Question from 6-16-10 S.D. event

[A Luttig]

Hello.

I have a question that hopefully some of you more experience mets can help me figure out. When a supercell is as stationary as the one yesterday, how does it not basically collapse on itself and cut off its own inflow with cold, stable downdraft?

My guess would be strong high level shear????

Thanks in advance for the help.

 

[Andrew Ryan]

Shear is definitely necessary to separate the downdraft from the updraft. But the shear was not very strong yesterday or else the storm would have took off rather than drift at about 8kts. But there was *just* enough to get the job done (create downdraft/updraft separation).

I'm sure someone more experienced can elaborate on this more.

 

[Jeff Snyder]

The important thing to remember (and you implied this) is that we need to look at vertical wind SHEAR. Imagine an environment with 30 kt easterly surface winds and 30 kt westerly mid-level winds . An initial storm motion may well be stationary, though the low-level shear may act to keep the storm's cold pool / outflow from spreading out too far and undercutting the updraft.

The 00z RAP sounding (HERE - for a short time) has 47 kts of 0-6 km wind vector difference (ok, we typically call this "deep-layer shear") and a Bunkers right-moving storm motion at 15 kts to the ENE. The SPC mesoanalysis valid 00z shows >=40 kts of 0-6 km "shear" (see HERE), which is certainly within the range that we typically look for when it comes to storm sustainability. The mesoanalysis also has high 0-3km (supporting updraft rotation) and 0-1km SRH (]see here, supporting significant low-level mesocyclones and tornadoes). The CAPE, low-level shear, and moderate LCL heights yielded high Significant Tornado Parameter values (see here), as well.

We don't really always have a good handle on storm motion, however, and I wasn't paying particularly close attention to this day, so I'm not sure if there was a quasi-stationary boundary around to help focus updraft development (in a 'backbuilding' sort of way). It probably helped that there was a decently deep layer of strong southeasterly flow to help the storm "manage" its outflow; a "forecast" sounding near Dupree, SD, from the 00z NAM initialization from that evening has >2 km of 20-30 kt southeasterly flow at and above the surface (see HERE).