From waves of strong tornadoes to repeated rounds of damaging thunderstorm winds, the past few months have produced an onslaught of severe weather in the Northeast. Sometimes overshadowed by the punishing heat waves and devastating fires in the West, this activity was nonetheless substantial.
Parts of the northern Mid-Atlantic and Northeast saw substantially more severe weather than the traditionally much stormier Central and Southern United States.
Philadelphia region receives tornadoes
Like a lot of the coastal Northeast, New Jersey and adjacent parts of Pennsylvania generally have little tornado activity. The National Oceanic and Atmospheric Administration pegs New Jersey’s yearly twister average at two, with only a fraction registering as “strong.”
Strong tornadoes are those with a damage-based score of at least EF2 on the scale for tornado intensity that ranges from EF0 to EF5. This typical scarcity makes the past months’ tornado activity even more interesting, as two of the worst regional tornado outbreaks in memory struck within five weeks.
The first, on July 29, came as a decaying storm complex blew through from the Midwest. Winds that turned and strengthened quickly with altitude worked with high-end moisture to spin up a half-dozen powerful, well-organized supercells or rotating thunderstorms.
The result: a cluster of 11 tornadoes, including an EF3 and two EF2s, along a swath from Allentown, Pa., to Red River, N.J. An uninterrupted line of red tornado warning polygons spanned the two cities, demonstrating the density and spread of tornadic storms.
The EF3 was the first tornado of such intensity anywhere in the Northeast since 2014, and the first in this part of the region since a Somerset County, N.J., twister in 1990. Additionally, the six tornadoes recorded in New Jersey meant July 29 was the second-most-prolific tornado outbreak on record for the state, behind only an infamous November 1989 event.
With a 6th NJ tornado confirmed for 7/29/2021, this further supports it as being the second highest known single day outbreak since 1950. Before, some considered it a tie with 1964 (5 tornadoes), which now doesn't matter. I don't think 1964 is accurate anyway and left off chart. pic.twitter.com/miLS7r0uNi— Anthony Masiello (@antmasiello) August 16, 2021
A second vicious outbreak struck the region on Sept. 1 as Hurricane Ida’s moisture-laden remnants climbed the Mid-Atlantic. The impressive wind field associated with the tropical system allowed scattered supercells, or rotating thunderstorms, to grow quite intense.
One well-organized storm triggered the Northeast’s first tornado emergency, reserved for the most dangerous situations, as it crawled through Bucks County. The storm ended up producing an EF3 twister in Gloucester, N.J. — the third in this part of the country in 31 years and the second in five weeks.
Cumulatively, the two outbreaks triggered the most tornado warnings on record for the region during July and September.
Widespread damaging wind gusts
Even outside tornado outbreaks, recent months have been unusually prone to damaging thunderstorm wind gusts. Data compilation data from the Weather Service’s Storm Prediction Center shows that, since 2000, this summer has seen the most reports of severe wind across the Northeast of any June-through-August period since at least 2000.
While it is important to be wary of trends in data because of increasing numbers of weather observers or storm spotters over time, the extent to which 2021 beats out prior years is striking.
The unusual density of damaging but non-tornadic storms can also be seen in the incredible number of severe thunderstorm warnings issued this summer across the Northeast. Between June 1 and Sept. 1, the top-five Weather Service offices for severe thunderstorm warning issuance were Washington-Baltimore, Pittsburgh, Mount Holly (serving the Philadelphia region), State College and Binghamton, demonstrating the striking regional concentration of storms this summer.
Why was the summer so active?
There is no simple answer to this question. Increasing temperatures and humidity that stem from human-caused climate change could hypothetically play a role in increasing energy for storms, though the most likely driver is the overall configuration of weather systems.
A double-barreled region of high pressure thousands of feet above the ground spent much of the summer with one center over the Four Corners and another over Bermuda. The clockwise circulation around these high-pressure domes regularly pumped moisture from the Gulf of Mexico and tropical Atlantic up the East Coast. This favorable setup paired with persistent low pressure over the Great Lakes, which helped lift the air, supported severe weather over the Northeast, time and time again.
Feuerstein is a college junior studying meteorology at Cornell University.