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Nocturnal Tornadoes

If this thread has been discussed at length elsewhere, feel free to re-direct.

I was reading about this event the other day and it had me wondering:

https://en.wikipedia.org/wiki/1998_Kissimmee_tornado_outbreak

Why do nocturnal tornadoes tend to occur with a greater frequency in Dixie Alley and Florida than elsewhere?

The only theories I could come up with were based on moisture (and maybe surface wind). Once there is a deficit of net incoming radiation, the surface of the Earth cools - causing the air above to cool as well, in a layer that becomes progressively deeper overnight - granted there are clear skies and light winds. This stable layer is thus resistant to lifting, and deep, moist convection becomes elevated. Moreover, the thermodynamic qualities of low level air is thought to play a crucial role in tornadogenesis, where it needs to be buoyant enough to be lifted and stretched (vorticity) from the surface to the cloud base. In the drier climates of the plains, and especially the high plains, nocturnal tornadoes (especially in the overnight and early morning hours, as opposed to late evening) are quite rare indeed.

Having a source of GoM moisture (mT air) readily available, and thus being climatologically more humid, tornadoes in the southeast would already seem to originate from storms with lower LCL heights than elsewhere in North America. At night, the LCLs would lower even more as the boundary layer cools, meaning that low level vorticity wouldn't have to be lifted as far to contribute to tornadogenesis. My guess is that high moisture quantities in the low levels of the troposphere would cause a lot of outgoing longwave radiation to be re-radiated back to Earth's surface, causing it to cool slower and delay the onset of boundary layer decoupling. Low level stratus could also play a similar role, as could mixing from strong low level winds associated with a synoptically energetic background environment.

Anyone else have any ideas?
 
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I believe most of it has to do with the peak tornado season along the Gulf Coast being in winter, when the days are shorter. Events close to the Gulf tend to be less diurnally-driven that time of year, meaning an event has an equal chance of happening at any time of the day. That is, you don't need a shortwave to come through with perfect timing (at peak heating). It can come through at night and produce an environment not dissimilar to one during the day.
 
Indeed, and that is why I am wondering what exactly is special about the environment that is enabling tornadoes to form at night in the southeast. Because as you said, timing of shortwaves in plains setups is an important factor - since strong disturbances in the overnight hours on the plains typically lead to elevated thunderstorms (and thus, rarely tornadoes).
 
Enviorments across the south east behave somewhat differently esspecially in the boundry layer. Winter time events tend to also be very strongly forced with impressive kinematic fields. This can in some ways make up for a lack of instabillity. See a paper by Ariel Cohen from SPC. Www.spc.noaa.gov/publications/cohen/pbl-waf.pdf

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Indeed, the so called "HSLC" setups (high shear low CAPE). I'm guessing then that strong low level winds keep the lower troposphere well-mixed, helping thunderstorms remain surface-based, and allowing for an environment more conducive to tornadoes - especially with the often tremendous low level shear already in place.
 
I'm pretty sure the majority of this explanation is moisture. Dixie Alley is very close to a strong moisture source - the Gulf. Thus there is always moisture nearby, and thus lack of moisture rarely limits an otherwise good setup like what happens in other parts of the world (e.g., the Plains). The only reason tornadoes are rare at night to begin with is because they require buoyancy near the ground, and buoyancy near the ground is usually the first thing to go once the sun sets. Loss of buoyancy can be mitigated in high moisture environments, however.
 
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It's also about the 'ease' or not of which thunderstorms can develop. In the Plains in Spring, typical setups include a SW flow aloft = an elevated mixed layer (EML) overruning the low-level moisture. Yes, this steepens lapse rates and can yield fat CAPE environments, but it also often puts a fairly stout capping inversion in place, which tends to be breached at or soon after peak heating, before strengthening again after dark. In the south-east, EMLs are less likely to be present in early season environments, and so storms will form day or night where strong low-level convergence and moisture are in place, which they often are in such winter storm environments. Also, in the early season, night is longer than day and so that's another reason after-dark tornadoes are more likely.
 
Dan makes an excellent point. With heating less important relative to spring, and otherwise equal chances all hours, winter has more nighttime hours. Thrown in the LLJ and overnight actually gets favored when daytime heating is less important. Still tough to forecast Dixie. Will it be an outbreak of tornadoes... or junk? Since I got a family now I hope for junk at night.
 
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