Hurricane-Tornado Threats

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I have been wondering this for a while and maybe you can help me out some. Why do tropical storms/hurricanes bring tornado threats? Is it because of 'embedded supercells'? And, why does the tornado threat only usually appear after it HAS made landfall, and not prior to that? I am asking these question because I have seen this happen so many times. Do the hurricanes make tornadoes wven when their out in the ocean? I'm thinking perhaps land has something to do with tornado development.
 
Originally posted by Andrew Khan
I have been wondering this for a while and maybe you can help me out some. Why do tropical storms/hurricanes bring tornado threats? Is it because of 'embedded supercells'? And, why does the tornado threat only usually appear after it HAS made landfall, and not prior to that? I am asking these question because I have seen this happen so many times. Do the hurricanes make tornadoes wven when their out in the ocean? I'm thinking perhaps land has something to do with tornado development.

I am sure it's still producing tornadoes over the water, but those would be called "waterspouts". The threat for tornadoes is a result of the extreme low level shear...
 
Originally posted by Andrew Khan
So your saying without low level shear, it can't spawn tornadoes?

Pretty much. Low-level shear is what makes conditions extremely favorable for tornadoes (along with other factors of course, but without strong LL shear it's hard to get tornadoes, especially strong ones).
 
Because of the wind shear. There is a whole lot of twisting going on in the atmosphere because of the extreme winds at the surface and aloft, movement of the storms relative to the flow themselves and the excellent "spinning" of the cells. These are not frequently too organized and kind of pop and dissipate quickly...although some have been loger lived under the right situation. But generally, they pop up and move so quickly that it's difficult to warn for them until they're picked up on radar as a TVS or Meso.
 
Yeah, without low-level shear your not going to get many tornadoes (if any at all). Some hurricanes will produce more then others... As some hurricane-spawned setups will allow for more insolation, which will allow for more SFC-heating/instability, which will enhance the tornado threat.

I'd have to wonder what Katrina would have done with the shear it had and 4000 SBCAPE :shock:
 
There are two known 'sources' of tornadic vortices associated with tropical storms. The first being small mini-supercell storms within the outer rainbands of the cyclone well away from the storm center. The storm environment is typically very moist with fairly steep low-level lapse rates. You also need very strong veering with height shear profiles - which is most often in the right front quadrant of the hurricane resulting from the combined steering environment and hurricance circulations. In essence, there is strong low-level stretching from buoyancy combined with extremely favorable shear profiles. Some studies suggest that dry air entrainment into a hurricane can significantly increase the likelihood of outer rainband tornadoes (likely improving the lapse rates following lifting, as the entrained air is inward of the outer rainband)- and the more caught up the system is in the westerlies (larger background shear) the more likely outer rainband tornadoes from climatology. The favorable shear can last days after a tropical system makes landfall - as was a common occurance with a number of the landfalling systems producing tornadoes last season. The other type of tornado in tropical systems is called a core-type tornado, and these are not clearly tied to individual thunderstorms such as those described above, but occur within the banded largely stratiform rain within the core region and eyewall. Fairly little is known about the mechanisms for these, but they can be quite intense as was noted in damage surveys following Hurricane Andrew (1992). There has been at least one study (mine :) ) to suggest that fine-scale banding within the hurricane core, generally only seen in cat 3 or greater intensity storms, is related to the core tornado threat, though there is still considerable variability and most post-hurricane damage surveys will fail to distinguish tornado damage from hurricane wind damage.

Glen
 
Thats a nice detailed statement. I never knew there was a core tornado. I thought they all occurred in the outer bands. Thanks for informing me.
 
Also noteworthy is that the kinematic environment in the northeast quadrant of many landfalling hurricanes can be a prime example of how an environment with strong flow in the low levels and moderate flow in the mid levels generates significant SRH despite little vertical veering being present.

This is significant in light of the mention in previous posts of extreme low-level shear with hurricanes being a big factor in the tornado threat. But then again, the minor vertical veering that does occur in NE quadrants of hurricanes seems to be frequently found in the 0-2km layer (from what I've seen anyway) which certainly does add to the SRH magnitude.
 
Originally posted by afischer
.... how an environment with strong flow in the low levels and moderate flow in the mid levels generates significant SRH despite little vertical veering being present.

This is true because srh doesn't take mid-level winds into consideration. the normal depths where the srh is measured is either 0-3 km or 0-1 km, about the only mid-level contribution is in the 'storm-relative' part as the storm motion estimate, depnding on which one is used, often takes mid-level winds into account.

Glen
 
Yes, but one can definitely get augmented SRH from a faster storm motion. A typically clockwise hodograph in a tornado set-up in Kansas would have much higher SRH given an incoming 70kt 500mb windmax than with a 40kt 500mb windmax, all else being equal... due to the "S-R" part, as u mentioned. And in this light, moderate mid-level flow in a NE hurricane quadrant definitely doesn't "hurt" either.

I mainly offered my previous post because sometimes set-ups occur in the plains/midwest where SRH will support tornadoes despite "unidirectional-ish" flow; there seems to be a focus on veering needed between 850mb and 500mb when it isn't always required for tornadoes. NE quadrant hurricane hodographs are on the same page kinda.
 
Originally posted by Chris Rozoff

Hey Glen - I admittedly haven't followed Andrew as much as I should, but do you think it is possible mesovortices resulting from Andrew's eyewall could have mixed stronger eyewall winds to do those infamous wind swaths? Have tornadoes been physically observed in this region? Or were the tornadoes associated with bands just outside the eyewall? If tornadoes do develop, It would be hard to not imagine just outside of the eyewall, intense horizontal shear plays a role in spinning up vortices.

Hi Chris - mesovortices and fine-scale banding are probably closely related, and the tornado from Andrew that was documented by Wakimoto and Black was very close to the track of the eye - so I'd easily be sold on this being within the eye. I don't think there was any visual documentation of the tornado itself - it would have been very difficult to see - beyond just rain-wrapped - lol. But, there are other events with considerable core tornado production - and some is clearly not from eyewall convection but outside of it. More often, these are weak and short-lived tornadoes, but the Andrew event is the only well-documented F3 core tornado that I've ever come across in the literature. Hurricanes this intense generally has few people out-and-about filming - so imagine trying to storm chase by just picking a spot and seeing what comes by, so I'm not surprised there are few visual reports. That said - the damage patterns are pretty clearly distinct from straightline wind damage - so aerial surveys could be quite easily employed to check for tornado tracks following hurricane landfalls - but generally I think folks care more about the extent of damage that occurred than what actually caused it.

Glen
 
Originally posted by afischer
Yes, but one can definitely get augmented SRH from a faster storm motion. .

Not a one-to-one relationship. remember that helicity is a measure of area on a hodograph. If you move the storm motion point - you may or may not get more area, in fact you can just as easily end up with less. That said - predicting storm motions is hardly an exact science, so the whole storm-relative thing is a bit arbitrary anyway. We are getting off-topic here though, so I won't elaborate here. Should be plenty of examples elsewhere on this.

Glen
 
Originally posted by afischer
Yes, but one can definitely get augmented SRH from a faster storm motion. A typically clockwise hodograph in a tornado set-up in Kansas would have much higher SRH given an incoming 70kt 500mb windmax than with a 40kt 500mb windmax, all else being equal... due to the \"S-R\" part, as u mentioned. And in this light, moderate mid-level flow in a NE hurricane quadrant definitely doesn't \"hurt\" either.

Actually, no, not always. Slower storm motions can increase SRH depending upon the hodograph, particularly for a "classic" clockwise looping hodograph. Note that there is usually an increase in rotation when a supercell slows down and turns to the right (of mean storm motion).

faststormmotion.gif

Say this is the SRH with the "mean" storm motion.

slowstormmotion.gif

This represents the SRH experienced by a storm with a slower storm motion. Note that the SRH is higher.

veryfaststormmotion.gif

And this represents that SRH for a storm with a much faster storm motion. Note that the negative SRH nearly equals the positive SRH, so there is very little net SRH.
 
For the record, I clearly didn't say increased storm speed means increased SRH. And I'm not sure what my "increasing midlevel flow 30 knots potentially augments SRH magnitude" example... has to do with a supercell turning right and slowing down... I'll stop now since we are well off-topic. 8)
 
Originally posted by afischer
For the record, I clearly didn't say increased storm speed means increased SRH. And I'm not sure what my \"increasing midlevel flow 30 knots potentially augments SRH magnitude\" example... has to do with a supercell turning right and slowing down... that certainly won't help your SRH any! I'll stop now since we are well off-topic. 8)

Since SRH is most often computed from 0-3km or 0-1km, increase mid-level flow will likely to little other than increase storm motion. As I have shown, increased storm motion often DECREASES SRH. If weaker mid-level flow yields slower storm motion, and slower storm motion often increases SRH, then one can indirectly conclude that (given all other favorable conditions and mid-level flow still strong enough to yield adequate deeplayer shear) weaker mid-level flow can help yield higher SRH. You imply (or state) stronger mid-level flow increases SRH, which I argue is exactly the opposite of what really happens (for many supercells). As Glen noted above, computing storm motion (or observing it for that matter) is hardly an exact science, so even forecast SRH has inherent errors associated with errors in storm motion forecasts.

EDIT: I talk of clockwise-looping hodographs and cyclonically-rotating supercells (which is often what we chasers jump for).
 
Thanks for the hodographs... I definitely didn't consider negative SRH in my blanket statement about storm speed's affect on SRH. :oops:
 
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