Predicting the next heat wave

Staying cool during DC's summer heat

Although 90° heat doesn’t become “average” in the nation’s capital until July, the first 90-degree day of the season often occurs much earlier than that. The month of May, for example, has averaged between one and two 90-degree days over the last 30 years, according to NOAA. True to form, the nation's capital had its first 90-degree day of 2024 on May 2 with a high of 91° at National Airport.

There is often a high degree of variance from one year to the next of when the first 90-degree heat of the season occurs. For the 30-year period from 1994 - 2023, DC's first 90-degree heat of the year occurred 15 times in May. That's on par with what the data shows and what longtime Washingtonians might expect. 

However, there has been 90-degree heat in April eight times over the last 30 years, most recently in 2017. DC residents even experienced three consecutive days of 90-degree heat in April 2002. On the other hand, DC's first 90-degree heat of the year occurred in June seven times over the last 30 years, most recently last year with a high of 94° on June 2. 

Not having 90-degree heat until May or June has no bearing on how hot the summer might ultimately be. A good example of this was in 2016 when DC residents didn't experience the first 90-degree temperature of the year until it was 96° on June 11. The summer of 2016 went on to become one of DC's hottest on record with a total of 58 days in the 90s or hotter. 

Having the first 90-degree heat of the season earlier also doesn't indicate an increased likelihood of a warmer than average summer. Such was the case during the summer of 2000.  DC's first 90-degree heat of 2000 occurred on May 7 (90°). The summer of 2000 went on to become one of the coolest in recent memory. The three-month period of June, July and August of 2000 featured more days with highs in the 70s (23), than with highs in the 90s (9).

Early indications from NOAA's Climate Prediction Center are for a warmer than average summer in the nation's capital. The first 90-degree heat of 2024 is already in the books. My colleagues and I on the WUSA9 Weather Team, however, don't currently expect any 90-degree heat over the next seven to 10 days.