Seven states — Virginia (tie), Maryland, Delaware, Pennsylvania (tie), New Jersey, Connecticut (tie) and New Hampshire — all clinched the top spot for their sweltering July heat. Records date back to 1895.
The heat was ubiquitous along the East Coast, with states east of the Appalachians recording a top five warmest July except Georgia and South Carolina.
Even in places that are normally hot — such as Florida — records went like hotcakes. Miami recorded not only its warmest July on record, but its all-time hottest month.
High pressure stalls with sizzling heat
Paramount to the heat was a sprawling dome of high pressure, which deflected cooling weather systems to the north. Westerly or northwesterly winds at the mid levels of the atmosphere helped pump in air from the core of this “heat dome,” also suppressing cloud cover or a cooling onshore flow.
Overall, the magnitude of the heat wasn’t overly impressive for most of the Northeast, but its duration was surprising. Manchester, N.H., hit 90 degrees 15 days out of the month, including on July 19, when the high was 98. That’s close to the average number of 90 degrees that would occur in a year.
New York City spent 14 days with highs of 90 degrees or greater; in D.C., all but three days hit that mark.
The nation’s capital broke a record for the number of 90 degree days it could cram into one month, at 28 days. The previous record was 25 in July of 2011.
Richmond spent three days in the triple digits, tying a daily record on July 19. Their July wound up about 3.6 degrees above average.
Exceptional heat even in areas used to it
In Miami, June wrapped up having set a record for the hottest week ever observed in the city. Dry, hot mid-level air drifting west from the Sahara eradicated thunderstorms and eroded cloud cover, allowing sunlight to pour in and heat the ground without relief.
That stretch of anomalous heat continued into July, with half the month topping 94 degrees or greater for an afternoon high.
Overnight lows failed to drop much at night either. The morning low on July 31 was 84 degrees. That ties for Miami’s hottest nighttime low on record for the fourth time this year.
Nine of Miami’s top 10 warmest nights have all occurred since 2017.
Warming overnight lows
Due largely to climate change, overnight lows are warming disproportionately faster than daytime highs. That’s partially owed to the urban heat island effect, or the tendency for pavement and cement to trap heat. But rising humidity levels, which slows nighttime cooling and the trend toward rising temperatures overall, are linked to greenhouse gas-induced climate change from human activities.
In D.C., the month of July passed without the temperature ever dropping below 71 degrees. That’s the first time on record for a month to never dip beneath 70. Average July low temperatures have increased more than four degrees since the 1940s.
Meanwhile, heat over the eastern United States doesn’t look to relent any time soon. The western limb of the Bermuda High, an offshore high-pressure system anchored over the Atlantic, continues to nudge west, bringing warm, humid weather to the Eastern Seaboard.
The National Weather Service Climate Prediction Center anticipates weather skewed warm over the East for the foreseeable future.