Ethan Lang
EF2
I have been forecasting my own chases ever since I began chasing and one thing I have always been confused about is backed vs. veered shear profiles. Can someone help explain this to me?
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Backed vs. Veered
- Thread starter Ethan Lang
- Start date
StephenHenry
EF3
I'll take a stab at some specifics (but also open myself up to corrections if I misstate anything). I'm assuming we're talking in the context of storm forecasting in the northern hemisphere with a nice N-S boundary like a dryline. In all of these cases, the drawn hodograph is veer-ING, (clockwise curvature), but at least from shorthand when most chasers chat about forecasts, I think we would classify as a backed vs a veered profile, in that it's mainly in the orientation of the surface winds.
Andy Wehrle
EF5
I suck at reading hodographs, but I do know that you generally want winds to be "backed" at lower levels (more southeasterly, or at least more counterclockwise relative to the winds at higher levels) and to veer (turn clockwise) with increasing height to have shear profiles favorable for tornadic supercells.
Spencer Dant
EF0
In the US, veered winds are WSWerly and backed winds are ESEerly. We want veered winds aloft and backed winds at the surface for the sake of a turning wind profile, which as Stephen's image makes clear promotes stronger streamwise vorticity. Counterclockwise spin results in positive vorticity in the northern hemisphere, which means storms rotate more freely. Within that scope, backed winds at the surface provide horizontal convergence by meeting and colliding with winds from the west. Horizontal convergence at and around the surface can result in some pretty extreme vertical stretching and, naturally, strong low level rotation. When you have backed winds at the surface and a good turning wind profile, you have a chance, even in otherwise marginal conditions.
Veered winds aloft not only permit the updraft to rotate with backed winds at the surface, but they often allow storms to be steered more easily by west-to-east boundaries like warm fronts. With westerly winds aloft, a storm moving NE will have a much easier time making a sudden right turn and latching onto a surface boundary than a storm moving NE with southwesterly winds aloft, which encourage the storm to continue on a northern track.
Veered winds aloft not only permit the updraft to rotate with backed winds at the surface, but they often allow storms to be steered more easily by west-to-east boundaries like warm fronts. With westerly winds aloft, a storm moving NE will have a much easier time making a sudden right turn and latching onto a surface boundary than a storm moving NE with southwesterly winds aloft, which encourage the storm to continue on a northern track.
Ethan Lang
EF2
You guys explained this very well. I have always somewhat understood hodos in that they take a top down look at the wind profile. But you guys definitely cleared this up for me and made hodos more understandable. Now lets bring on a great 2019 season. Thanks!
Devin Pitts
EF2
I want to add that "backed" surface winds is a relative term, when in reference flow direction aloft. Take NW Flow type events during the summer months in the Midwest. Technically straight southerly surface winds could be considered "backed" relative to the flow aloft in a NW flow type event, where the hodo would look something like the first one in the table in Stephen's post, but rotated slightly along the center axis of the hodograph. This hodograph from 6-22-16 in Illinois should illustrate that pretty well.
I feel like everyone who has responded here has glossed over the fundamental definition, and is therefore missing the technical aspect of this discussion, which I would like to correct.
Veering and backing are relative terms (not absolute):
-veering means "turning clockwise" (either with height or time)
-backing means "turning counterclockwise"
Saying "winds are backed" is technically ambiguous unless proper context accompanies the description, or unless everyone already agrees on the context without saying it. In the chasing world, this tends to be the latter, but for those who are new to the science, this is not at all clear, and one can end up misunderstanding the true concept underlying the jargon. Southeasterly winds are not, by definition, "backed", but they are backed compared to southerly or southwesterly winds. Likewise, a westerly wind is not "veered" all by itself, but instead is veered relative to a southwesterly or southerly wind, for example. Meteorologists use jargon like "backed" and "veered" winds as shorthand for saying "favorable flow for...(whatever...moisture return, adequate directional shear, slow storm speeds etc.)".
So in essence, the phrase "backed shear profile" is pretty meaningless without context. Same with "veered shear profile". Better terms to use would be "winds backing with height" or "veering wind profile". Outside of the context of the mid-latitudes of the northern hemisphere, the meanings that others have described above is much different, and may be perfectly backwards (in Argentina, for example).
I believe @StephenHenry generally knows what he's talking about, but I disagree with some of the descriptions in his post. All hodographs he showed are effectively the same as far as their impact on existing convective storms (remove the axes and coordinate markings and you would be hard pressed to distinguish between them). As he correctly mentioned, winds veer with height in each profile and thus I would consider them all "veering wind profiles". But they all have about the same SRH, so the descriptions of "more streamwise vorticity" under the "backed" graphic is factually incorrect (especially without markings indicating the distribution of the shear in those hodographs; I presume the red means 0-3 km AGL, as is pretty common on many websites, but that is not some sort of official standard or anything; if the distribution of the shear in those profiles differed, then we could talk about differences in their impacts on storms, but based on the color distributions, they don't appear to differ). What differs between them is the location of the storm motion vector and how that would impact a chase strategy. A storm chaser would generally prefer "backed" low level winds because it would not only enhance SRH (assuming the mid- and upper-level flow does not also shift, which is never a given), but would move the storm motion vector closer to the origin, thus implying slower storm speed, which is much easier to chase in a car from a road-relative perspective (which we humans are glued to, but storms are not).
The responses from others in this thread continues from here regarding the learning process and meaning of "backed" and "veered" as they pertain to forecasting severe storms.
Veering and backing are relative terms (not absolute):
-veering means "turning clockwise" (either with height or time)
-backing means "turning counterclockwise"
Saying "winds are backed" is technically ambiguous unless proper context accompanies the description, or unless everyone already agrees on the context without saying it. In the chasing world, this tends to be the latter, but for those who are new to the science, this is not at all clear, and one can end up misunderstanding the true concept underlying the jargon. Southeasterly winds are not, by definition, "backed", but they are backed compared to southerly or southwesterly winds. Likewise, a westerly wind is not "veered" all by itself, but instead is veered relative to a southwesterly or southerly wind, for example. Meteorologists use jargon like "backed" and "veered" winds as shorthand for saying "favorable flow for...(whatever...moisture return, adequate directional shear, slow storm speeds etc.)".
So in essence, the phrase "backed shear profile" is pretty meaningless without context. Same with "veered shear profile". Better terms to use would be "winds backing with height" or "veering wind profile". Outside of the context of the mid-latitudes of the northern hemisphere, the meanings that others have described above is much different, and may be perfectly backwards (in Argentina, for example).
I believe @StephenHenry generally knows what he's talking about, but I disagree with some of the descriptions in his post. All hodographs he showed are effectively the same as far as their impact on existing convective storms (remove the axes and coordinate markings and you would be hard pressed to distinguish between them). As he correctly mentioned, winds veer with height in each profile and thus I would consider them all "veering wind profiles". But they all have about the same SRH, so the descriptions of "more streamwise vorticity" under the "backed" graphic is factually incorrect (especially without markings indicating the distribution of the shear in those hodographs; I presume the red means 0-3 km AGL, as is pretty common on many websites, but that is not some sort of official standard or anything; if the distribution of the shear in those profiles differed, then we could talk about differences in their impacts on storms, but based on the color distributions, they don't appear to differ). What differs between them is the location of the storm motion vector and how that would impact a chase strategy. A storm chaser would generally prefer "backed" low level winds because it would not only enhance SRH (assuming the mid- and upper-level flow does not also shift, which is never a given), but would move the storm motion vector closer to the origin, thus implying slower storm speed, which is much easier to chase in a car from a road-relative perspective (which we humans are glued to, but storms are not).
The responses from others in this thread continues from here regarding the learning process and meaning of "backed" and "veered" as they pertain to forecasting severe storms.
Last edited:
Ethan Lang
EF2
Thanks to all for your responses, I understand this much better now especially from a operational standpoint, now in order to keep this thread alive how can one look at a wind profile (hodo) and figure out SRH and streamwise vorticity?
SRH is mathematically computed as the area on the U-wind/V-wind coordinate axes bounded by the wind profile and the line connecting the storm-motion vector to whatever vertical levels on the wind profile you are integrating between (in the example below, 0-3 km AGL, and c is the storm motion vector). But due to scaling of axes and poor human judgment, you will not be able to quantify this by sight. Just envision the area to get a sense. But any website that shows model soundings will probably also label the calculated SRH as well.
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StephenHenry
EF3
Yep this is definitely the crux of where I tried to cheat by side-stepping the whole relativity issue for the sake of some simplicity. My first panel was meant to suppose that the same storm motion vector applied to each of the 3 hodos - thus locking in some relativity. But of course you can't just magically pick an storm motion vector, it's itself driven by the hodo. Great clarifications Jeff! Also love the nugget about storm speeds, great additional insight.
I was hoping if I overstated or just plain misstated, Cunningham's Law would kick in and I'd improve my own mental models in the process. Don't lie though, you know the left hodo gives you tingles and the right one makes you go meh. Or is that just me and have my neurons been back-propagating on the wrong training set all these years?
Don't lie though, you know the left hodo gives you tingles and the right one makes you go meh. Or is that just me and have my neurons been back-propagating on the wrong training set all these years?
No, I agree, I would prefer the left hodograph, but only because of the slower SMV. Also, I'll admit my argument neglected some very minor alterations among those hodographs. I think you "cheated" and tried to make the "backed" one look better by veering it just a little harder than the other profiles
Ethan Lang
EF2
I too like the left just for the sake of storm motion. I chase a lot in areas with poor connection and road networks so a nice slow mover is preferred.
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