The Northeast is no stranger to heatwaves, with many cities up and down the I-95 corridor (especially in Maryland) already having a handful of days in the 90s since the beginning of June. This most recent heatwave brought the highest temperatures we've seen so far this year. What made it noteworthy, however, was the rather tropical air mass that kept dewpoints in the 70s and brought heat indices well into the 100s.
500mb Upper level chart, July 21st
We certainly can put the blame on this short but definitely not sweet stretch of hot weather on a rather anomalous ridge of high pressure that stretched all across the Eastern Seaboard from the 19th to the 22nd of July, as seen by the upper level map above. The ridge not only brought us the heat, but also a rather tropical air mass, leading to higher than normal dewpoints. Typically ridges of high pressure result in calmer and drier weather patterns, however, hot and humid conditions helped to keep things unstable and led to scattered showers and thunderstorms.
The worst of the heat wave was felt on the weekend of the 20th - 21st. Some places, like Newark and Boston, saw temps reach 98 - 99° on Sunday, but Baltimore and Hartford were the winners, topping out at 100° degrees. With dewpoints more common of southern Florida in place and temperatures each day solidly in the middle to even upper 90s, heat indicies for much of the area rose well into the 100 - 115° range!
Daily High temperatures (Peak Heat index in red)
The end of July through the first week of August is typically where we see the hottest weather of the year. However, to put these past high temperatures into perspective, the average high for this time of the year in some of the bigger cities ranges from 82° - 86° degrees! As we start getting closer to the end of summer, we may still have another heatwave or two up our sleeve, but last month's heat was certainly impressive.