Southeastern Supercells: How Different from Prairie Cousins?

[Steve Holmes]

I believe I have a good grasp, at a "See Spot Run" level, of the basics of a Great Plains supercell: Moisture, cap, lift, shear, horizontal roll, updraft, vertical roll. What about Southeastern supercells, like the ones that produced the tornadoes in Albany and Adel, Georgia last month and in New Orleans today. Are the dynamics different?

I am specifically wondering about the role of the cap. The storms that rolled through south Georgia on 1/22 came up from the Gulf (not sure about today's storm). Where would they have gotten the dry air for a cap? As to the low cloud bases, is it that the cloud bases are really low or that the terrain is hillier than on the prairie?

Thank you.

 

[Andy Wehrle]

Tornadic supercells anywhere need the same basic set of ingredients (lift, instability and shear), but as we know there are many different combinations that will work, and many that look like they should produce but don't (as Tom Grazulis put it, we know the ingredients but are many years from having the recipe). You're right in that Dixie events to tend to feature weaker capping, usually because it is farther from the EML source region. They also typically rely on strong forcing and destabilization via cooling aloft (vs. heating at the surface). They can thus be mitigated by storm interference/messy convective mode, but can also produce intense tornadoes at any time of day as we saw today.

Storms in the Gulf coast region often have visually low cloud bases and wide ("wedge") appearing tornadoes due to the high relative humidity near the Gulf. The terrain can also play a role, as people who think they're seeing the tornado from a distance are actually seeing the wall cloud/sometimes the entire updraft, which is much larger than the tornado itself which may be hidden behind hills and trees.

* By the way, if a Dixie Alley setup with everything else in place ever does get a classic Plains-style EML to advect into the area, watch out. (4/27/11 being the most obvious and extreme example)

Just my 2 cents.

 

[Steve Holmes]

Thank you, Andy. I'm anxious to see what VORTEX-SE has to say about this when its project is over. From its website: "“The southeastern United States commonly experiences devastating tornadoes under variables and conditions that differ considerably from the Midwest where conditions for tornado research have historically been focused." I've heard SE meteorologists talk with resentment of the attention given to Midwestern storms. This will help even things out.

 

[Paul Knightley]

It does seem that a lot of research over the years has focussed on supercell/mesocyclone-spawned tornadoes, and for good reason - they are responsible for a disproportionately high number of deaths, injuries, and the amount of damage they cause. However, as you mention, some regions are more prone to this kind of tornado over others (e.g. the Plains). Here in the UK, probably over 1/2 of our tornadoes are from lines/segments, etc, forming in low CAPE/high shear/strongly forced environments, in the cool season. At least some evidence might hint that, at the spot of the tornado, the conditions are not so dissimilar in some of these set-ups than when compared within a mesocyclone in a supercell. But as study of these weaker, more elusive/transient tornadoes isn't so far advanced, it will take a while before firmer details emerge.

As Andy mentioned above, though, when conditions present themselves then you can get classic supercells, wherever you are - and the April 27th, 2011, example is one of those conditions. Large warm-season supercells are rare here in the UK, but we get them from time to time - June 28th, 2012, was a day where the conditions presented themselves and we saw several supercells ongoing at the same time, one producing 10cm diameter hail - several tornadoes occurred too.

 

[Steve Holmes]

Thank you, Paul. I've followed TORRO's work over the years. Is it true that, per 10,000 square kilometers, England has more tornadoes than Oklahoma?

In places other than the Great Plains, how much of a role does the dryline play?

 

[Devin Pitts]

Yeah there are many environments that can support rotating thunderstorms outside of the classic supercell ingredients needed in the plains, and Central California is a good example of this. Pretty much every supercell there forms in the cool season as a low-topped "mini" supercell, and they tend to initiate in the cold pool after the passage of a wet cold front in the late morning/early afternoon(if the cloud cover clears after the front passes through at least). As mentioned above with Southeastern setups, they tend to form when you have low CAPE(200-800 J/kg) and high shear in the lower levels. These storms tend to make use of natural convergence zones and a Lee trough that forms on the western side of the valley rather than cold/warm fronts, OFB's, and drylines.

 

[Paul Knightley]

Thank you, Paul. I've followed TORRO's work over the years. Is it true that, per 10,000 square kilometers, England has more tornadoes than Oklahoma?

In places other than the Great Plains, how much of a role does the dryline play?

That's great to hear, Steve, re: TORRO.

Regarding your question - it is often quoted that the UK has more tornadoes per unit area than any other country. That may be true, but I think it's a somewhat disingenuous statement - for example, the USA is much, much larger than the UK, and so it's quite likely that, averaged over the whole area, the UK 'wins'. However, if you take a parcel of land in the UK and one in, say, central Oklahoma, the latter will 'win'. In addition, the latter has a much higher chance of strong-violent tornadoes, and on all other types of severe convective weather too. Indeed, the two are hardly comparable at all!