EF2 tornado, Flourtown to (Nxx)Nas Willow Grov, Pennsylvania, September 1, 2021

1. EF065 to 85 mph

   Light damage. Surface peeled off some roofs, gutters and siding damaged, branches broken, shallow-rooted trees pushed over.

2. EF186 to 110 mph

   Moderate damage. Roofs badly stripped, mobile homes overturned or badly damaged, exterior doors lost, windows broken.

3. EF2111 to 135 mphthis tornado

   Considerable damage. Roofs torn off well-built houses, frame homes shifted on their foundations, mobile homes destroyed, large trees snapped or uprooted, cars lifted off the ground.

4. EF3136 to 165 mph

   Severe damage. Whole stories of well-built houses destroyed, large buildings badly damaged, trains overturned, trees debarked, heavy cars lifted and thrown.

5. EF4166 to 200 mph

   Devastating damage. Well-built houses leveled, structures with weak foundations moved some distance, cars thrown and large missiles generated.

6. EF5over 200 mph

   Incredible damage. Strong frame houses swept off their foundations, car-sized missiles thrown more than 100 meters, steel-reinforced concrete badly damaged, high-rise buildings deformed.

7. Unrated

   The survey recorded a tornado but could not put a number on it, most often because it crossed open country and damaged nothing a rating can be read from.

Rating

EF2

Date

September 1, 2021

Touchdown

5:35 PM EDT (21:35 UTC)

Lifted

5:49 PM EDT (21:49 UTC)

On the ground

14 minutes

Path length

8.3 miles (13.3 km)

Path width

400 yards (366 m)

Deaths

1

Injuries

2

Property damage

$5.0 million

County

Montgomery County

Touchdown point

40.107, -75.238

Lift-off point

40.210, -75.158

Record number

2109011535-01

47 damage points were recorded along this path by NWS PHI, in the Damage Assessment Toolkit the survey teams carry. The strongest indicator was rated EF2. The highest wind the damage implies is 132 mph.

• Relatively small trees snapped.
• Roof damage at veterinary hospital.
• Decent size limbs with at least one small uproot
• May be a mix of flooding damage and wind damage in this area.

Survey points are preliminary and are the raw material the published rating was built from. Source: NWS Damage Assessment Toolkit.

The remnants of Hurricane Ida impacted the mid-Atlantic on September 1, 2021. As it moved northward, Ida began transitioning to a strong mid-latitude cyclone with pronounced frontal features, but with continued tropical moisture. On a regional level, the remnants of Ida became one of the most severe natural disasters in US history, due to a combination of several tornadoes, some of them strong, as well as catastrophic flooding over a large and densely populated area. The severe weather threat unfolded as the system's developing warm front lifted northward from the Delmarva region into Pennsylvania and New Jersey. South of the warm front, partial clearing and moderate to strong instability developed. Meanwhile, very strong shear and forcing, along with tropical moisture, were present within the warm sector. The result was widespread convection in the form of both supercell and quasi-linear thunderstorms. A number of storms became severe across the region, producing several tornadoes and other instances of wind damage. This continued an exceptionally active stretch of high end severe weather in the region, coming barely a month after the tornado outbreak of July 29.

A semi-discrete supercell thunderstorm began to exhibit rotation |over Delaware County, PA around 5:15 PM on September 1. The |rotation continued north-northeast into Montgomery County, and |eventually strengthened enough to produce a tornado. Minor tree |damage was first observed along Stenton Ave in Fort Washington in |the area of the Philadelphia Cricket Club. A path of tree damage |was observed heading northeast along Joshua Rd and through Fort |Washington State Park. Some minor shingle and siding damage to |houses was also seen. ||Damage became more severe after the tornado crossed the |Pennsylvania Turnpike. At this time the tornado also moved into a |more residential area. Damage was observed along Montgomery Ave, |Spring Ave, and Prospect Ave and numerous crossing streets of |those. This mainly consisted of numerous snapped and uprooted |trees with minor to moderate roofing and siding damage to a number|of homes. The worst damage was observed slightly farther |northeast in the area of Stuart Creek in Upper Dublin. A number of|homes suffered partial to near total loss of roofing with many |damaged to the point of being uninhabitable. At least one home |experienced partial collapse of exterior walls consistent with EF2|tornado damage. Nearly all trees were snapped or heavily damaged |in this area as well. ||Damage continued along Route 309 to the area of Upper Dublin High|School. The high school itself suffered roofing damage, and |significant roofing loss occurred to an Upper Dublin Township |building adjacent to the high school. A number of residences along|Loch Alsh Ave and several surrounding streets were also damaged. |A woman was killed when a tree fell on her home on Kenyon Dr, the |only known tornado-related fatality from this tornado or any |tornado during this regional outbreak.||Further northeast, damage occurred on and around the Temple |University Ambler Campus, again consisting of considerable tree |damage and partial roofing loss on a number of homes and |university buildings. The tornado then moved into Maple Glen, |crossing a heavily developed area near Norristown Rd at Limekiln |Pike. Snapped trees and mostly minor roofing and siding damage to |several commercial buildings was observed in this area. The |tornado remained continuous but did begin to lose some intensity |as it moved into Horsham Twp, traveling east of Limekiln Pike. A |number of trees were snapped along Babylon Rd. Near Horsham Rd, |roofing damage occurred to a veterinary hospital, along with |additional large trees snapped. Damage appeared to end not long |after that point. The tornado crossed Horsham Rd, but little to no|damage was observed in a couple of parks across the road, |approaching the Bucks County line. The tornado is believed to have|lifted in that area.

The deaths

1 direct deaths are recorded for this tornado, each with what Storm Data knows of it.

From NOAA NCEI's Storm Events Database, the narrative the local forecast office wrote for Storm Data.

20 minutes of warning. NWS PHI issued a tornado warning for this county at 21:15 UTC, 20 minutes before the tornado touched down, in force until 21:45 UTC.

The county was under a tornado watch, issued at 16:41 UTC and running to 01:54 UTC.

Every warning and watch covering the touchdown point that day, from the Iowa Environmental Mesonet's archive of National Weather Service products. Lead time is measured from the issue of the warning in force to the touchdown time in the database, both of which were recorded to the minute by different people.

The Storm Prediction Center had this point in a Enhanced Risk, with a 0.10 chance of a tornado within 25 miles. The outlook in force was issued at 19:46 UTC.

SPC's Day 1 convective outlook as it stood when the tornado began, read at the touchdown point. The same archive is on the radar archive.

The radiosonde launches on either side of the tornado, from the nearest station that was launching then, drawn and analyzed the way Storm Lab does: the same diagram, the same parcel and shear definitions SPC's mesoanalysis uses.

The Philadelphia radar (KDIX), 71 km away, was scanning. The volume nearest the touchdown began at 21:35 UTC.

Open the radar archive at this moment, over the track

Reflectivity and velocity, decoded from the Level II archive in the browser. Volumes take a few seconds to open the first time.

Local storm reports within 60 km of the path, in the three hours either side.

From the Iowa Environmental Mesonet's archive of National Weather Service local storm reports.

The live radar over the trackEvery 2021 tornado in PennsylvaniaEvery tornado in Pennsylvania since 1950Warnings in force in Pennsylvania nowThe forecast where you are

10 more were confirmed in the United States that day.

Ratings are the survey's, read off the damage rather than measured; the rating scale is above. Tornado counts rise through the record as reporting improved, so a year in the 1950s is not comparable with one today except for the strongest tornadoes, which were always noticed.

NOAA Storm Prediction Center tornado database, 1950-2025, the official record kept by NOAA's Storm Prediction Center.