Deep Dive: July 29th, 2021 Tornado Outbreak

One of the most significant severe weather outbreaks of the summer impacted the Mid-Atlantic on Thursday, July 29^th. Several rotating thunderstorms (called supercells) brought corridors of damaging wind, large hail, and a number of tornadoes to the region. Check out this map of storm reports collected across the region from local National Weather Service offices and the Storm Prediction Center:

 

Storm reports complied by the NWS and SPC. Blue W’s indicate thunderstorm wind damage, green H’s indicate 1.00” or larger hail, and red T’s indicate a tornado

The main focus will be the unusual tornado outbreak that occurred from northern MD into eastern PA and parts of NJ, but it’s worth noting that supercell thunderstorms began producing wind damage and tornadoes around midday in western PA. This was before activity re-intensified east of the mountains later in the afternoon, closer to the I-95 corridor. Also note that scattered severe storms also brought swaths of wind, hail, and a few tornadoes to the Ohio Valley ahead of a cold front pushing out of the Great Lakes.

Video of a tornado near Bensalem, PA on the evening of July 29^th, 2021, courtesy of Stacy DaSilva

By the numbers, the official counts of tornadoes are rather impressive for areas such as eastern PA and NJ. For instance, the tornado (in the above video) that impacted the Trevose and Bensalem, PA area in southern Bucks County was rated as an EF-3 tornado, with peak winds estimated at 140 MPH. This is the strongest tornado in PA since 2004. Five tornadoes were confirmed by the National Weather Service to have touched down in NJ, which is the second most on record in one day; the most on record in one day occurred on November 16^th, 1989, when a remarkable seven tornadoes occurred in the state! It’s not ruled out that one or two more tornadoes are added to the final tally if additional damage surveys are completed; either way, the number of tornadoes for the region was quite unusual…plus, another tornado occurred in Maryland, with several more occurring farther west into the Ohio Valley as well.

What sparked this unusual tornado activity across the Mid-Atlantic? Let’s start with a look at the surface map from 5 PM Thursday afternoon, courtesy of the Weather Prediction Center:

Surface analysis valid at 5 PM on July 29^th, 2021, courtesy of the Weather Prediction Center

A low pressure was centered near Lake Ontario, with a cold front draped to the west into the Ohio Valley, which sparked its own round of severe weather. There was a warm front positioned near the NY/NJ/PA boarder, which are known to enhance “turning” of the winds in the atmosphere, that can aid in tornado development. However, it wasn’t one of these fronts that sparked the storms over eastern PA, NJ, MD, VA and DE; it was the combination of a weak wind shift (called a pre-frontal trough) over central PA, marked by the thin tan lines on the map above, and an old cluster of thunderstorms from the night before. In the summer, when larger-scale weather systems tend to be weaker, it’s not uncommon for subtle wind shifts, boundaries, or even leftover thunderstorms from the night before to spark new thunderstorms.

Satellite image valid at 1 PM on July 29^th, 2021, courtesy of College of DuPage 

What was a little bit unusual about this event was that it was mostly cloudy ahead of the thunderstorms over much of PA and NJ. However, dew points in the low-mid 70s coupled with southerly winds transporting warmth from sunny areas farther south, allowed for the atmosphere to become unstable and supportive of severe thunderstorms. Here’s a look at some of the key atmospheric indicators we look at for potential severe weather / tornado development, valid late in the afternoon on July 29^th:

 

Analyzed mixed-layer CAPE, a measure of instability, valid at 5 PM on July 29^th, courtesy of the SPC

Let’s start with instability. Despite the cloud cover, CAPE (which stands for convective available potential energy) values rose to 1000-2000 across areas like MD, eastern PA, DE and most of NJ. Values over 1000 begin becoming conducive for severe weather; while these values aren’t extreme, they were high enough for severe weather to occur. 

Analyzed shear between the ground and 6km of altitude, valid 5 PM on July 29^th, courtesy of the SPC

While the instability present for this event was adequate, what was more compelling was the amount of wind shear (change in wind speed / direction with height) over the region. Over 50 knots (58 MPH) of shear was analyzed in the lowest 6km (roughly 20,000 feet) of the atmosphere over eastern PA and NJ as the tornadic thunderstorms began developing late in the afternoon! Typically, values over 30-35 knots are considered strong enough for more organized and potentially rotating thunderstorms. 

Analyzed “storm relative helicity” over the lowest 3km of altitude, valid at 7 PM on July 29^th, courtesy of the SPC

Another measure of shear is “storm relative helicity”, which incorporates the changes in wind speed / direction with height along with the motion of thunderstorms in the region. Values of helicity in the lowest 3km (approximately 10,000 feet) of the atmosphere greater than 200 are favorable for rotating thunderstorms and tornado potential; values as the event was peaking over southeast PA and into NJ were analyzed at 300-400+. Values were relatively lower into MD and VA and while supercells occurred here, only one brief tornado resulted, as opposed to the several tornadoes that occurred in PA and NJ where the helicity was higher. 

Analyzed lifted condensation level height, valid at 5 PM on July 29^th, courtesy of the SPC

A final parameter we’ll examine is the “lifted condensation level” height; basically, this tells us how far off the ground clouds will start forming if a thunderstorm develops. Values below 750 meters are most favorable for tornadoes, with values above 1500 meters unfavorable for tornadoes. Basically, if the cloud bases are too high, that indicates a lot of potential for colder, drier air to choke off a developing tornado. Note how over most of eastern PA and NJ, values were below 750m; meanwhile, into MD and VA, values were higher. While severe thunderstorms occurred there, they were mainly non-tornadic, unlike what occurred farther north. Ironically, the cloud cover over PA and NJ kept temperatures closer to the sultry dew points, which kept the LCL heights lower. In this instance, it can be argued that cloud cover actually enhanced tornado potential in eastern PA and NJ. 

Radar imagery valid at around 3 PM on July 29^th, courtesy of College of DuPage

On top of the broader environmental considerations discussed above, I want to touch on the importance of the thunderstorms from the prior night. The radar image valid in the mid-afternoon is still rather unimpressive, but there’s clearly a remnant batch of showers and storms over PA into western VA that turned severe as it moved into stronger instability east of the mountains. Another interesting note is that there were three well-defined little “spins” with this batch of rain, called “Mesoscale Convective Vortices” or MCVs. Each MCV directly sparked severe weather:

1. The northern MCV sparked the storms that produced tornadoes from eastern PA into NJ
2. The middle MCV sparked the storms that produced damaging winds and one tornado over eastern MD and near DC
3. The southern MCV sparked a severe storm that brought significant straight-line wind damage and large hail to the Fredericksburg, VA area

Hopefully, this gives insight on the environmental factors we use to anticipate severe weather / tornadoes, and some of the smaller-scale things we look for when issuing shorter-term forecasts for thunderstorm development. The resulting thunderstorms in this environment produced a lot of severe weather, and looked like something we’d see in a textbook (or perhaps the Plains during spring):

Radar image of a supercell over Bucks County, PA and Hunterdon and Mercer Counties, NJ valid just before 6 PM on July 29^th

This radar image captures a supercell thunderstorm shortly before it dropped a strong (EF-2) tornado near New Hope, PA that tracked into western Mercer County, NJ. Note on the left, a classic “hook echo” indicating a rotating supercell thunderstorm. On the right, note the greens and reds located next to each other, which is the radar directly seeing “spin” in this thunderstorm. With an environment conducive for tornadoes and thunderstorms that looked like THAT on radar, it’s no wonder so many tornadoes occurred across the region.

Here’s how that storm structure looked from the ground:

This tweet is from Weatherworks’ own Joe Slezak; what you’re seeing isn’t a tornado itself, but it’s the larger, rotating updraft associated with the supercell that dropped an EF-2 tornado near Barnegat Bay shortly after 9 PM. With much of the Northeast being filled with trees, buildings, and hilly terrain, it’s very rare to see that kind of structure around these parts. But on rare occasions, when the atmosphere is more like something we’d see in the Plains, we can get pictures like that from anywhere, including the Jersey shore.