Theoretical question concerning cold-weather tornadoes.

[David Draun]

I'm sure this has been addressed before, but I thought I'd start a new discussion on it. Is it theoretically possible for a tornadic supercell to exist in near-freezing surface conditions? This would definitely limit or eliminate surface instability, but keeping in mind that all other ingredients are in place such as directional and speed shear, with SE winds in the surface layer bringing in somewhat warmer air. This particular storm would be oriented on or near a warm-front. Could the storm be able to tap surface-based parcels, despite being in a near-freezing surface environment? I've seen supercells in sub-freezing conditions which were elevated. Multiple severe warned storms in NC IL just a couple weeks ago exhibited supercell characteristics with hook echoes and persistent MESO indicators. Albeit, these storms were elevated and posed no tornado threat whatsoever.

http://stormtrack.org/forum/showthread.php?t=15128

The precip cores had produced brief thundersnow mixed with some severe-criteria hail in Lee and Dekalb counties in IL. The surface temps were between 30 and 34 degrees at the time of the activity.

Now given a storm like that, would it be able to tap surface based parcels and possibly produce a tornado? I had read a thread once on here about tornadic mini-supercells in Britain that had produced a strong tornado or two with the precipitation starting as snow.

 

[Jeff Snyder]

If the near-surface layer is very stable, I don't think it's likely that a supercell would be able to ingest surface parcels. It's not uncommon for the lower-part of a supercell updraft to be negatively buoyant if vertical perturbation pressure gradients associated with vertical wind shear (i.e.a veering wind profile) are strong enough. Heck, that's partly what happens when a storm exists in an environment with a cap or CINH layer. That said, unless the mid-level temp is extremely cold (yielding potential instability for a surface parcel -- something that we assumed we not present), it's unlikely that a very cold surface layer (particularly <32F) would lead to convective potential instability. Again, I'm not really talking about cold-core events or other events where the surface temps are cold but still "warm enough" compared to mid-level temps to lead to positive CAPE for such a surface parcel.

Now, that's not to say that the tornado potential would be negligible. What happens if a very strong but "warmer" RFD was able to penetrate to the surface? I find it feasible to think that the airflow around and in the downdraft may be able to penetrate through the very stable layer. If this happens, the storm may essentially "dislodge" that very stable surface layer immediately beneath the supercell, which may allow for the development of a tornado. Remember -- very strong low-level shear is often associated with stable layers, and it's important to remember that it's quite unlikely that such a stable layer (depending upon all sorts of things, including the depth and magnitude of said stability) would actually be entrained into the supercell. This is partially why the "effective deep-layer shear" and "effective storm-relative helicity" that are shown on the SPC/RUC mesoanalysis graphics are useful.

 

[Bill Tabor]

Good / interesting question and nice commentary Jeff. I'm not the expert here (that would be Jeff), but along the lines of his RFD comments I'm thinking also perhaps as you mention along the boundary and lets say a very thin surface cold frontal boundary layer has just penetrated a geographic area and let's say the sup is feeding primarily above sfc say 925mb up. Seems to me if you had a very unusual extreme upper instability situation of cold temps aloft and extreme lapse rates then perhaps....what do you think Jeff?

 

[Paul Knightley]

We have had storms in the UK which may well have been supercells with temps not far off of zero - however, these are generally near the coast in polar maritime type airmasses rather than being of a frontal nature.

 

[Rich Thompson]

If you assume a saturated lifted parcel from a surface temperature of roughly 32 F and a surface pressure of ~975 mb, your lifted parcel temperature is ~-18 C at 700 mb and ~-38 C at 500 mb. Those are quite cold temperatures that are not observed in tandem too often with a surface temperature of 32 F, and you'd need environmental temperatures at least a little colder than that to have any buoyancy. Some buoyancy is necessary for vortex stretching, since you'd be hard pressed to establish the typical supercell pressure perturbations and forced low-level ascent in the complete absence of instability.

There have been reports of tornado-like vortices within lake effect snow bands. Such environments have very steep low-level lapse rates and some CAPE driven by the underlying "warm" lake. However, vertical shear is normally weak in the cloud bearing layer for strong lake effect bands, so it's doubtful that the swirlies have much to do with supercell processes.

I've heard of a few cases of tornadoes occuring along the NW coast of Japan in the winter. Some of these tornadoes have occurred with snow cover and have been sufficiently strong to knock over train cars, but I don't know if the parent convection was supercellular.

Rich T.

p.s. The case of an elevated supercell atop a shallow cold layer is a different situation altogether. Such conditions are somewhat common, but tornadoes rarely occur for the reasons Jeff mentioned above.

 

[Donald Giuliano]

Several tornadoes that may have formed in a way similar to that suggested to Jeff Snyder occurred near Altus, OK on Feb. 22, 1975. Here is a surface map valid 0600 UTC that morning, with the counties affected by tornadoes highlighted.

--Don