It’s the hottest May ever in Miami. Heat index ‘completely off the charts’

It’s already the hottest May in Miami, ever — at least judging by the heat index, a “feels like” measure that combines temperature and humidity.

Last weekend’s record temps jacked up the average heat index into a record for May, according to Brian McNoldy, a senior research associate at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science.

“The type of heat and humidity we had this weekend would’ve been exceptional even in another three months,” said McNoldy. “These temperatures in May are completely off the charts.”

McNoldy created an online chart that updates daily with the cumulative amount of time the heat index spent above various heat index thresholds. The reading in 2024 already rivals or tops nearly all end-of-summer 108°+ and 110°+ marks.

Usually, the hottest time of the year is the first and second weeks of August but this weekend temperatures peaked at 112 degrees heat index— that’s a stunning six degrees hotter than any previous May heat index recorded.

Early-season heat events have some of the highest rates of heat illness and heat-related deaths because people are not prepared for it. Nearly 1,200 people die from heat every year, according to NOAA, and record-breaking heat waves fueled by climate change add to that threat.

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Climate change makes things like these record highs more likely. But over the weekend McNoldy said there was also the “perfect combination” of a high pressure ridge (where air sinks and warms), fewer clouds and moist air coming in from the southwest.

Other records were broken over the weekend too. Sunday’s nighttime temperatures averaged (the average of the high and low temperature) to 89 degrees. That is a tie for the third-highest daily nighttime average temperature ever recorded in Miami, and that’s never happened as soon as May.

As of Monday, there had also been four new high daily average temperature records and record-high humidity levels in the past five days.

The National Weather Service is predicting that the record-breaking heat will ease in the coming week, thanks in part to the increasing relief of rain. But it also signals the potential for another scorching summer ahead. Summer 2023 was the hottest on record in Miami.

“What this looks like for June, July, August? Who knows,” McNoldy said. “But it’s not off to a promising start.”

Ashley Miznazi is a climate change reporter for the Miami Herald funded by the Lynn and Louis Wolfson II Family Foundation in partnership with Journalism Funding Partners.