Sioux City, Iowa, topped a ranking of U.S. cities for the most abnormal October heat

The Missouri River town of Sioux City, Iowa, spent October 2026 running 9.0°F warmer than usual, and no other American city in a national comparison ran hotter against its own normal. Climate Central, the nonprofit research group, put Sioux City first among 192 cities it analyzed. A monthly average that far above the 1991-2020 baseline is the sort of departure that usually belongs to a heat wave in July, not a month that is supposed to cool.

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The ranking is a measure of how unusual the month was, not of how hot it was in absolute terms: Sioux City’s October average was 59.3°F, cooler than several cities lower on the list. Its neighbors in the Plains and Upper Midwest packed the rest of the top ten, and Sioux City’s own warming trend since 1970, 1.0°F in the report’s table, is the smallest in that group, so the October gap owes more to the weather of the month than to long-run drift at that one station.

The ranking, city by city

Climate Central’s monthly attribution overview for October 2026, published Nov. 3, ranks cities by the gap between their October average temperature and their 1991-2020 normal, using NOAA daily station data. Sioux City led at 9.0°F, with Cedar Rapids, Iowa, a tenth of a degree behind at 8.9°F and Des Moines at 8.6°F. Minneapolis and Quincy, Illinois, tied at 8.3°F. Cheyenne, Wyoming; Lincoln, Nebraska; and Denver followed at 8.1°F each, and Springfield, Missouri, and Omaha, Nebraska, closed out the top ten at 8.0°F.

All five of the top cities sit in Iowa, Minnesota or Illinois, which is why the report singles out the Upper Midwest as the hottest of the nine NOAA climate regions, more than 7.3°F above normal. The margins between ranks are tiny. A tenth of a degree separates first from second, so Sioux City won a close contest among a cluster of cities that were all running hot.

Scale of the warmth across 192 stations

Across the whole analysis the average anomaly was 3.5°F. Of the cities analyzed, 188 finished warmer than average and five cooler, and 33 of them recorded one of their five hottest Octobers on record, in station series that average back to 1893. The report body gives the analysis as 192 cities, though one bullet in its second section says 194 locations; the page does not reconcile the two, and the 188-plus-five count lands on neither.

Nearby National Weather Service numbers fit the same picture. The Chicago office’s October summary had O’Hare at 60.8°F, 6.8 degrees above normal and tied for sixth-warmest October on record, with a record 88°F on Oct. 12. Chicago, in other words, ran very warm and still sat outside Climate Central’s top ten. The comparison shows how widely the warmth spread: a month that put O’Hare in the record books for October did not even reach the leaderboard once the Plains cities were counted.

The Climate Shift Index and the attribution question

The report is built on Climate Central’s Climate Shift Index, a scale from minus 5 to plus 5 that estimates how much human-caused warming has changed the likelihood of a given day’s temperature. A level 5 reading marks a temperature occurring at least five times more often than in a world without carbon pollution. Climate Central launched the index in 2022 and rests it on peer-reviewed attribution work.

The October overview reports elevated index values in eight of the nine climate regions, with the Upper Midwest the exception even though it was the hottest region. That seeming contradiction is a reminder that a big anomaly and a large climate-change signal are separate quantities: the Upper Midwest’s warmth was extreme, but the index methods the report applies did not flag the same level of added likelihood there. The report’s methodology matches each station to the nearest 0.25-degree grid cell of the ERA5 reanalysis for those index levels, so the attribution layer and the station temperatures come from different data.

One warm month inside a global record

Sioux City’s month sits inside a global one. The European Union’s Copernicus service reported that October 2026 was the warmest October on record worldwide, putting the month 1.75°C warmer than preindustrial times and 0.90°C warmer than the 1991-2020 norm. A strong El Niño has been building through the autumn; the NOAA Climate Prediction Center’s Oct. 8 discussion described a strong-to-very strong event as likely through January-March 2027.

Climate Central’s pages name no individual scientist behind the ranking, so the findings stand as the organization’s analysis rather than a quoted expert’s judgment. The next test of the ranking is the final tally: NOAA’s own monthly national report will carry its station-based October figures for the Corn Belt, and the gap between that and the 192-city list will show how firm the order of the top ten really is.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.