upper level winds and tornado production

[jshields]

i've noticed that with this weeks storm system coming out on wednesday, the upper level winds (250,300) are from the southeast near the triple point across northern kansas and from the south across southern kansas so the winds are backing at those levels vs the winds at 500. i know at this level these are normally the anvil level winds, so besides making some very interesting looking hodo's, do they have any effect at all on storms ability to produce a tornado? i know i normally see people talk about winds up to the 500 hPa level, but I was curious on whether people look at wind direction at these levels or just wind speed?

12Z hodo example: http://www.twisterdata.com/index.php?sounding.x=454&sounding.y=326&prog=forecast&model=NAM&grid=221&model_yyyy=2011&model_mm=05&model_dd=09&model_init_hh=12&fhour=60&parameter=WSPD&level=250&unit=MB&maximize=n&mode=singlemap&sounding=n&output=image&view=large&archive=false&sounding=y&sndclick=y

 

[Chip Redmond]

I don't see how it would really influence tornado production at one point from the skew-t. Amount of upper level divergence would still be the biggest factor in creating convergence at the surface and therefore impacting tornado production and really thunderstorm production of any kind. The only way other way I can think it would influence tornadoes is perhaps placing the decaying anvil away from the warm sector/moisture source of the storm. This would aid in surface heating in this region instead of overspreading it with cloud cover and creating a weak cold pool, prohibiting supercell sustainment. Even then, the low level and mid level winds are the biggest key to producing long-lived storms/tornado production and steering cold pools, though they are still influenced by the location to a jetstreak.

Chip

 

[Brett Roberts]

The shear vector (vector difference between the wind above and below a point on the sounding) is one factor which determines the distribution of pressure perturbations in and around a supercell, and the type of hodograph you posted does raise some significant concerns. In an idealized half-circle hodograph, the shear vector turns smoothly clockwise by about 180° from the surface to the anvil level, and this helps to enhance the right flank of a supercell via the associated vertical perturbation pressure gradient force (VPPGF). In the forecast hodographs for Wednesday, particularly from about I-40 northward, the shear vector abruptly reverses direction in the mid-troposphere (due to winds abruptly backing with height above H5, after smoothly veering with height below there). The resultant three-dimensional VPPGF distribution is much more convoluted and less conducive to right-flank enhancement. I will try to dig up some relevant paper(s) explaining this in more detail later, if someone else doesn't beat me to the punch.

EDIT: Here's a good paper on the subject: http://twister.ou.edu/MM2005/Klemp87Review.pdf (take a look at Fig. 7b on pg. 16)

In practical terms, I know many chasers loathe seeing "S-shaped hodographs" based on real-life experiences, and I'd have to agree. One event I can recall in which this veer-back-veer profile was an issue is April 26, 2009. The HIGH risk area (driven by 30% hatched tornado probs) for that case saw only a couple tornadoes, and IIRC, many of the storms exhibited only borderline/intermittent supercell structures that day. Now, there are several possible reasons for why that case failed to produce a significant tornado outbreak, including early initiation and excessive storm coverage. Even so, it is quite possible that the strung-out, non-classic look of most storms that day in OK might be attributable to the whacked-out shear profile. I'm sure we have some more experienced chasers here who might be able to offer additional past chase days of significance on which S-shaped hodographs were a factor.

Aside from the VPPGF concerns, I'd also be worried about heightened potential for anvil seeding given very meridional winds at 200-300 mb.