(This article, first published Wednesday afternoon, was updated Thursday morning.)
A line of violent thunderstorms barreled through southeast Pennsylvania and southern New Jersey between midmorning and midday Wednesday, unleashing destructive gusts of 60 to 90 mph and knocking out power to more than a half-million customers. The Philadelphia region caught the brunt of the storms, where wind damage was widespread.
The National Weather Service warned of an “extremely dangerous situation” as the squall line approached the city and moved into New Jersey. According to the National Weather Service, the squall line met the criteria of a derecho, a fast-moving line of violent storms that produces widespread damage along its path.
NBC Philadelphia reported three deaths resulted from falling trees during the storm in Montgomery County, Pennsylvania, just northwest of the city.
The fierce storm complex was the first of several rounds of storms expected for parts of the Mid-Atlantic and Northeast through Saturday.
On Wednesday night, the derecho was followed by a vigorous supercell or rotating thunderstorm, that prompted a tornado warning in the Philadelphia area and brought further wind damage.
Between the derecho and the supercell, more than 250 reports of wind damage were logged by the Weather Service office serving southeast Pennsylvania and southern New Jersey Wednesday.
An active weather pattern will bring continued chances of strong to severe thunderstorms for many areas until at least Saturday. The Weather Service Storm Prediction Center has placed the zone from Washington to Philadelphia under a level 2 out of 5 risk for severe thunderstorms on Thursday.
This same volatile weather pattern was also responsible for damaging thunderstorms from southeast Minnesota through Wisconsin and Michigan on Tuesday night and from Nebraska to Missouri on Wednesday.
Derecho hammers Philadelphia region, southern New Jersey
Storms over the Great Lakes Wednesday morning intensified as they swept southeastward and encountered warm, humid air over the East Coast. When they arrived in southeast Pennsylvania, they unleashed an 83 mph gust at the Reading Regional Airport in Pennsylvania. Pottstown, Pa., saw winds gust to 76 mph, while gusts exceeding 75 mph were reported in Malvern, Pa.
Doppler radar indicated winds of 77 mph just 100 to 200 feet above the ground over downtown Philadelphia, with a number of high-rise buildings or skyscrapers probably seeing wind gusts up to 80 mph. Philadelphia International Airport recorded a gust to 61 mph, while northeast Philadelphia Airport clocked a gust to 67 mph.
The storms roared southeastward at upward of 75 mph, their breakneck forward pace transforming a calm, tranquil summer day into a vicious windstorm in minutes. The storm complex progressed about 110 miles in 90 minutes.
Just after 2 p.m. Wednesday, the website PowerOutage.us reported more than 360,000 customers without power in Pennsylvania, mostly in the southeast part of the state, and nearly 180,000 outages in New Jersey.
Philadelphia’s west and northwest suburbs were especially hard hit, with the greatest concentration of outages in Montgomery and Berks counties. The National Weather Service’s Storm Prediction Center logged scores of damage reports in these counties, mainly from downed trees and power lines. Some trees and power lines collapsed on homes and buildings.
A second area of widespread outages and damage reports focused east of Philadelphia in southern New Jersey in Burlington and Ocean counties, where trees and power lines were down in numerous locations.
The Weather Service reported gusts to nearly 90 mph in Beach Haven, N.J.
As they charged from northwest to southeast, the storms were teeming with lightning, visible from weather satellites in space.
Those storms exited the coast shortly after the lunch hour.
Supercell storm Wednesday evening follows derecho
In the wake of the first round of storms, the atmosphere was already reloading to the west over central Pennsylvania. Into the evening Wednesday, a spattering of intense thunderstorms then charged toward southeast Pennsylvania.
Philadelphia was placed under a tornado warning at 7:21 p.m. and the rotating storm unleashed a gust of 68 mph at Philadelphia International Airport.
“We are getting widespread reports of damage across southeast PA from these thunderstorms,” tweeted the Weather Service office serving the Philadelphia area. “This is a real situation and we urge people to take cover to protect themselves now.”
The storms then rumbled through southern New Jersey where warnings were issued as far east as Atlantic City.
No tornadoes were confirmed in Wednesday’s second round of violent storms. “All indications is the damage was from straight line winds with wind gusts of at least 60-70 mph,” the Weather Service wrote in a tweet.
The Storm Prediction Center had placed the northern Mid-Atlantic in a 2 out of 5 risk zone for severe storms which extended along a serpentine path all the way to Rapid City, S.D. stretching through Columbus, Ohio, Indianapolis and Kansas City as well. A stagnant west-to-east cold front — part of a “ring of fire” weather pattern dominating the eastern Lower 48 — was the focus for a lengthy corridor of windy severe thunderstorms.
Near the Kansas-Missouri border, two large storm complexes along this ring of fire drew together like magnets, appearing to fuse together at one point.
“45 years of combined meteorology experience on this shift and... yep... this is a new one,” tweeted the Weather Service office serving Kansas City.
Why Wednesday’s morning’s storm complex was a derecho
Those who have lived in the Mid-Atlantic long enough have probably heard the term “derecho” before, often in reference to the infamous windstorm that rocked the nation’s capital on the night of June 29, 2012.
Widespread 60 to 80 mph gusts accompanied a band of thunderstorms that tracked 800 miles from Indiana to the East Coast, knocking out power to the Washington area for up to two weeks. Hot temperatures that lasted into the night and ample moisture allowed thunderstorms to swell into an arcing line with beastly winds.
So were Wednesday’s storms in Philadelphia a derecho? Yes.
The American Meteorological Society notes that for a storm to be a derecho, “damage must be incurred either continuously or intermittently over a swath of at least [400 miles].” Other sources stipulate damage must occur along a 250-mile span.
The National Weather Service solely states that a derecho is “a widespread and usually fast-moving windstorm associated with [thunderstorms].” Looking just at those definitions, it would appear that Wednesday’s storms would be a derecho.
Damage first occurred in western Pennsylvania, and with scattered to increasingly more numerous reports of damage as storms neared the coast, that’s a 300-mile path of damage. The significant winds continued even after moving out to sea, at least according to Doppler radar. By the time the storms arrived at the coast, an 89 mph gust was clocked southwest of Beach Haven, N.J.
Radar suggests a few pockets of 90 to 95 mph gusts probably occurred. Structurally, the storms fit the bill of a derecho. While instability, or the propensity for air to rise, was less impressive than during the 2012 Mid-Atlantic derecho, wind dynamics in the atmosphere compensated.
The American Meteorological Society notes that “[derechos] have sustained bow echoes with bookend vortices and/or rear-inflow jets.” A bow echo forms when storms curve in the shape of a backward “c,” owing to descending air surging ahead and causing the line of storms to arc. That was plainly observed in the Philadelphia area.
A bookend vortex occurs in a bow echo when this effect is dramatic enough that the northern and southern ends of the squall line curve back around and sort of wrap into the storm. The rear inflow jet, or a current of air racing into the storms from behind and accelerating the strong winds, amplifies that mechanism. Both were seen, especially along the northern end of the storms, as they continued east through New Jersey.
“The storm complex had a very impressive radar presentation; it’s the type of structure, with a bow echo and channel of high wind entering the rear portion, that structurally defines a derecho,” wrote Jeff Halverson, Capital Weather Gang’s severe weather expert.
The only limiting factor that some might argue would preclude this from having been a derecho would be its length of track. It simply didn’t meet the AMS’s 400-mile criterion. But many other publications cite a 250-mile value, which would qualify what happened Wednesday as a bona fide derecho.