The 7-29-2021 Tornado Outbreak & General Tornadogenesis Information

[hm2]

One of the coolest questions left in meteorology with a frontier-like feel to it is, "why do tornadoes exist." Research in the last 20 years has applied field/case studies to figure out why some rather organized supercells under pristine conditions do not produce tornadoes and others do. In the last 5-10 years, there have been some really good papers on this topic, focusing on the lowest 500m of the near-structure/under-structure environment. The importance of 0-500m SRH & 0-3km MLCAPE cannot be stressed enough. Assuming all the usual environmental conditions are there and favorable for supercells, the lowest layer shear structure (few hundred meters) seems to be a strong factor in determining tornadogenesis. The more streamwise the component of wind shear is here, the better the chance subtornadic vertical vortices can link up favorably with the supercell's mesocyclone.

The reason I'm starting out with this paragraph is because I see a lot of attention by the community, particularly outside of the Plains, to focus on the larger-scale ingredients to determine tornadogenesis probabilities. But more often than not, they are describing a checklist to predict general supercell potential, and other types of regional severe, which may or may not also predict a potential tornado. If you have a proper streamwise component to the wind shear with positive buoyancy in the lowest few hundred meters, even a low-topped supercell or modest mesocyclone can organize a tornado. Heck, stretching alone can deliver a tornado or landspout (non-supercellular tornadoes happen here, think back to 2019 Mount Laurel tornado).

Refer to the following papers for more information:

Coffer & Parker 2017 https://journals.ametsoc.org/view/journals/mwre/145/1/mwr-d-16-0226.1.xml

Coffer & Parker 2018 https://journals.ametsoc.org/view/journals/mwre/146/8/mwr-d-18-0050.1.xml

Coffer et al. 2019 https://journals.ametsoc.org/view/journals/wefo/34/5/waf-d-19-0115_1.xml

The 7-29-2021 event was marked by an environment that was full of wind shear, and nobody was surprised by that and its potential. But it was the way things unfolded that caused uncertainty in the forecasting community, esp. midday. The rapid arrival of dangerous conditions would catch most off guard and give a false sense of security prior to its arrival (i.e. the cloud cover red herring and shower activity ahead of disturbance). I think an image that best captures what I'm talking about is the SPC's 0-3CAPE/Bulk Shear overlay. Watch how both the stronger deep layer winds and the off-the-charts 0-3km CAPE arrive nearly simultaneously between 18z-00z:

 

Why did the clouds not matter as much? For starters, the lifting mechanisms in place (approaching disturbance, nearby warm front), or heading in, were substantial and interacting with deep low level moisture. Watch how the lowest 100mb mean mixing ratio exceeds 16 g/kg before the wind convergence arrives from the West:

 

The impressively low LCL and LFC heights marked an environment favorable for deep convection, should even modest lifting approach (which it did, i.e. incoming wave from old MCS). Having the sun come out completely wasn't necessary to initiate updrafts (weak convection was igniting with warm front early). The logic I used during the day to assess this, besides the environmental conditions, was that the mixed HREF/CAM solutions (some that produced severe weather and some that didn't) all had 1 thing in common: they all had the residual cloud cover from upstream MCS in their output. It was a constant on all the solutions and didn't seem to stop supercells from building on the more prolific runs.

 

 

Proper instability is not a function of the dry bulb alone. Conditions on the 29th were quite unstable when you factored in the low level moisture (virtual temperature vs. regular temperature) to the equation. The boundaries and moisture around with the exceptionally low LCL/LFC heights were just sitting there waiting for a trigger. In addition, these moist environments put a lid on how strong/cold downdrafts can be that overwhelm with their cold pools. This is important for tornadogenesis. You could almost argue that too much sunshine would have ruined the tornado threat, allowing the air to mix out and balloon the LCL heights. Clearly, the warming that took place, mixed with the plentiful (less dense) water vapor, was more than sufficient for a tornado outbreak. I suppose a few hours of additional sunshine would have made things worse, but it's a fine-line sometimes. 🤷‍♂️

Here's a 0-3 CAPE and surface vorticity loop showing their overlap nicely for potentially quick-developing updrafts. Values this high are quite impressive around here. My eyebrows raise when we start getting into the 50-100 range.

 

Let's get back to the wind shear for a minute. From the Coffer et al. 2019 paper, here are a few figures on the importance of 0-500mb SRH. Notice the rather high TSS score for this parameter in the Northeast, as well, in second figure:

 

 

 

 

Values exceeding 100 are to be respected with effective buoyancy. Values exceeded 200 in parts of the area by 00z 7/30-->

 

 

In addition, the 0-2km storm relative winds were perfectly in range to support supercells:

 

 

The 00z WAL/OKX soundings are the closest to the area, which I will provide here, but neither were quite in the most tornadic zone (OKX closer than WAL). However, we can really learn a lot from these, esp. 00z 7/30 OKX sounding and the DIX VAD wind profilers.

 

 

The OKX sounding had over 400 effective SRH with a rapid rise in low level wind ~ 1 km of 35-40 KTS. In addition, 3CAPE was ~ 200 j/kg!! This is an impressive sounding, even with the rain-cooled appearance. While they were mainly north of the main show, this gives insight into how ridiculous the soundings got there for a few hours between 6-9 PM. If the soundings look unclear in this thread, head to this link for clearer image: https://www.spc.noaa.gov/exper/archive/event.php?date=20210729

Heck, just look at the ramp-up in streamwise component shear (SRH & critical angle) on the DIX VAD wind profiles. From 4:40 to 6:30 PM, 0-500m SRH climbs to 200 with critical angle dropping from 110° to 85°!

 

Here's a mean sounding loop from 18z 3km NAM for C NJ/SE PA 5-10 PM. While the model was a bit inaccurate with convective evolution, pay attention to how the forecast profile trends increasingly more alarming with time. The wind accelerates in the 1-3KM range and veers more, enlarging hodograph and there's a lack of CIN. In addition, the mid level dry air moves in behind system, continuing high potential buoyancy well into the evening.

 

 

Finally, here's the 18z NAM BUFKIT for PNE at 23z 7/29. Winds exceeding 40 KTS ~ 2KM arrive with strong SRH being anticipated (0-1km SRH over 200 here):

 

 

Summary:

1. Conditions were tornadic, albeit more typical of region, prior to the arrival of the upstream disturbance with some positive buoyancy, lifting mechanisms in place, and veering profiles. However, the modest heating mixed with the weaker wind aloft tended to keep the initial warm frontal convection under severe limits. But where updrafts were assisted more, supercells quickly developed. Clues were there with what occurred over Delaware and esp. across W-C PA earlier in the day (the residual MCS was producing supercells/tornadoes embedded in the rain across C PA where instability was poorer than here).

2. The rapidity of conditions elevating the marginal tornado risk to significant happened in just a couple hours. Between 20-22z, low to mid level winds increased substantially and there was hardly any CIN being modeled.

3. Most parameters between 6-9 PM were favorable for significant tornadoes. In particular, 0-500m SRH and 0-3km CAPE were high-end. They suggested updrafts would quickly organize, rotate, and go tornadic. This knowledge can also help increase warning time knowing environment allowed for that sort of thing.

[JimCaruso]

Thanks for this excellent and detailed analysis, I am going to have to read it again several times, it will help my forecasting for Plains chasing as well. I am going to post a link to this on Stormtrack, where there is also a thread for this event.

[PRINCETON ANGLER]

Thanks Anthony. I didn’t hear many reports of significant straight line wind damage. Your explanation seems to say that strong downdrafts were inhibited by the moist environment in the mid-levels. In Princeton, we were north of the tornado, but had an intense thunderstorm in terms of rain and lightning but no wind. Safe to say that strong downdrafts inhibit tornadoes?
 

I guess this explains the experience I’ve heard where people describe how calm things are pre tornado.

[ACwx]

Fantastic work here. I genuinely hope NWS employees can see this as it will help with warning times like you said. Helps answer some of my questions from the other day lol.

Logically it makes sense that the lowest levels of the atmosphere would be important with determining whether a rotating circulation aloft would connect with the surface. I appreciate your points about the sun perhaps being an inhibitor if it actually came out that day, seems counterintuitive at first but it makes sense when you walk through it. I'll have to keep these ideas in mind next time our region is under the threat of tornadoes from a non-tropical system (hopefully not anytime soon).

I look forward to seeing your dew point project regarding climate variability with tornadoes.

[JimCaruso]

6 hours ago, PRINCETON ANGLER said:

Thanks Anthony. I didn’t hear many reports of significant straight line wind damage. Your explanation seems to say that strong downdrafts were inhibited by the moist environment in the mid-levels. In Princeton, we were north of the tornado, but had an intense thunderstorm in terms of rain and lightning but no wind. Safe to say that strong downdrafts inhibit tornadoes?
 

I guess this explains the experience I’ve heard where people describe how calm things are pre tornado.

Responding not as a meteorologist but as a long-time Plains chaser (and painfully aware of how much I don’t know)… Writing this more to test my own understanding and hope  hm2 and others will set me straight if needed…

It’s not literally correct to say that strong downdrafts inhibit tornados. Like other parameters, a balance is required. A supercell that becomes outflow dominant undercuts it’s own updraft. Or the rear flank downdraft (RFD) ultimately occludes the mesocyclone and can spell the end of a tornado. When cells do not remain discrete, the outflow from one can disrupt another, or their outflows merge into cold pools that ultimately help foster upscale growth into an MCS. Supercells do generally have very strong updrafts, especially the RFD, so it’s still surprising there wasn’t more straight-line wind damage. In fact, tornadogenesis theories say that the RFD is a necessary ingredient for tornados - specifically, a warm/moist RFD (note Anthony mentioned conditions helping to prevent downdrafts that were too strong *and cold*). But any tornado warned supercell on the Plains is going to also be warned for winds of ~60 mph in addition to the tornado. 
 

Things are calm pre-tornado because in the updraft region the air is going UP, so there is no sensation of horizontal wind unless you are in the inflow jet. In my experience, in a classic Plains supercell that is not completely rain-wrapped, there is usually a bit of “space” between the tornado on the ground and the broader circulation from the RFD that wraps around the mesocyclone. There is also separation between the forward flank downdraft and the mesocyclone.

[Chubbs]

Very nice write-up. As I recall, the July 27, 1994 mid-atlantic outbreak (Limerick+Avondale F3s) had some similarities:  moist, not overly warm, low cloud condensation level, front nearby.

[irishbri74]

Wow, what a read! Absolutely great read. Can learn a lot from this. Will keep it pinned for a while. 
 

Thanks for taking time outta your day to explain and post this @hm2

[hm2]

On 8/9/2021 at 7:35 AM, PRINCETON ANGLER said:

Thanks Anthony. I didn’t hear many reports of significant straight line wind damage. Your explanation seems to say that strong downdrafts were inhibited by the moist environment in the mid-levels. In Princeton, we were north of the tornado, but had an intense thunderstorm in terms of rain and lightning but no wind. Safe to say that strong downdrafts inhibit tornadoes?
 

I guess this explains the experience I’ve heard where people describe how calm things are pre tornado.

 

23 hours ago, JimCaruso said:

Responding not as a meteorologist but as a long-time Plains chaser (and painfully aware of how much I don’t know)… Writing this more to test my own understanding and hope  hm2 and others will set me straight if needed…

It’s not literally correct to say that strong downdrafts inhibit tornados. Like other parameters, a balance is required. A supercell that becomes outflow dominant undercuts it’s own updraft. Or the rear flank downdraft (RFD) ultimately occludes the mesocyclone and can spell the end of a tornado. When cells do not remain discrete, the outflow from one can disrupt another, or their outflows merge into cold pools that ultimately help foster upscale growth into an MCS. Supercells do generally have very strong updrafts, especially the RFD, so it’s still surprising there wasn’t more straight-line wind damage. In fact, tornadogenesis theories say that the RFD is a necessary ingredient for tornados - specifically, a warm/moist RFD (note Anthony mentioned conditions helping to prevent downdrafts that were too strong *and cold*). But any tornado warned supercell on the Plains is going to also be warned for winds of ~60 mph in addition to the tornado. 
 

Things are calm pre-tornado because in the updraft region the air is going UP, so there is no sensation of horizontal wind unless you are in the inflow jet. In my experience, in a classic Plains supercell that is not completely rain-wrapped, there is usually a bit of “space” between the tornado on the ground and the broader circulation from the RFD that wraps around the mesocyclone. There is also separation between the forward flank downdraft and the mesocyclone.

Jim, your thoughts look good to me. The RFD and FFD contributions to tornadogenesis remain a big research topic. Since tornadoes have been both observed and not observed in warm/cold RFDs, it appears that it doesn't solely determine tornado potential. In fact, occluding (sometimes more than once) is often associated with the mature stage of tornadoes (and then their death) and new tornadoes can quickly spin up next in cycle if the vorticity is channelized in the buoyant inflow properly. It's when the shear is not as streamwise where trouble starts with the mesocyclone and sub-tornadic vortices not pairing up. In addition, the RFDs end up overpowering in this type of shear and preventing  tornadogenesis. There are also non-buoyant ways a supercell can lift horizontal vorticity and stretch column via the vertical pressure perturbation (environmental shear induced process with mesocyclone). Finally, contributions from the FFD via baroclinic instability can induce sub-tornadic vortices that end up producing tornadoes.

Having said that, in this area especially, we are usually concerned with RFDs and storm interactions. We usually lack a traditional EML/cap, and interacting cold pools are usually part of the routine. Having a very moist profile reduces the ability for storms to produce a dominating, cold RFD, and therefore widespread damaging winds (the mid level drying and steeper lapse rates arrived late here but are usually part of the equation in the Plains, creating much stronger RFDs). This process helped on 7/29 for sure to maintain the supercell threat, though. In addition, we had a strong supply of low level helical flow and buoyancy that sustained the mode/system of supercells. Without that very moist inflow with buoyancy and helical motion, the storms would have easily transformed into multicell chaos.

[hm2]

On 8/9/2021 at 11:41 AM, ACwx said:

Fantastic work here. I genuinely hope NWS employees can see this as it will help with warning times like you said. Helps answer some of my questions from the other day lol.

Logically it makes sense that the lowest levels of the atmosphere would be important with determining whether a rotating circulation aloft would connect with the surface. I appreciate your points about the sun perhaps being an inhibitor if it actually came out that day, seems counterintuitive at first but it makes sense when you walk through it. I'll have to keep these ideas in mind next time our region is under the threat of tornadoes from a non-tropical system (hopefully not anytime soon).

I look forward to seeing your dew point project regarding climate variability with tornadoes.

Thanks!

The sun point is just to get us to think about the various factors and what it could have meant with things like the LCL height, e.g. In the DC area, they still had supercells with some producing tornadoes, after receiving more heating than our area. Of course, their low level winds veered out more being away from the warm front and their LCLs rocketed upward. So, perhaps we would have seen something more like VA/DC with more damaging wind, perhaps significant, with a tornado or two, but the outbreak scenario may have shifted north to wherever the gradient/front ended up.

I'll keep you posted on the dew point stuff.

[hm2]

21 hours ago, Chubbs said:

Very nice write-up. As I recall, the July 27, 1994 mid-atlantic outbreak (Limerick+Avondale F3s) had some similarities:  moist, not overly warm, low cloud condensation level, front nearby.

If we were able to somehow build a climo of types of setups here, this would definitely be 1 of the ways we would do it. The others would be tropical systems or systems with cold-season like dynamics/winds that produce powerful squall lines and embedded tornadoes (i.e. autumn of 1989 and 2003). It seems that the Ekster EML setups usually end up favoring interior areas into New England more than here, but we've seen those come down our way as well. In fact, NE NJ usually ends up participating in those. The Mid Atlantic threats with upper lows over the Central Appalachians usually end up to our south but can sometimes include far S NJ into DE. Those usually bust colder/rainier here.

[Rainshadow]

Anthony,

Before I retired I remember being told (or maybe reading) about the importance of a lower LCL for tornadogenesis. Mike Gorse used to always say never trust a warm front.  It looks like this and many pieces of swiss cheese aligned to make this day possible. Fantastic write-up. 

[hm2]

1 hour ago, Rainshadow101 said:

Anthony,

Before I retired I remember being told (or maybe reading) about the importance of a lower LCL for tornadogenesis. Mike Gorse used to always say never trust a warm front.  It looks like this and many pieces of swiss cheese aligned to make this day possible. Fantastic write-up. 

Wise words indeed.

[JimCaruso]

17 hours ago, hm2 said:

Having said that, in this area especially, we are usually concerned with RFDs and storm interactions. We usually lack a traditional EML/cap, and interacting cold pools are usually part of the routine. Having a very moist profile reduces the ability for storms to produce a dominating, cold RFD, and therefore widespread damaging winds (the mid level drying and steeper lapse rates arrived late here but are usually part of the equation in the Plains, creating much stronger RFDs). This process helped on 7/29 for sure to maintain the supercell threat, though. In addition, we had a strong supply of low level helical flow and buoyancy that sustained the mode/system of supercells. Without that very moist inflow with buoyancy and helical motion, the storms would have easily transformed into multicell chaos.

Hi Anthony, thanks for the additional info. Question, when you wrote “This process helped on 7/29...” were you referring to the moist profile limiting dominating/cold RFDs, OR the arrival of the mid-level drying and steeper lapse rates?

[hm2]

6 hours ago, JimCaruso said:

Hi Anthony, thanks for the additional info. Question, when you wrote “This process helped on 7/29...” were you referring to the moist profile limiting dominating/cold RFDs, OR the arrival of the mid-level drying and steeper lapse rates?

It was referring to the moist profiles preventing dominating RFDs point.

[tombo82685]

Anthony, awesome write up, thank you for posting this. I was one of them who dismissed this that afternoon due to the lack of sun around and gave you all the credit in the world on Twitter for this. Just was a very unusual setup for this area which I believe caught so many people off guard. 
 

what was the surface or I guess h5 setup that led to the strong low level shear? 

[hm2]

1 hour ago, tombo82685 said:

Anthony, awesome write up, thank you for posting this. I was one of them who dismissed this that afternoon due to the lack of sun around and gave you all the credit in the world on Twitter for this. Just was a very unusual setup for this area which I believe caught so many people off guard. 
 

what was the surface or I guess h5 setup that led to the strong low level shear? 

Tom, you didn't do as bad as you think you did, but I admire your approach to verification. It doesn't do anyone any good to live in a reality where they think they always do well (some big accounts on Twitter act like they always get it right). In fact, you were one of the few being honest about the day's confusing set of conditions and not writing off anything totally. And here's the thing, perhaps if we repeated this setup with similar conditions another 10 times, it wouldn't work out quite to the extent that this one did. Your approach being objective and listing the pros/cons is a good one that will work overall over time. Like I said in the other thread, I was torn on how I felt about this one. Sometimes a freakish event like the largest SE PA/NJ tornado outbreak is simply not going to be anticipated well 6-12+ hours ahead of time. This is perhaps best exemplified by the fact that the SPC issued quite a low probability tornado watch for our area (and who better understands this stuff than the SPC). That's why I think it would be a good idea to work on a more localized climo for the area instead of being lumped into the broad Northeast or Mid Atlantic ones that exist. We have always felt like the in-between-spot in terms of severe climo.

So, I know most are well aware of the MCS/MCV and general synoptic setup. They know that we had an incoming mid level jet, NW flow regime on the mouth of an incoming upper jet. If your question is about what happened synoptically that could give clues, I think one thing I haven't seen mentioned is how this cyclone developed. The overall digging of SLP / low level circulation into NY/New England before rapidly deepening off the coast I think helped intensify the 0-2km SR flow. The gradient from the very hot Corn Belt to the cold that was over Canada I think made the cyclogenesis event impressive for the time of year. In terms of the low level shear, I think a lot of that is because of the prior day's MCS that developed a strong LLJ. This helped increase the 0-2km flow overall and it was superimposed on an intensifying background gradient wind.

[MGorse]

This is a fantastic thread! Thanks for the awesome analysis and discussion.

[tombo82685]

On 8/11/2021 at 3:18 PM, hm2 said:

Tom, you didn't do as bad as you think you did, but I admire your approach to verification. It doesn't do anyone any good to live in a reality where they think they always do well (some big accounts on Twitter act like they always get it right). In fact, you were one of the few being honest about the day's confusing set of conditions and not writing off anything totally. And here's the thing, perhaps if we repeated this setup with similar conditions another 10 times, it wouldn't work out quite to the extent that this one did. Your approach being objective and listing the pros/cons is a good one that will work overall over time. Like I said in the other thread, I was torn on how I felt about this one. Sometimes a freakish event like the largest SE PA/NJ tornado outbreak is simply not going to be anticipated well 6-12+ hours ahead of time. This is perhaps best exemplified by the fact that the SPC issued quite a low probability tornado watch for our area (and who better understands this stuff than the SPC). That's why I think it would be a good idea to work on a more localized climo for the area instead of being lumped into the broad Northeast or Mid Atlantic ones that exist. We have always felt like the in-between-spot in terms of severe climo.

So, I know most are well aware of the MCS/MCV and general synoptic setup. They know that we had an incoming mid level jet, NW flow regime on the mouth of an incoming upper jet. If your question is about what happened synoptically that could give clues, I think one thing I haven't seen mentioned is how this cyclone developed. The overall digging of SLP / low level circulation into NY/New England before rapidly deepening off the coast I think helped intensify the 0-2km SR flow. The gradient from the very hot Corn Belt to the cold that was over Canada I think made the cyclogenesis event impressive for the time of year. In terms of the low level shear, I think a lot of that is because of the prior day's MCS that developed a strong LLJ. This helped increase the 0-2km flow overall and it was superimposed on an intensifying background gradient wind.

Yea I saw the shear present and that certainly raised my eyebrow. I certainly put the notion out there that it could be a bad svr outbreak but just got caught up in the lack of sun which proved to be wrong. I try to be up front with people and share my thoughts. Not on there to be a know it all and what not, just want to be real with people. 
 

I’m not sure if you have been told this but you are definitely the GOAT with weather. Have learned a ton from what you have posted and really appreciate the threads you have here 

[hm2]

On 8/10/2021 at 1:14 PM, hm2 said:

 

Jim, your thoughts look good to me. The RFD and FFD contributions to tornadogenesis remain a big research topic. Since tornadoes have been both observed and not observed in warm/cold RFDs, it appears that it doesn't solely determine tornado potential. In fact, occluding (sometimes more than once) is often associated with the mature stage of tornadoes (and then their death) and new tornadoes can quickly spin up next in cycle if the vorticity is channelized in the buoyant inflow properly. It's when the shear is not as streamwise where trouble starts with the mesocyclone and sub-tornadic vortices not pairing up. In addition, the RFDs end up overpowering in this type of shear and preventing  tornadogenesis. There are also non-buoyant ways a supercell can lift horizontal vorticity and stretch column via the vertical pressure perturbation (environmental shear induced process with mesocyclone). Finally, contributions from the FFD via baroclinic instability can induce sub-tornadic vortices that end up producing tornadoes.

Having said that, in this area especially, we are usually concerned with RFDs and storm interactions. We usually lack a traditional EML/cap, and interacting cold pools are usually part of the routine. Having a very moist profile reduces the ability for storms to produce a dominating, cold RFD, and therefore widespread damaging winds (the mid level drying and steeper lapse rates arrived late here but are usually part of the equation in the Plains, creating much stronger RFDs). This process helped on 7/29 for sure to maintain the supercell threat, though. In addition, we had a strong supply of low level helical flow and buoyancy that sustained the mode/system of supercells. Without that very moist inflow with buoyancy and helical motion, the storms would have easily transformed into multicell chaos.

Check out Leigh Orf's work to learn more about the importance of the streamwise vorticity current (SVC) that originates from the front flank downdraft. Here's a good paper that talks about it, but visit the main page too.

https://journals.ametsoc.org/view/journals/bams/98/1/bams-d-15-00073.1.xm

https://orf.media/

[SouthernNJ]

I sent a message to Mount Holly on Saturday with an inquiry into any additional tornadoes from the event, specifically the damage near Hellertown and the TDS west of Lakehurst.  They said that, unless new info came in, that the structural damage near Hellertown would be classified as wind damage and not a tornado.  They said that the images from above the forest were still being looked at west of Lakehurst.  Interestingly enough, not even 12 hours after the message, a PNS was sent out stating that they had confirmed the Lakehurst forest tornado.

 

With the surveys seeming to be completed now, I can do a final summary.  If something else should arrive, I will just update it later.  The 10 tornadoes are the most in a single day in the Mount Holly CWA history, passing the 9 on 11/16/89 and 7/27/94.  If you count the Lebanon and Essex county tornadoes nearby, 12 tornadoes occurred in a 5-hour window roughly between 4 and 9 PM across the region.  14 tornado warnings were issued on 7/29/21, which is not close to the record of 20 issued on 8/4/20 with the system of Isaias moving through the area.  8 tornadoes were confirmed with Isaias.  3 of the 10 tornadoes were rated EF-2 or higher (Washington Crossing area, High Bar Harbor area, Trevose area).  9 out of 14 tornado warnings verified, with 3 of the false alarms having either waterspouts, funnel clouds, or tornado-like wind damage occurring in them.  Although a couple were late, all 10 tornadoes were warned prior to the ending points.  Mount Holly has only ever verified more than 9 warnings once in an entire year (2020), let alone one day.  The New Hope-Washington Crossing EF-2 had a 27-minute notice before it began, and the Barnegat-High Bar Harbor EF-2 had a 24-minute notice.  The EF-3 was the latest, with the warning sent out 3 minutes after the tornado was in progress.

 

Interesting note, the Trevose EF-3 and NE Philly EF-0 were occurring simultaneously at 7:07 and 7:08 PM, from two very different parts of the supercell thunderstorm.  The EF-0 was said to be spinning anticyclonic, which is less than two percent of all tornadoes.  In the first image from KDIX at 7:09 PM, you can see the NE Philly tornado formed inside of the more standard appendage, yet the Trevose tornado rapidly blossomed inside of the bookend vortex.  Usually more common with bowing-line segments, seeing a bookend tornado occur within a supercell that is also producing one in its main region during the same time was fascinating to say the least.

With TPHL at 7:07 PM, you can see the "reverse" couplet compared to the one above it, denoting where the apparent anticyclonic tornado occurred.

[hm2]

This is a nice summary by the NWS and the radar tab has excellent information. It makes you think about how much worse this could have been.

https://www.weather.gov/phi/eventreview20210729

[phillyflip]

17 hours ago, hm2 said:

This is a nice summary by the NWS and the radar tab has excellent information. It makes you think about how much worse this could have been.

https://www.weather.gov/phi/eventreview20210729

Thanks for posting this.  A quick question, the overview states there was "unseasonably strong wind sheer."  That statement leads me to believe it wouldn't have been considered as strong in a different season, but I'm not sure when that would be other than maybe early Fall?  My guess is the wind sheer experienced that day would've been considered unseasonably strong in any season, but the fact that they called it out makes me ask the question.  Thanks for any insights.

[hm2]

12 hours ago, phillyflip said:

Thanks for posting this.  A quick question, the overview states there was "unseasonably strong wind sheer."  That statement leads me to believe it wouldn't have been considered as strong in a different season, but I'm not sure when that would be other than maybe early Fall?  My guess is the wind sheer experienced that day would've been considered unseasonably strong in any season, but the fact that they called it out makes me ask the question.  Thanks for any insights.

Regardless of how you define the shear type, in general you'll see the climatological minimum in late July into August (and maximum in the winter). This is the time the westerlies are at their most poleward in the summer. You can get sounding climatology here: https://www.spc.noaa.gov/exper/soundingclimo/

I've attached the bulk shear 0-6km OKX July 30th plot for example purposes.

[phillyflip]

11 hours ago, hm2 said:

Regardless of how you define the shear type, in general you'll see the climatological minimum in late July into August (and maximum in the winter). This is the time the westerlies are at their most poleward in the summer. You can get sounding climatology here: https://www.spc.noaa.gov/exper/soundingclimo/

I've attached the bulk shear 0-6km OKX July 30th plot for example purposes.

Very interesting and kind of the exact opposite of the way I had it in my mind.  Thanks for clarifying and sharing the source data.