in second place. The jet stream spent a lot of time over the northern U.S. during the month, bringing storm after storm to those regions.

An additional “hotspot” of wetter than normal can be seen where Tropical Storm Imelda dropped upward of 40 inches of rain on the Golden Triangle of southeast Texas.

Alaska and Hawaii

Places outside of the Lower 48 felt the heat, too. Alaska had its third-warmest September on record, coming on the heels of an extreme summer in the Final Frontier. As one example, Utqiagvik, the United States’ northernmost city, saw its warmest September on record as a loss of sea ice allowed the North Slope to heat up in a way it never has before. Averaged throughout the month, Utquiagvik ran nearly 9 degrees above normal.

More freak weather comes to Alaska, which has had an unprecedented summer

Hawaii, meanwhile, likely saw its warmest summer on record, as well. Lihue tied or broke record highs at least 20 days in a row. The nighttime highs were even more impressive. From 1950 to 2018, only 14 nights in Honolulu failed to drop below 80 degrees. This year has featured 19 such nights.

Inside Hawaii’s wild summer of broken high-temperature records

Think all this is coincidence? Think again. All the extreme heat fits into a pattern in which climate change is tipping the scales toward more and persistent record heat. As the world continues to warm and the atmosphere responds accordingly, events once considered extreme may become the new normal.