NYC's first heat wave of 2023 happens in... September?

This week, the Tri-State dealt with its worst hot stretch of the season. On both Tuesday and Wednesday, high temperatures reached the low 90s in Central Park, and on Thursday, the preliminary data shows a high temperature of 93°, the hottest it's been in the city since July 5th. Further, with 3 days in a row of 90+ degrees, this will be the first heat wave of 2023 in New York City. That means our first heat wave will have taken place during Meteorological Fall! 

Heat index forecast for Friday, September 8th, which will be the coolest day since Sunday.
Graphic courtesy of the National Weather Service office in Upton, NY

We got close a couple of times; in early July, when we reached 93°, we just barely missed heat wave criteria as the temperature failed to reach 90° for three consecutive days. Overall, this summer is part of a larger trend in the Tri-State, as recent summers have not even come close to some of the scorchers of the later 20th Century. 

Our local Weather Service office keeps record of every heat wave at the official NYC observation station in Central Park. Below are the stats on the longest heat waves the city has seen: 

Chart of longest heat waves in NYC from the National Weather Service office in Upton, NY

Other than a particularly nasty stretch in 2002 and a long stretch of heat in 2013, this century has been devoid of long-lasting periods of intense heat. It's even easier to see the change in 90° frequency on this chart, also from NWS New York: 

Chart of 90°+ days in NYC from the National Weather Service office in Upton, NY

In the 20th Century, it was fairly common for the city to deal with 90s for basically an entire month of the summer. But here's the kicker: despite the lack of heat waves, our summers haven't gotten any cooler, because the average temperature has actually gone up. 

The chart below shows the average temperatures during Meteorological Summer at Central Park's observation station, dating back to the 1880s. The average temperature is computed using the daily high and daily low from every day in June, July, and August. In the 1880s, the average was a bit above 72°, and so far in the 2020s, the average is about 4 degrees warmer- and the upward tick has been pretty consistent from one decade to the next. 

The upward trend can be attributed to two main factors: fewer cooler than average days (such as highs in the 60s), and warmer overnight temperatures. However, the warmer nighttime lows are the bigger contributor of the two. According to Climate Central, "summer minimum temperatures across the US have warmed on average at a rate ... nearly twice as fast as the warming rate observed for US summer daytime highs over the same period". Higher humidity is largely to blame. As the bodies of water surrounding the Tri-State get warmer, the evaporation rate increases, introducing more moisture into the air. Humid air heats and cools more slowly than dry air, meaning that it's harder to cool down at night. But it's also harder to get really hot. Take a look at the change in 100° days in NYC. After peaking in the mid 20th Century, we've largely failed to reach the triple digits ever since. 

So, even when we're not breaking record highs, we're still feeling the summer heat in the Tri-State, and that's largely due to higher humidity. For this reason, some researchers suggest that heat waves should be declared based on heat index, not just the number on the thermometer. For more on nighttime heat, check out my post from June, and you can get tips on beating the heat from the National Weather Service here.