The Storm-Relative Hodograph

Supercell Appearance, Behavior and Impacts

First off: we’re going to take a deep-dive into how storms “feel” the winds around them. We will go in-depth at times; if you feel it getting too technical for your liking, just scroll down to the “In the Wild” section, where you’ll find a practical field guide to many of the different types of supercells you’ll encounter as a spotter/forecaster.

Motivation

Supercells 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 that poster-child spaceship-like stationary supercell, 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?

Screenshot from 2018-09-02 13-26-09
A cold-season significant-to-violent tornadic hodograph, observed in close spatio-temporal proximity to a long-track EF-4-producing supercell.

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. Diagnosing these from a storm-relative sense has been largely lead by understanding basic hodograph shapes (“curved” vs. “straight”), and by using derived quantities such as Storm-Relative Helicity (SRH). Certain standalone “rules of thumb” have also made their way into the forecast process (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).

However, in a daily forecast routine, aside from analyzing maps of SRH, knowing these basic hodograph shapes and their associations, and occasional 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? 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.

Why don’t we look at hodographs this way? 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.

Methodology

Want me to just explain it verbally? Check out the first few minutes of this walkthrough!

***Note my error at 8:10; upper-level wind should be at our backs!

Creating an along-storm-relative hodograph is easy:

  1. Take the standard hodograph and subtract storm motion; this gives you the north-relative storm-relative hodograph
  2. Take the north-relative storm-relative hodograph and rotate it by the direction of storm motion from north; this gives you the along-storm-motion-relative hodograph

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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
    • (Sure it looks weird our storms are moving northwest, but do we care?)
  • Abnormally-oriented hodographs created by non-classic flow patterns
    • (We don’t always need “backed surface winds” for tornadoes)
  • Wind profiles that run “off the chart”, hiding valuable upper-level flow data
    • (Did you realize it stops at 4km? Do you think that matters? (YES))

The result is a powerful tool; it normalizes all of the seemingly-infinite shear profile possibilities into what the supercell actually experiences. 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 shape of a storm, and interestingly, a number of patterns begin presenting themselves. (But don’t worry, I kept a standard hodograph on the bottom right in case of too much head tilting).

(Special thanks to SharpPy for providing me with the means to explore these. All of the following hodographs are RAP model point soundings, with storm motion corrected for observed when necessary.)

There’s likely a lot of new info on here (we’ll trial by fire first and explain later), so for now just focus on the red (right-mover) hodograph line:

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A rather “quintessential” cold-season significant-to-violent tornadic hodograph, resulting in a long-track EF-4 tornado near Henryville, IN. Note strong storm-relative flow throughout, suggestive of a powerful synoptic system. Not also how the standard hodograph (bottom right) fails to show you any information above 4km! (**THIS IS VALUABLE INFORMATION!!**)
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In contrast, a “quintessential” warm-season significant-to-violent tornadic hodograph, resulting in twin EF-4s near Pilger, NE. Note weak storm-relative flow in the upper-levels caused by more subtle synoptic flow, and the necessity for a large, rounded hodograph structure in the low-levels to achieve similarly-supportive SRH.

Three (3) Hodographs in One?

Why are there 3 hodographs?? A typical hodograph displays one wind profile (centered at the origin) along with the right-moving (RM), mean wind (MW), and left-moving (LM) storm vectors. This method is inversed, showing all three storm motions (centered at the origin) along with their associated hodographs.

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A standard splitting supercell hodograph resulting in large hail near the DFW metroplex, illustrating nicely how a mean wind-moving storm with little to no SRH may split, with both right- and left-moving counterparts acquiring more SRH as they deviate from this hodograph over time.

Supercell structure and ventilation change significantly depending on their motion relative to the same hodograph; for proof of this, check out this image taken from a paper by Lindsey & Bunkers, 2005 (https://www.weather.gov/media/unr/soo/scm/LB05.pdf). These formed in a similar environment to the hodograph above; now we can visualize this process!

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Even with a constant westerly jet stream over the dryline, thunderstorm anvils are all over the place! This is observational proof of just one profound impact the storm-relative hodograph has on rotating updrafts.

We’re Using SRH Wrong.

As an important aside, if you have ever pulled up a plot of 0-3km “SRH” before to diagnose the potential for rotating storms, you too have only further supported my bold statement. But you’re not alone; “SRH” is referenced ubiquitously in weather discussions as the be-all, end-all for supercell formation. For instance, doesn’t “with storm-relative helicities approaching 350 m2/s2, initial supercellular mode appears likely” sound familiar?

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Indeed; here is an example (Storm Prediction Center (SPC), 5/27/17 – Day 1 1630z).

Now, it is not my goal to publicly criticize the time-driven decisions made by the meteorology community’s objectively best team of severe convective weather forecasters – especially when claims such as this have been backed for years off research showing considerable correlation between high right-moving SRH and supercell environments; however, it is my goal to examine ways we can improve these sort of statements.

Firstly, SRH throughout what depth? That’s pretty important (SPC has us covered in this case with “effective”, but I have personally witnessed “layer-less” SRH).

Secondly, SRH relative to what storm motion? 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. This is a problem; we’re trying to determine if supercell formation is possible, yet we’re assuming that storms are already deviantly propagating (supercellular).

The hard reality is that before an updraft acquires rotation, it is *not* propagating with Bunkers Right (what every supercell-based parameter in existence operates on). But is it okay to say “supercells could be particularly robust today given very high 0-3km SRH”? Of course! Bunkers Right SRH is the best measure of the potential SRH that a mature, deviant supercell will obtain (indeed, significant relationships have been found between this SRH and tornado potential).

Here’s how we could improve

In my humble opinion, it is foolish and essentially meaningless to use any derivative of this vaguely-colloquialized “SRH” to diagnose the likelihood that updrafts become supercells, because we know that no updraft can have this amount of vorticity available to them from the get-go. Curious as to why there are discrepancies around whether a clear threshold of “SRH” exists that separates non-supercell environments from supercell environments? This may speak to the answer.

What’s the solution then? Perhaps our best attempt to determining the potential for supercells would be to examine the “Mean Wind (MW)” hodograph. The available “MW SRH” may then have a significantly more robust relationship to differentiating supercell environments from non-supercell environments, though published studies have not yet been done.

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Look at that storm-relative veer-back! Cases like this with “N-shaped” low-level hodographs get tricky with regards to how much positive vorticity gets ingested by cells; sure, a right-moving supercell may have positive SRH from 0-1km; but the initial mean wind cell? A whole 2 kilometers worth (1-3km) of negative SRH. No supercells evolved from this environment. Perhaps the most fun part? Check out that ground-relative hodograph. You may not believe me at first, but there is absolutely no backing of the winds, but rather perfectly constant veering with height up to 7km, with some weakness in between. If you go by the veering with height rule, you are going to have a bad time…

Getting deeper into the theory here, because a storm acquires its vorticity in both streamwise- and crosswise-fashioned components, though, a measure of storm-relative crosswise vorticity ingestion would also need to be used as an addition. This has not caught on as a parameter, but would especially have use in straight hodograph environments, where minimal MW SRH exists but significant crosswise vorticity is ingested. For now, our use of bulk shear by nature approximates this, but still cannot give us the full story (plus, comparing knots to m2/s2 is a bit like comparing apples to oranges).

In a perfect world, I would prefer that we explored these measures of available streamwise, crosswise and total vorticities in their most basic form – vorticity. When dealing with severe convective weather, our daily routine may have us mashing shear (knots), SRH (m2/s2) and even near-surface vertical vorticity (/s) together! Yes, we’ve built our expertise and pattern recognitions around these apples, oranges and even bananas. But that doesn’t stop the fact that vorticity is vorticity; SRH is derived from vorticity, a change in wind with height can give us vorticity, and in its purest form a rotating storm is simply the result of the contribution of all of these sources.

Plotting Thermodynamics on a Hodograph?

Is this illegal? I hope not. But just as we have been trained to view wind profiles and certain shear indices with respect to a skew-T, it is also quite valuable to view important thermodynamic variables with respect to the hodograph. One of the most important examples is the equilibrium level. Quite literally, a storm can only be so tall; in the cold season months especially, if a storm tops out at only 9km, the wind above that simply doesn’t matter to a large extent. So having awareness of which parts of your wind profile can be “thrown out” is critical – this may completely change its effective shape!

As you may have guessed, LCL/LFC height may also be useful, especially in diagnosing boundary layer vs. deep-layer shear, etc.

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As the evening of a tornado outbreak in northern IL wore on, a sinking tropopause squished this supercell we were following! At this time, EL heights were around 7km, so this storm was not able to take advantage of the huge speed shear in the stratosphere above 8km. Forgetting this and factoring in this extra shear would have given a very different outcome.

Tweaking the Storm-Relative Winds

From now on let’s assume we have a right-moving supercell that moves with the RM (red) hodograph.

Remember, in the along-storm-relative hodograph, storm motion is upward (along the y-axis). What likely first stood out in the above 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 bottom left quadrant (if you are the storm, blowing in from your front right)
  • a 2-5km wind in the front 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)

Winds ~5km and up are a bit less uniform, however, but tell you increasingly more about different storm types. All we’re doing in this layer is rotating where (and with what strength) precipitation is vented out of the storm – but this has many implications. 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. However, the direction of this wind can also say a lot about where this precipitation is placed:

Veering

A veering of the hodograph into the right-front quadrant wraps rain clear around the front of the storm, shielding much of its updraft from an observer who stands in its path; these storms will consequently move into their own forward flank downdrafts.

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Watching this EF-3-producing New Orleans supercell approach would have been a bit of a pain; the “veer angle” greater than 180 degrees suggests that 0-6km storm relative winds veer (and wrap rain) well around the storm, obscuring most of the view. The storm would then be moving into its own precipitation core (“into core”).

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Keep in mind that hodographs do change over time! We can watch the effects of a rapid dose of veering on this tail-end supercell as it encounters a strong low-level jet. Not only does its motion deviate to the *left* with the added southerly flow, but its echo structure appears to rotate clockwise such that it starts moving into its own core.

Backing

A backing of the hodograph into the left-front quadrant shoves precipitation well away of the front of the updraft and “inflow region”, unraveling spectacular views of the entire mesocyclone as it approaches; these storms move into pure, uncontaminated air.

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On the contrary, this smaller hodograph features significant veer-back, with a “veer angle” much less than 180 degrees suggesting only a smaller fraction of the area surrounding the updraft receives rain. The result was this stellar supercell near Guthrie, TX whose features were visible from miles away!
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Photographers – don’t let these kind of hodographs go unnoticed. The beautiful Simla, CO supercell in its mature stages (https://www.youtube.com/watch?v=yVIkStupD8w) rode slowly south towards its observers with all precipitation pushed behind the storm, revealing the entirety of its barber-pole mesocyclone. This supercell moved directly into clear air (“into clear”).

Neutral

A neutral hodograph sticking to the front-center will offer the more classic view of rain and hail to the right, with half the mesocyclone emerging from the left. These storms move along their own forward flank precipitation gradients and have access to both rain-cooled downdraft air and warm inflow air, which may assist the tornado process (explained in more detail later).

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A rather neutral hodograph structure with respect to veering/backing.
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A little veer-back? No problem! Storms near Pampa, TX were still able to take advantage of very high low-level SRH and produce a couple powerful tornadoes; in fact, *many* especially long-track, violent tornadoes are produced from a similar hodograph, which I still like to call “classic”.
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If you had to guess which way this supercell was moving based solely off intuition, what would you say? If thought “to the right”, you’re correct, and not alone! Long inflow tails such as these are visual representations of baroclinic vorticity generated along the forward flank as a supercell moves along an axis where a pool of cooler, moist FFD air has become established to the left of warm inflow air. For this to happen, the storm-relative hodograph’s upper levels must extend nearly perfectly toward front-center, so the storm can move along its forward flank precipitation gradient. The vast majority of significant-to-violent tornadoes occur with a similar configuration.
Kanorado Kansas Mothership Supercell
Which way is this supercell moving? Right towards you actually! The beauty of storm-relative hodographs that extend towards left-center with height is that forward flank precipitation is neatly vented away, sometimes even tucked behind the storm, so mesocyclone viewing is prime. Storms with this configuration rarely produce significant tornadoes, though.

What about the mid-levels?

Mid-level storm-relative flow is also quite important to storm structures, and can begin bending the rules between what is considered “low-precipitation” or “high precipitation”.

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The Oakley, KS tornadic supercell (https://www.youtube.com/watch?v=AnAvULYPnM4) had an undeniably HP forward flank tendency; however, it also featured an expansive rear rain-free base. Aided by its speedy forward motion, winds from the surface even up to ~6km are pushing against the storm! The result? Much of its precipitation was pushed behind it, in a similar fashion as would a fast-moving bow echo! This was confirmed with timelapse imagery of the supercell, with rain curtains shunted over 90 degrees around and behind the storm as it pushed forward.
hodo8
In contrast, the infamous Bennington, KS supercell of 2013 (https://www.wku.edu/news/articles/index.php?view=article&articleid=2295) had the rather odd pairing of a very high-precipitation RFD region and a quite expansive low-precipitation forward flank region. Although weak upper-level storm-relative winds were encouraging of high-precipitation mode, very strong mid-level flow (compared to most supercell cases) may have encouraged this clean forward flank region.

Mesocyclone Tendencies

Though a supercell is synonymous with the word “rotation”, it is often perplexing at first to think that in many cases a mesocyclone is not a vortex. Commonly, air parcels rise into the supercell via its inflow and are immediately ejected out and away with strong upper-level storm-relative flow. The supercell in that case is simply a couplet of fast velocities and faster velocities. However, some supercells can indeed enter what can be considered like a sort of “solid body rotation”, where parcels rotate around the mesocyclone over time. In this latter case, nearly-pure streamwise vorticity needs to be ingested through a layer sufficiently deep enough to contain most of the mesocyclone.

“Relative Helicity” (http://tornado.sfsu.edu/geosciences/classes/m500/Shear_Helicity/Helicity.htm) is the ratio of streamwise vorticity over total integrated vorticity throughout a layer – essentially, a measure of the “purity” of streamwise vorticity. An environment with high streamwise purity will feature a nearly perfect quarter to half-circle (or more) hodograph around storm motion (the wider the circle or the fuller the circle, the more streamwise vorticity ingested).

The most readily-observable impacts of high streamwise purity are on the visual shape of the updraft. Supercells with cloud-layer ratios > 0.50 take on at least half their vorticity as streamwise, thus their updrafts will appear to have at least some semblance of weak corkscrew motion. Supercells in ratios > 0.75 tend to show definite visual cues of rotation, while supercells in ratios > 0.90 often take on near-perfectly cylindrical, sculpted “mothership” structure with well-formed striations. Supercells with ratios < 0.50 are characterized by more crosswise vorticity tilting than streamwise, thus their updrafts deceptively show little to no sign of rotation, and appear sheared or leaning.

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The most extreme example (ratio not shown on hodograph) I have come across of nearly-pure streamwise vorticity ingestion up to more than 6km (minus the ~1-2km layer), the Douglass, KS supercell (https://twitter.com/JohnRichTX/status/1011797378343624704) was a particularly stunning, cylindrical and striated “mothership”.
6/12/17 Supercell and funnel near Bushnell, NE
Supercells influenced by large proportions of streamwise vorticity tend to display obvious signs of rotation, such as circular bases and striations; complex carouseling mesocyclones also tend to be reserved for these hodographs, such as the three areas of rotation pictured here.
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Though a weak tornado producer with a classic rain-free base structure, this supercell was rather awkward and linear in structure, due in part to nearly half the vorticity ingestion in the mid-levels being crosswise.
2/28/17 Tornado near Washburn, IL
Supercells influenced by large proportions of crosswise vorticity tend to take on elongated shapes that don’t usually scream the storm is rotating. Sometimes a clear-cut distinction between the rotating wall cloud and the rest of the storm is even difficult.

Cyclic Mesocyclones

Though downdraft thermodynamics play an increasingly major role in the re-generation of low-level mesocyclones, supercells ingesting nearly pure streamwise vorticity through a deep layer are often characterized by steadily “carouseling” low-level mesocyclones, with multiple tornadoes that may form and circle around the main mesocyclone in quick succession.

On the contrary, it is my observation that supercells ingesting a higher percentage of crosswise vorticity (or less vorticity entirely!) in the mid-levels retain in contrast more steady-state mesocyclones, and in turn potentially steadier-state (and longer-tracked) tornadoes. This is, however, just one possible mechanism for explaining cyclic mesocyclogenesis, which will not be discussed further here.

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Did someone say carouseling? This study wouldn’t be complete without a proximity sounding from the Pilger, NE “twin” violent tornadoes.
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Not satisfied? Here’s Pilger’s little brother, Dodge City ’16. Notice the resemblance? Both have very pure streamwise vorticity even up to 7km, though magnitudes of relative winds differ. It is from my limited (by nature) sample size that nearly all simultaneous-tornado-producing supercells are birthed from similarly deeply streamwise hodographs. (Next time I see this, I’m getting the hell into Dodge).

Internal Boundary Orientations

Warning – this gets technical. Skip to the next section if you just want some real-world examples already!

Hearkening back to our exploration of precipitation placement, the relative hodograph structure can also be used to approximate the preferred orientations of internal/peripheral boundaries. Of course, thermodynamics play an increasingly important rule here, but I’ve began to note that hodograph structure may have the final say in determining “preferred” or equilibrium-state behaviors for the RFGF, forward-flank precipitation gradient and SVC.

RFGF

The Rear Flank Gust Front (RFGF), though tied to the thermodynamics and microphysics of the supercell’s cold pool, appears to (and theoretically should) also have a behavior that hinges largely on hodograph structure. Think of the relationship of the RFGF to a low-level shear vector like that of a QLCS to a low-layer shear vector. Line segments especially toward the northern portion of the QLCS that bow out perpendicular to the shear are said to be downshear propagating, and have been found to deal the strongest winds. On the contrary, segments that drape down especially toward the south into a shear-parallel position are said to be upshear propagating, and tend to simply translate, only dealing severe winds if this translation speed is sufficiently fast. I believe there to be some similar relationship between the RFDs of supercells and a low-level shear vector. With supercells, the RFD at full strength is almost always bowed like a horseshoe. Theoretically, if the entire length of the RFD is assumed to generate the same amount of baroclinic horizontal vorticity, the portion perpendicular to the low-level shear vector would have its baroclinic and barotropic vorticity intake most balanced, ensuring that its updraft/downdraft interface is most balanced, which would generate the strongest resulting horizontal vorticity and forward surging / damaging wind potential. Conversely, the portion parallel to the low-level shear vector would be weak, generating little storm-relative outflow wind.

Determining the likely preferred orientation of a surging RFD may have direct implications on supercell damaging wind potential and even upscale growth or lack thereof. In the vast majority of supercell cases, this low-level shear vector is closely aligned with storm motion; the resulting RFD winds would be strongest along a very localized region on the apex of this surging bow with the additive properties of storm motion, then the storm runs the risk of cycling or becoming occluded, which has been observed to be true. From my experience alone, orientations more perpendicular to storm motion have actually been found to lead to more long-lived, steady-state supercells producing long, continuous swaths of damaging wind when storm motions are fast; this may be because their RFDs generate comparatively little storm-relative outflow wind, thus the supercell can remain propagating steady-state without occluding.

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The Classic Tornadic supercell (top) has an RFD that surges (spiked portion) forward, thus storm motion provides an additive effect to ensure localized damaging wind. However, this makes it quick to occlude.                                                                                    The Classic Upslope supercell (bottom) has an RFD that surges due left of storm motion, thus cross-wise storm motion negates much damaging wind along this portion. Because the RFD does not surge ahead of the storm, these are frequently very long-lived despite often very high bases that would typically render them outflow-dominant.

FFPG

The Forward Flank Precipitation Gradient (FFPG) orientation may also be found by examining upper-level shear profiles, as forward flank precipitation placement is a direct product of storm-relative ventilation. The classification “FFPG” will be used here as opposed to Forward Flank Convergence Boundary (FFCB), as it is through my experience that we may be reaching a grey area in knowledge between what is the FFCB, and what is the SVC (they both may be similar features / have the same location). Upon examination of a variety of cases (thanks to Matt Wilson of UNL), it appears that the best estimation of the orientation of the FFPG is found with the ~0-8km shear vector. It is unclear via our current state of simulation science as to what extent this precipitation gradient and size sorting is important, but some exploration has been done on precipitation or anvil shadow baroclinic zones that suggest their importance. It could be hypothesized, then, that a supercell moving directly along its FFPG (parallel to the ~0-8km shear vector) contributes to most significantly tornadic supercells, as this configuration encourages a nearly stationary baroclinic boundary to form ahead of the storm that it will later ride along; however, this has observed to not be the case, thus it appears that a different mechanism is at play.

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As a very general rule of thumb, the inner bound of a supercell’s reflectivity signature at steady-state maturity reveals the hodograph that created it.

FFCB / SVC

The rather newly-documented Streamwise Vorticity Current (SVC) is a quasi-steady-state, laminar feature within a supercell that is the direct result of baroclinic vorticity generation within the forward flank region, though it too appears to have orientations tied to that of a low-level shear vector. It has been modeled to mark the most substantial baroclinic zone between inflow air and FFD air (even though it generally does *not* set up along the forward flank precipitation gradient but further back within the storm) and has been found to maintain or potentially strengthen tornadoes. It from my experience assumes the position of what others have termed the Forward Flank Convergence Boundary (FFCB). Finding its orientation with respect to storm motion may or may not have implications on the amount of total streamwise vorticity available to a developing tornado – barotropic plus baroclinic, or when this added baroclinic vorticity is enough to supplement weak barotropic vorticity. For instance, if two supercells existed with identical near-surface SRH, yet one travelled along its SVC and the other perpendicular to it, it would seem surely plausible that the supercell travelling along its SVC would reap the benefit of an extra substantial source of vorticity that the other could not, and that this would have a non-trivial effect on low-level mesocyclone strength and longevity, especially if the storm is travelling slowly (such that more time is allowed for evaporative cooling to establish a baroclinic zone over a local area). This, however, has not been studied.

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A rather typical orientation of internal boundaries within a post-split supercell; the FFPG in red, the RFGF in blue, the SVC in pink.

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The orientation of internal boundaries within a special case of supercell; though highly uncommon, it was simulated and studied rather intensely as it betrayed many of the above relationships.

In The Wild

Now that we have some concept of the multitude of possible storm-relative hodograph structures, there’s a lot of patterns and “subspecies” out there; let’s see if we can find them in their natural habitats! For this exercise, I thought up just about the wildest severe weather setup imaginable (so don’t expect this to happen anytime soon):

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In this fantasy setup, a couple waves exist, with strong dynamics driven by upper-level (light blue), mid-level (blue) and low-level (green) jets. A hurricane is also present. Low pressure centers and jet streak arrow heads are sized according to their relative strengths.

Yikes!!!! The Storm Prediction Center is going to have a fun time with this one. But so are we. Let’s assume here that there is reasonable buoyancy levels, sufficient shear, and perfect forcing for discrete along *all* boundaries and open warm sector. I’ve labeled a variety of positions. Should a supercell mature in these locations, here’s what to expect.

Group 1 – Classic Tornadic

1a. Classic “Textbook”

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Structure: classic/LP tilted corkscrew

Spotting: stairstepping roads to keep up, otherwise easy viewing, slow storm evolution

Hazards: (significant) tornadoes, large hail, damaging winds

Environment: close to Low beneath approach of strong upper trough, with moderate low-level shear and very strong upper-level shear; most typical in Plains to Midwest states

ex. Pampa, TX 11/16/15; Loami, IL 3/15/16; Salina, KS 4/14/12;

1b. Classic Long-Track

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Structure: classic, perhaps HP at times, linear with less obvious visual cues of deep-layer rotation

Spotting: picking an intercept point south of the FFD, following it on E/W roads until it outpaces you

Hazards: (violent) long-lived tornadoes, large hail, damaging winds

Environment: well south of Low with often slightly veered surface winds, beneath powerful upper jet, with very strong mid-level shear

ex. Henryville, IN 3/2/12; Atkins, AR 2/5/08; Washburn, IL 2/28/17

1c. “Dangerous”

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Structure: expansive low base, far tornado separation from FFD, often no real mid-level structure

Spotting: picking an intercept point, getting cored, then not being able to catch up, fast storm evolution (dangerous)

Hazards: (violent) long-lived tornadoes, (significant) large hail, damaging winds

Environment: (rare) deep within warm sector at intersection point of strong low-level jet and strong upper jet, with very strong low-level shear and moderate mid-level shear; typically reserved for more compact systems especially in the Southeast

ex. Tuscaloosa, AL 4/27/11; Moore, OK 5/20/13; Beauregard, AL 3/3/19

1d. “Surprise” Long-Track

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Chasing: classic though often not noteworthy structure

Spotting: generally slow with laid-back E/W road following, less worry of precipitation

Hazards: surprisingly long-lived (significant) tornadoes, large hail, damaging winds

Environment: relatively less dynamic airmass with comparatively small low-level hodograph bulk, positioned away from strongest forcing/jets

ex. Chapman, KS 5/25/16; Chetek, WI 5/16/17; Rochelle, IL 4/9/15

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Group 2 – Mini-Supercell

2a. Classic Cold-Season Mini-Supercell

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Structure: small, crisp, highly-visible leaning towers, no discernible mid-level striations

Spotting: picking an intercept point, following briefly until it outpaces you, generally having no visual cues to suggest when tornadogenesis will occur

Hazards: perhaps a (long-tracked) tornado, varying largely in potential strength

Environment: near very strong, often occluding Low with little appreciable change in wind speed from low-level to upper-level jet; also perhaps the leading hodograph for strong mid-Atlantic tornado events with more “sheared-out” troughs with unidirectional flow with height

ex. Washington, IL 11/17/13; Bruington, VA 2/24/16

2b. “Surprise” Cyclic Mini-Supercell

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Structure: not-as-mini, beautiful combination of classic shallow barrel structure beneath tilted tower, highly visible bases

Spotting: stairstepping roads to keep up, otherwise easy viewing, being aware of  mesocyclone hand-offs despite pattern recognition that typically suggests otherwise

Hazards: a succession of tornadoes, large hail

Environment: (very rare) along occluded front of deeply occluded low, with heavily backed near-surface flow and strong, backing upper jet aloft

ex. Havana, IL 12/1/18; Eads, CO 5/9/15

2c. Classic Warm-Season Mini-Supercell

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Structure: compact, low-based, murky, shallow smooth barrel structure, many inflow features

Spotting: generally needing to be close to circulation to discern features, otherwise easy with slow motions and not much obscuring precipitation

Hazards: brief tornadoes

Environment: positioned along/within ridging and very weak upper flow, with low-level wind fields enhanced by fronts/outflow boundaries or perhaps remnant Mesoscale Convective Vortices; most typical in Midwest during summer months

ex. Van Wert, OH 8/24/16; Homer, IL 9/9/16

2d. Hurricane Mini-Supercells

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Structure: compact, low-based, murky, non-classic structure orientation

Spotting: fast speeds, gradual leftward deviations, tornadoes happen early on (due to low-level SRH becoming weaker with deviation)

Hazards: brief (weak) tornadoes

Environment: front-right quadrant of tropical cyclone (or, less commonly, rapidly-deepening mid-latitude cyclone or developing Mesoscale Convective Vortex), with powerful low-level flow weakening upwards

ex. Houston, TX 8/26/17; Morehead City, NC 9/15/18

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Group 3 – Veered

3a. Classic “Northwest Flow”

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Structure: often low, murky bases, often no real mid-level structure

Spotting: generally requires positioning near “notch” close to circulation to keep up, yet surprisingly lower-precipitation hook region, with moderately-paced storm motion

Hazards: a succession of (significant) tornadoes, (significant) large hail, damaging winds

Environment: strong northwest flow regime with strong warm-air advection near warm front; most typical in the Southeast

ex. New Orleans, LA 2/7/17

3b. “The Notch”

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Structure: low, murky bases, thick inflow bands, often no real mid-level structure

Spotting: requires being very close to circulation in “notch” or even in core to discern features (often dangerous)

Hazards, a succession of (significant) tornadoes, damaging winds, large hail

Environment: weak northwest flow regime with weak mid-level shear and strong warm-air advection near warm front or outflow boundary, most typical in Midwest; also common of early-season Plains setups with strong southwesterly mid-level flow and minimal shear above 3km

ex. Sublette, IL 6/22/15; West Brooklyn, IL 6/22/16; Hollis, OK 4/4/19

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Group 4 – Streamwise

4a. Classic Cyclic

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Structure: classic smooth deep barrel or shallow saucer, often mid/high base

Spotting: stairstepping roads to keep up, otherwise easy viewing

Hazards: a succession of (significant) tornadoes, (significant) large hail, damaging winds

Environment: often a day pre-trough with continuously veering wind profile and strong mid-level support despite more moderate upper jet, most typical in high Plains; more classic supercells may tend to transition into this form with the evening low-level jet, which may accompany a *leftward* deviation

ex. Kimball, NE 6/12/17; Camp Crook, SD 6/28/18; Silverton, TX 3/28/07

4b. Stationary “Mothership”

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Structure: typically beautiful deep mothership structure

Spotting: following from a distance behind / to the south, very relaxed paced, tornadoes happen early on typically before structure improves (due to deviation often reducing low-level SRH but increasing mid-level SRH)

Hazards: large hail, damaging winds, perhaps a tornado

Environment: often a day pre-trough with continuously veering wind profile and powerful low-level jet despite weak upper jet, most typical in central / High Plains; also seen when moving supercells become stationary “bookend vortices” to developing Mesoscale Convective Vortices, a byproduct of rapid accelerations of mid-level flow usually undetectable in numerical models

ex. Leoti, KS 5/21/17; Douglass, KS 6/27/18; West Point, NE 6/14/13

4c. Tilted/Mothership Hybrid

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Structure: shallow mothership structure contorted to linear tilted in mid-levels

Spotting: stairstepping roads to keep up, otherwise easy viewing

Hazards: large hail, damaging winds, perhaps a tornado

Environment: fairly classic supercell environment a bit removed from strongest forcing, with gradual veering and not-too-powerful upper jet

ex. Climax, KS 5/10/14

4d. Marginal Saucer

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Structure: shallow saucer structure below upright tower

Spotting: very easy with slow storm motions and classic views

Hazards: large hail, damaging winds, perhaps a tornado

Environment: very weak upper-level flow and similarly weak low-level jet, along outflow boundary where residual turning is required for supercell formation

ex. Burwell, NE 6/16/14; Willow, OK 3/18/12; Selden, KS 6/4/15

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Group 5 – Cyclic

5a. “Occluding” Cyclic

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Structure: expansive low broad but pinched base, no real mid-level structure

Spotting: positioning in large gap between RFD and FFD, being aware of rapid mesocyclone hand-offs spurred by RFD surges, with rather instant / continuous short-lived tornado production

Hazards: rapid succession of tornadoes, large hail, damaging winds

Environment: (very rare) strong warm-air advection with substantial near-surface veering, under weak or backed mid and upper jet

ex. Greensburg, KS 5/4/07, Winona, KS 6/8/19, Geary, OK 5/29/04

5b. “Carouseling” Cyclic

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Structure: expansive bases with often no real mid-level structure, except on occasions where mothership structure is visible

Spotting: positioning under expansive circular rain-free base, following it northward, being aware of multiple long-lived circulations ongoing with two or more tornadoes possible at once, steadily carouseling around the main mesocyclone

Hazards: simultaneous (significant) tornadoes, (significant) large hail, damaging winds

Environment: (rare) continuous veering with strong mid-level flow to ensure near-perfectly circular hodographs; either serendipitously created in strong flow scenarios or, more often, along remnant outflow boundaries in moderate flow in warm season

ex. Pilger, NE 6/16/14, Dodge City, KS 5/24/16

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Group 6 – Full-Structure

6a. Classic Upslope

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Structure: often classic shallow barrel structure beneath tilted tower, often low tops and virga/mammatus, highly visible bases

Spotting: waiting for storm to slowly come down from N/NW, full structure visible upon approach

Hazards: damaging winds, tornadoes, large hail

Environment: decent-clip upslope surface winds on high Plains with weak mid-level flow and moderate/strong upper jet; storms can take on a nearly due southwest deviation in late summer northerly flow regimes

ex. Simla, CO 6/4/15; Ulysses, KS 6/29/17

6b. “Surging” RFD

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Structure: expansive broad, often high base, shallow saucer structure

Spotting: full structure visible upon approach, rapidly accelerating storm speeds, powerful RFD that appears to “kick out” (quite similar to QLCS bulges/notches!)

Hazards: (significant) damaging winds, (significant) large hail

Environment: (rare) weak low-level flow under strong upper jet; this hodograph is only created if a storm accelerates / deviates much farther from the hodograph than predicted by Bunkers’ Right; most common in higher-LCL, weak surface flow scenarios especially around the Rapid City to Goodland area

ex. Tahoka, TX 5/5/19; Goodland, KS 6/19/18; Oakley, KS 5/25/17

6c. Post-frontal “Mothership”

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Structure: full deep “mothership” structure, often low tops and virga/mammatus

Spotting: positioning ahead toward the east, full structure visible upon approach, moderate storm speed, clear post-frontal skies with crisp structure

Hazards: (significant) large hail, damaging winds

Environment: (rare) elevated atop sagging weak cold front, with strong northerly winds quickly veering above shallow cold layer; almost exclusive to the TX/OK panhandle region where cold frontal forcing / undercutting is weaker

ex. Boise City, OK 5/27/17

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Group 7 – Veer-Back

7a. Classic Splitting

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Structure: classic/LP tilted corkscrew structure, linear with less obvious visual cues of deep-layer rotation; a higher ratio of 1-6km shear to 0-1km shear regularly results in classic “flying eagle” radar signatures

Spotting: positioning ahead to the east, generally no precipitation in the way

Hazards: large hail, damaging winds, perhaps a tornado

Environment: approach of strong upper-level system with some backing aloft, close to Low

ex. Meridian, TX 3/18/18

7b. Outflow Dominant

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Structure: classic tilted corkscrew structure, linear with less obvious visual cues of deep-layer rotation, “slides” outward RFD-first, appears outflow-dominant but are yet often long-lived

Chasing: full structure visible upon approach, moderate storm speeds

Hazards: damaging winds, large hail

Environment: veered near-surface flow, with relatively unidirectional/backing flow into the upper levels, often along cold front but occasionally seen in upslope regimes especially in the northern high Plains; it is unclear whether the rotated 0-2km segment is encouraged by undercut, post-frontal storm motions, or if the intrusive RFD placement is a result of this rotated 0-2km segment

ex. Cohagen, MT 6/9/16

7c. “Awkward” Structure

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Structure: quite unclassic tilted structure, often with the appearance that lower half of updraft has been twisted at a right angle of where it should be; bent, classic “flying eagle” radar signatures

Chasing: full structure visible upon approach, slow storm speeds

Hazards: (significant) large hail, damaging winds

Environment: (very rare) perhaps most common within subsidence inversions under strong meridional flow, where recovering airmass and backed near-surface winds contribute to large low-level hodographs which sharply back above; most common in southern Plains

ex. Guthrie, TX 6/14/16

Hopefully this re-imagining of hodograph structure sheds much more light on those seen every day by forecasters and storm spotters alike. Again, all this can be gleamed from them, but now after a bit of head-tilting I bet they became much more obvious.

I’d love any constructive feedback you may have on how I could further pursue this deconstructing of the anatomy of a rotating storm.

Special thanks to: Matt Wilson (UNL), Alex Schueth (TTU), and Will Wight (DTN)