hm2 Posted August 9, 2021 Report Share Posted August 9, 2021 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. 25 Quote Link to comment Share on other sites More sharing options...
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