Tropical cyclones and tornadoes

[ChristianT]

I feel this is appropriate given the landfall of Tropical Storm Bill along the Texas coast today. I know tropical tornadoes are a little bit different than traditional supercell spawned tornadoes so I have a few questions for anyone who might know. Are the tornadoes spawned by low-topped supercells on the outer bands of the cyclone or can they come out of any storm in the cyclone? Also, is there the presence of a wall cloud like in a traditional thunderstorm? I also know they are more difficult to detect than supercell tornadoes. How can one at least get a clue via radar that one of these quick tornadoes may be about to form? Does the radar give enough clues via velocity scans usually?

 

[Jeff Duda]

Are the tornadoes spawned by low-topped supercells on the outer bands of the cyclone or can they come out of any storm in the cyclone?

Yes and yes. Either can happen.

Also, is there the presence of a wall cloud like in a traditional thunderstorm?

I don't know, but my guess would be that many tornadoes spawned from TCs are not accompanied by a traditional supercellular wall cloud.

I also know they are more difficult to detect than supercell tornadoes. How can one at least get a clue via radar that one of these quick tornadoes may be about to form? Does the radar give enough clues via velocity scans usually?

Due to the shallow nature of tropical convection, many tornadoes are difficult to see coming or warn for until after they have already occurred. At large distances from the radar, the beam will be well above the area where a tornado would be most likely to be detected since it is the frictional drag that enables the strong low level shear needed for tornadoes. Additionally, tornadic storms in TCs usually don't have a mid-level mesocyclone characteristic of a supercell thunderstorm as there is usually very little deep layer shear in a TC (at least above the surface).

 

[Dan Robinson]

While tornadoes can happen in the outer bands of a TC, the best TC tornado days happen after the storm is well inland and a dry slot has worked into the circulation. This allows for heating of the boundary layer and a more classic supercell environment to develop, usually in the southeast or eastern quadrant of the cyclone. These storms can exhibit classic supercell structure and tornadoes, including wall cloud, rain-free base and RFD clear slot.

The feature to keep an eye on is that dry slot - if one can entrain into the cyclone and clear the clouds out near/in the center of the circulation, watch out!

 

[Boris Konon]

Yes, what Dan says about the dry slot is key. TC environments have plenty of low level shear, but little CAPE due to moist adiabatic lapse rates. Now, introduce a bit of dry air at mid levels, and that steepens the lapse rates. All you need 500-1000 CAPE and you can get swarms of small tornadoes.

 

[Randy Jennings]

FWD had an informative AFD on Bill tonight. While it doesn't cover tornados, it is a good primer on tropical systems for those of us who don't see them to often. You can find it at: http://mesonet.agron.iastate.edu/wx/afos/p.php?pil=AFDFWD&e=201506170233

 

[Dan Robinson]

Interesting that we had this thread a few days before an actual TC tornado event here. Here is a list of some of the videos of tornadoes from this event in Illinois and Indiana, some from locals and one from me. The pronounced RFD I saw on the Enfield storm surprised me, carving a cylindrical meso around the tornado circulation.

Again, this day produced because the NE quadrant of the cyclone cleared out in the morning and destabilized. That's the big cue that you need to be out chasing! One big tip with chasing TC tornadoes is that the parent storm does not have to be very strong. Any area of reflectivity with a break to its south is a player. The Enfield storm appeared rather weak on radar, but it had a big break of clear air to its south. The Macedonia tornado was imbedded inside a shield of rain closer to the center of the TC circulation. High-res radar is helpful with these, as the circulations are usually too small for WxWorx to pick up. None of these had rotation markers in WxWorx, but the couplets were clearly visible on velocity images, even more so on SRV. I tend to use SRV more with these types of events as it's a little easier to see weaker circulations.

Macedonia, IL:
https://www.facebook.com/wsilnews3/videos/10153498716434880/?fref=nf

Lawrenceville, IL:
https://pbs.twimg.com/media/CH5DshNWoAE_JB-.jpg:large

Troy, Indiana
https://twitter.com/RyanHokeWAVE3/status/612020035842916352

Enfield, IL: