What the storm feels
Supercell storms come in all shapes and sizes. Over time, pattern recognition and intuition might begin to tell you that gentle flow over western Kansas the last week of May with a rocking low-level jet may signal the arrival of those otherworldly stationary “mothership” supercells, while a powerful negatively-tilted trough digging over 60s dewpoints in the Midwest the last week of November likely means that any long-track, strong tornadoes will sit beneath anticlimactically grungy, linear walls of cloud that rarely show the classic visual cues of rotation.
To the experienced forecaster and storm spotter, even a still photo of a supercell cloud structure or a frame of radar reflectivity will provoke insights into how this supercell will operate; for example, if shown nothing but a radar likeness of the linear, “string-bean”-style supercell found in a late November lightning-fast tornado outbreak, a seasoned chaser would likely not tell you that he envisions a “MINDBLOWING STATIONARY MOTHERSHIP“. You probably wouldn’t either. Why?
Do we use hodographs to their full potential?
The hodograph has for some time now been the primary way to visualize wind shear in the atmosphere, and how it might relate to severe convective storms and their morphology. Much has already been explored regarding its influence on supercellular processes, including splitting, deviant propagation, and tornado potential. Aside from the recognition of some basic common shapes (“curved” vs. “straight”), operational use of the hodograph in the storm-relative sense has been largely limited to using derived quantities such as Storm-Relative Helicity (SRH), and a few “rules of thumb” (for example, anvil-level flow’s link to the precipitation spectrum (LP vs. HP), or near-surface storm-relative flow’s association with tornado potential).
In the daily forecast routine, aside from analyzing maps of SRH, knowing these basic hodograph shapes and their associations, and occasionally referencing these other rules of thumb for a particularly savvy forecast, the storm-relative hodograph in all its glory is largely shoved under the carpet. Why? After all, the storm-relative wind profile is the wind profile that by definition governs each and every supercell and is thus overwhelmingly responsible for its behavior.
Wind shear organizes all convection. But as if in fitting metaphor to the lack of value we have given the hodograph, common practice has been to shove this all-knowing little tool into the corner, where the important details are lost.
Why don’t we look at hodographs in storm-relative? Well, the short answer is you are already doing so. Everything’s right there in front of you on your familiar standard hodograph. However, you must first build an intuitive concept of the storm-relative hodograph before you can see it.
Seeing in storm-relative
Creating an along-storm-relative hodograph is easy:
- Start with the standard, ground-relative hodograph:
2. Take this ground-relative hodograph and subtract storm motion; this gives you the storm-relative hodograph (north-relative):
(look, we’ve recovered the missing upper-level winds!)
3. Lastly, take this storm-relative hodograph and rotate it by the direction of storm motion; this gives you the along-storm-motion-relative hodograph:
That’s it. Doing so minimizes any biases/distractions from:
- Abnormally fast or slow storm motions
- (Fast storm motion does not always mean high shear!)
- Abnormal direction of storm motion
- (It’s always weird to see the hodographs of northwest-moving supercells, but in reality, it’s only the shape that matters to the storm!)
- Abnormally-oriented hodographs created by non-classic flow patterns
- (We don’t always need “backed surface winds” for tornadoes; storm-relative hodographs allow us to forget these silly myths!)
- Wind profiles that run “off the chart”, hiding valuable upper-level flow data
- (Did you realize in some cases the wind is cut off as low as 4km? Do you think that matters? (it does!))
The result is a powerful tool; it normalizes all of the seemingly-infinite hodograph shapes into what the supercell actually experiences… and whereas no two ground-relative hodographs are alike, you’ll find that patterns may actually repeat themselves here! A supercell is an engine; here, every single turn and kink with height (or lack of segment altogether!) has a profound impact on the wind-sculpted supercell, its appearance, behavior, and impacts.
Now, let’s take a look at some hodographs through this new lens!
Tweaking the storm-relative winds
Remember, in the along-storm-relative hodograph, storm motion is upward (along the y-axis). What likely first stood out to you in the above few along-storm-relative hodographs is that, for the vast majority of right-moving supercell cases, a common orientation is as follows:
- a 0-1km wind in the lower left quadrant (if you are the storm, blowing in from your front right)
- a 1-6km wind in the upper left quadrant, typically veering toward the front center (if you are the storm, blowing in from your right side then becoming more from your back with height)
- a 6-12km wind somewhere in the upper quadrant with a variety of shapes
Now that we’ve got a standardized hodograph, let’s start tweaking certain parts of it to see what they do.
Upper-level storm-relative winds
The upper-level storm-relative winds (here, 6-12km) are perhaps the easiest to visualize; since these winds influence how precipitation gets vented out of the storm, they impact everything from radar echoes to anvil orientations, and even whether we perceive a storm as low- or high-precipitation.
The speed of the upper-level storm-relative winds influence how far precipitation is vented out of the storm. Consequently, studies have already shown that weak storm-relative wind speeds in this layer encourage high-precipitation (HP) supercells, while strong storm-relative winds help promote low-precipitation (LP) supercells.
The direction of this wind, however, can also say a lot about where this precipitation is placed.
Weak upper-level storm-relative winds
Weak upper-level winds reduce the ventilation of precipitation away from the updraft, so most of the rain and hail will fall close to the storm. This typically results in RFDs filled with rain and hail, and “high precipitation” storms. The author’s preferred “danger zone” for HP storms is the 20 kt range ring; storms with most of their 6-12 km storm-relative winds less than 20 kt are likely to be HP.
Strong upper-level storm-relative winds
Strong upper-level winds vent precipitation well away from the updraft, yielding crisp rain-free bases, dry RFDs, and “low precipitation” storms. The author’s preferred threshold for LP storms is the 40 kt range ring; storms with at least some of their 6-12 km storm-relative winds greater than 40 kt will tend to be more LP, even in very moist environments.
A veering of the hodograph into the right-front quadrant wraps rain clear around the front of the storm, creating large precipitation shields which block much of the updraft from an observer who stands in its path; these storms will consequently move into their own forward flank downdrafts.
A backing of the hodograph into the left-front quadrant keeps precipitation from wrapping around the storm, creating narrower precipitation shields and yielding excellent views of the updraft.
Mid-level storm-relative winds
Mid-level storm-relative winds (here, 1-6km) are perhaps hardest to visualize, but play arguably the most important role in supercell behavior.
The strength of the shear in this layer (*especially* the lower half of it) is most important for the maintenance of broad, persistent mesocyclones. Shear above the boundary layer is critical for supercell storms. However, “supercells” (explored below) are still possible without it.
The streamwise and crosswise nature of the storm-relative winds also have profound impacts on supercell structure that are quite easy to visualize.
Let’s take a look at how mid-level winds control supercell dynamics.
Weak mid-level shear
Weak mid-level shear does little to support broad, persistent mesocyclones. However, “supercells” can still form in weak-to-zero mid-level shear given enough low-level shear. “Supercells” are distinguished here by their compact or “mini” mesocyclones and “pulse-like” behavior; often, they consist only of a strong tornado cyclone and a precipitation core. Once the tornado dissipates, then, it looses its rotation, and becomes mostly multicellular and non-severe. These are most commonly found in the Southeast U.S. and Mid-Atlantic.
Strong mid-level shear
Strong mid-level shear can support broad, persistent mesocyclones, even in unfavorable thermodynamics. Even with a lack of low-level shear, supercells born in strong mid-level shear can become very large and well-organized. These may “cycle” through numerous tornado cyclones, while continuously producing all hazards. These are most commonly found in the Great Plains.
Streamwise mid-level storm-relative winds
Streamwise storm-relative winds are winds which blow parallel to the environmental shear vorticity vector (which points to the left of each shear vector). In other words, storm-relative winds are streamwise if they intersect the hodograph at a 90 degree angle. In streamwise shear profiles, the updraft rotates as a whole.
Crosswise mid-level storm-relative winds
On the contrary, crosswise storm-relative winds are winds which blow perpendicular to the environmental shear vorticity vector. In other words, storm-relative winds are crosswise if they point in the same direction as the hodograph. In crosswise shear profiles, the updraft does not rotate as a whole, but develops rotating flanks. Intense mesocyclones and tornadoes are still possible, however, especially if ample streamwise vorticity exists near the surface.
Remember, supercells need horizontal vorticity, but it does not necessarily need to be streamwise. Supercells can still occur with zero streamwise vorticity (0 SRH).
What really is SRH?
(and how you’re likely using it wrong)
As an important aside, let’s break down this most commonly used storm-relative quantity. It’s quite useful; however, it’s just as commonly abused.
If you have ever pulled up a plot of 0-3km “SRH” before to diagnose the potential for rotating storms, you too have been an abuser. Of course, SRH is referenced to ubiquitously in weather discussions to predict storm mode and supercell formation. For instance, you’ve probably heard something along these lines:
So, what’s actually wrong here? This is definitely the way it’s been done. And of course, years of research have backed the concept that there exists a considerable correlation between high SRH and supercell environments. However, let’s talk about when and how this breaks down, and what to turn to instead.
First, SRH throughout what depth? That’s pretty important (the real-life example has us covered, sort of, with “effective”, but there are also innumerable examples of “layer-less” SRH out there). As discussed above, for example, the 0-1km layer can have profoundly different impacts than the 1-3km layer. While 0-3km SRH can make for good suggestions of overall mesocyclone strength, the behaviors of storms can be wildly different depending on the distribution of this SRH.
Second, SRH relative to what storm? When in doubt, “SRH” has become the standard shorthand we’ve created for the storm-relative helicity obtained by the Bunkers Right deviant storm motion. So what if we’re trying to predict if supercell formation is possible? This is a problem, because we’re assuming that storms have already gone deviant (supercellular). The irony here is that before an updraft acquires rotation, it is *not* propagating with Bunkers Right (what every supercell-based parameter you use operates on). Therefore, when diagnosing the potential for supercell storms, using SRH like we’re accustomed to is nothing but a big dynamic fallacy (and, you guessed it, won’t give you the best results)!
Third, though SRH is a single quantity proportional to the area under the curve, for the sake of discussion it can be thought of like a composite parameter, as it includes both the strength of the storm-relative winds as well the streamwise vorticity. Each of these do very different things (sort of like CAPE and SRH in the Energy Helicity Index), so the exact same amount of SRH in one case is unlikely to behave the same way in another case (just as the same EHI in one case is unlikely to behave the same way in another case).
Fourth, there is more to shear than just SRH. In some cases, using only SRH to forecast supercells may lead you into big trouble. That’s because we’re neglecting the other component of shear vorticity – crosswise. Supercells need vorticity from the shear to organize and maintain themselves; however, this shear need not be streamwise. When using SRH, we’re assessing only the streamwise shear, and none of the crosswise. Why we chose to neglect crosswise from the forecast process in the beginning is still a head-scratcher, but it’s no less important for supercell formation and maintenance. Of course, crosswise shear is much less effective at producing sustained, intense tornadic mesocyclones; however, storms dominated by crosswise shear can still become powerful and produce very large hail.
Using SRH (the right way!)
The good news is, there is lots of room to improve upon supercell forecasting, using just the above concepts.
First: better communication of the distribution of SRH will assist forecasters in recognizing the character of the storms expected. For instance, while “300 SRH” is pretty common notation, “300 0-1km SRH” immediately rings bells of strong, perhaps even intense tornadoes, while “300 1-3km SRH” means some particularly large and persistent parent supercells are possible.
Second: using mean wind SRH instead of right-mover SRH will significantly improve predictions of supercell formation, as well as minimize “busts”. Amongst the most particularly misleading are days where right-mover SRH is high but mean wind SRH is near-zero, or even negative. Knowing that mean wind SRH is high will give reassurance that quick formation of supercells is likely, while days where mean wind SRH is low may have a high potential for false-alarm forecasts in some situations.
As a separate forecast tool, right-moving SRH is still very dynamically useful in assessing the potential strength of mesocyclones. Since deviant storms are likely to take a trajectory similar to Bunkers’ Right, SRH is a dynamically correct tool to gauge the shear vorticity they have available to them. Think of it this way – while mean wind SRH is the actual SRH available to maturing storms, right-moving SRH is the potential SRH that they could achieve, thus an excellent measure of their potential intensity.
Third: better distinguishing of the effects of streamwise vorticity and storm-relative inflow will significantly improve predictions of supercell size and intensity. Current work has already shown the impacts of storm-relative inflow on supercell updraft size. Knowing when SRH is high due to high streamwise vorticity and weak storm-relative winds, or instead due to low streamwise vorticity and strong storm-relative winds, can make all the difference in a forecast.
Fourth: mainstream use of the crosswise component and/or total storm-relative vorticity will also significantly improve predictions of supercell formation and potential intensity. Currently, common practice is to use deep-layer shear alongside SRH to predict supercells. While this has achieved relative success, this is not dynamically correct. Use of total vorticity in addition to just the streamwise component will ensure that far fewer instances of robust supercells fly under the radar, with no increase in false alarm rate.
What is supercell shear?
Wind shear (technically, bulk wind difference) is commonly the only other shear quantity used to predict supercells and tornadoes. Assessing shear the right way can lead to some very well-informed predictions of supercells (technically better than SRH and even total vorticity, because it doesn’t rely on a storm motion assumption that might be wrong!). However, even simple shear can be abused in the forecast process.
We would benefit from breaking down wind shear like we did with SRH above, in order to better assess the potential for tornadoes versus supercells. Of course, we do this already, typically by using 0-1km shear and 0-6km shear. However, this 0-6km shear can run us into some problems. Since we’ve established above that the 0-1km shear is of most importance for tornadoes, and the 1-6km shear is crucial for supercells, what does the 0-6km shear actually do? Though this shear is a good long-range indicator of shear necessary for severe storms, here’s where using only 0-6km shear while anticipating supercells and their hazards may cause more harm than good:
As explored above, 40kt of 0-6km shear can mean, for example,
- 10kt of 0-1km shear and 30kt of 1-6km shear (great for supercells, poor for tornadoes),
- 30kt of 0-1km shear and 10kt of 1-6km shear (great for tornadoes, poor for supercells),
- 20kt of 0-1km shear and 20kt of 1-6km shear (not that great for either), and even
- 40kt of 0-1km shear and 40kt of 1-6km shear (particularly great for both!), depending on hodograph curvature
This is likely easier to see visually. The following hodographs are regular storm-relative hodographs, and all possess roughly 40kt 0-6km shear:
Though all hodographs have the same 0-6km shear, they still have wildly different shapes! So, while 0-6km shear may be high on a certain day, there can still be many scenarios where shear is in reality rather marginal for supercells, and other cases where the same amount of 0-6km shear may create much larger and stronger supercells than expected. Therefore, using 0-6km shear thresholds to forecast supercells may leave you with some pretty big surprises, both where supercells were much more marginal than predicted, or where they caused a lot more damage than anticipated.
While the author’s preferred secret weapon for forecasting supercells is the 1-3km shear, the main point here is to acknowledge that when using shear to forecast supercells, the 0-6km layer most often ends up hiding some of the most crucial details.
Common hodograph shapes
Here’s a few common hodograph shapes for supercell storms that you’ll likely see out in the wild. These are regular storm-relative hodographs, so they’ll look a bit more like you’re used to.
Though most are named by the region where they are most commonly found, it’s very important to recognize that these shapes can occur anywhere (video locations may reflect this).
Ohio Valley Outbreak
This type is found most often around the Ohio River Valley, especially from March-April (with a potential secondary peak around June) in organized severe weather outbreak scenarios, and is characterized by strong, mostly streamwise low-level shear and veering upper mid-level flow. Though lack of lower mid-level shear may make for some “grungier” and less organized storms at times, strong cloud-layer shear typically allows for longer-lived supercells with the potential for significant tornadoes and severe wind gusts.
This type is found most often in central OK and southern KS around May (though similar shapes may filter into the Ohio River Valley, especially from March-April), and is characterized by moderate, mostly streamwise low-level shear and strong mid-level shear. These supercells tend to be large and rather linear, and given robust supercell and tornado dynamics, this type often carries the potential to produce large, very long-lived, violent tornadoes, along with significant-severe hail and severe wind.
Nocturnal Low-Level Jet
This type is found most often from the TX/OK Panhandles into central KS, especially from May-June, and is characterized by moderate, partly crosswise low-level shear and strong mid-level shear, with otherwise weak deep-layer shear. These supercells tend to be large with a circular front and deep rearward occlusions, and is rapidly cyclic, with the potential for highly deviant, significant tornadoes and significant-severe hail.
This type is found most often in central KS, especially from May-June, and is characterized by weak low-level shear and strong, purely streamwise mid-level shear. These supercells tend to be large, nearly perfectly circular, and often almost stationary, and carry the potential to produce significant-severe hail, as well as a couple weak tornadoes.
This type is found most often from W TX northward into E CO, especially from late May-July, and is characterized by weak low-level shear and moderate, purely streamwise mid-level shear. These supercells may be small at times, but given robust supercell dynamics, may possess strong and tight mesocyclones with the potential to produce significant-severe to extreme hail, as well as weak-to-significant tornadoes (especially given a surface boundary).
High Plains Magic
This type is found most often from E CO northward into E WY and SE MT, especially from late May-July, and is characterized by moderate low-level shear and strong, mostly streamwise mid-level shear. These supercells tend to be large, and given particularly powerful supercell and tornado dynamics, this type often carries the potential to produce large, long-lived significant-to-violent tornadoes (even in hostile thermodynamic environments) in slowly cyclic fashion, along with significant-severe hail.
This type is found most often from E CO into the TX/OK Panhandles (though may also appear almost anywhere up the High Plains), especially from June-July, and is characterized by weak low-level shear but very strong, mostly streamwise mid-level shear. These supercells tend to be very large, with very robust supercell dynamics that allow for storms to persist well into barely hospitable thermodynamic environments, with the potential for significant-severe hail and significant-severe wind.
Northern Plains Hailer
This type is found most often from North Dakota into Nebraska, especially from July-August (though similar shapes may be found from western OK into north-central TX especially around May), and is characterized by weak, largely crosswise low-level shear and very strong mid-level shear. These supercells tend to be very large and carry the potential to produce extreme hail, and perhaps a couple weak tornadoes.
This type is found in a variety of locations, especially around the central US from W KS into C IL, at almost any time of the year, and is characterized by weak-to-moderate (at times largely crosswise) low-level shear, weak lower mid-level shear, and strong upper mid-level shear. These supercells tend to be small, and carry the potential to produce several weak-to-significant tornadoes, but are often otherwise only marginally severe.
This type is found most often in a variety of locations east of the Mississippi River, particularly from the corn belt into the Ohio River Valley, especially from July-August, and is characterized by weak-to-moderate, purely streamwise low-level shear, and weak, streamwise mid-level shear. These supercells tend to be very small, and carry the potential to produce several weak-to-significant tornadoes, but are usually otherwise non-severe (even failing to produce lightning).
This type is found in a variety of locations, especially around the south/central US from C OK/KS into the northern Ohio River Valley, especially bookending peak severe weather season from March-April and from October-December, and is characterized by strong, mostly streamwise low-level shear, and strong, crosswise mid-level shear. These supercells carry the potential to produce very long-lived, violent tornadoes, and severe hail.
This type is found most often in the southeast U.S., especially from MS into GA and occasionally up to TN, especially from November-April, and is characterized by strong low-level shear and strongly veering upper mid-level shear. These supercells may possess large precipitation shields and produce copious amounts of rain, and carry the potential to produce long-lived, significant-to-violent tornadoes.
This type is found up and down the East Coast, particularly from GA northward into MD, especially from March-April (though is also regularly found as far west as MS and as far north as S IL during the winter months), and is characterized by very strong low-level shear, and weak mid-level shear. These supercells may be very compact and consist mostly of a large tornado cyclone only (and be frequently associated with QLCSs), and carry the potential to produce very long-lived, significant-to-violent tornadoes, and severe wind, but struggle to remain organized and even dissipate when not producing a tornado.
Special thanks to: Matt Wilson (UNL), Alex Schueth (TTU), and Will Wight (DTN)
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