11/27/05 NOW: Central/Eastern Plains

[Helge Tuschy]

@ Mr. Thompson,

congratulations...I think you all did a really good job in forecasting this severe weather day. To be honest, I was sceptical ( only a little bit ) about your westward extension of the 15% TOR prob. in Kansas, but finally it was the right decision.
One question and I hope it sounds not too stupid...:

When considering "TOR watch versus PDS watch", does the topography also play a role in your decision? In NW Arkansas we are talking about some elevations 2 kft or higher, which could influence storms in either way . So do you now say that maybe the topography will be too rough for tornadoes becoming long-tracked /-lived ...or at least there will be the chance for 1 or 2 of them to become long-lived, but all in all there will be an higher chance for "shorter" tornado tracks due to the rough topography ----> issuing an high probability tornado watch, but not a PDS watch.

Good look in the next MDT area,

Helge, (Innsbruck, Austria )

 

[Andy Wehrle]

Helge,

It has been demonstrated time and time again that local topography plays little if any role in tornado potential. Hills, valleys, and river beds that were thought to protect certain areas have been traversed by tornadoes many times, often with devastating results.

 

[Greg Stumpf]

It has been demonstrated time and time again that local topography plays little if any role in tornado potential. Hills, valleys, and river beds that were thought to protect certain areas have been traversed by tornadoes many times, often with devastating results.

Not so fast!

Present science indicates that the wind shear in the lowest kilometer might be an imporant determination of whether a supercell is tornadic or not. And there have been a few mobile Doppler radar data sets indicating tornadic strength vortices just above ground level with no evidence of damage at the ground.

I think what can be stated is that the effects of local topography on tornadogenesis are not yet well understood, and that there is currently no definitive proof that topography can greatly enhance or supress tornado formation. Yet, there is ample evidence that topography cannot *completely* supress tornadoes (there are many other factors way up high in those storms). So if someone asks if the river or hill southwest of their town "protects" them from tornadoes, you should tell them not to be so trusting of that notion.

 

[Ronnie Bradley]

radar pics from the November 27, 2005 Arkansas outbreak

At last, some screengrabs...
2304Z level III bf/srv

Same volume; LII data:

0004Z level III. This was the supercell that later crossed I-40 and caused the casulty:

L2 data... Note the very tight couplet.

I'll post more pics to this post later on...
Here is a cross-section of the Morrilton / Plumerville supercell, 11282005_0004Z volume:

 

[Helge Tuschy]

Hi Andy,

I think you misunderstood me. I'll quote myself:

"So do you now say that maybe the topography will be too rough for tornadoes becoming long-tracked /-lived ...or at least there will be the chance for 1 or 2 of them to become long-lived, but all in all there will be an higher chance for "shorter" tornado tracks due to the rough topography "

I defintitively did not say that there won't be any long-tracked tornadoes in NW Arkansas where topography is a little bit more rough and I did not want to open this discussion again how strong the topography will influence tornadogenesis.

[ Fact is that I also had some good/bad experiences here in Germany, when forecasting tornadoes in rough topography...e.g.models depicted an area of enhanced LL shear in SE France on 22th Oct., but only weak LL shear over central Germany....one storm developed in an hilly region in CNTRL Germany and finally produced a F2 tornado....My opinion is that there CAN be some influence by topography ( stronger localized backing wind field ...) but our models here are too coarse for proving it ATM...Another interesting paper " Hailstorms in Switzerland...by R.A. Houze", where LL anticyclonic curvature in the lowest levels CAN influence splitting of TSTMs...I agree that a tornado can travel also over rough terrain // even long-tracked //, but this was not the point I discussed above ].

I only was interested if the SPC forecasters also included topography in this case, yesterday. The forecaster looks at the screen and watches a squalline with many discrete cells over W and NW Arkansas... Now he asks himself if he should issue a PDS or a "normal" tornado watch with higher probabilities. That's the point where I would be interested if the topography also played any influence for his final decision. E.g. the decision for issuing a 60%-60% tornado watch:
The forecaster shows with these high probabilites, that the environment is very conducive for tornadoes, even 1 or 2 significant one. Now....maybe one point for not issuing a PDS watch could be that he saw the rough terrain over NW Arkansas and comes to the final decision that this could be one negative factor amongst other for long-tracked/significant tornadoes, maybe lowering the confidence in a PDS watch....maybe as a result of not really knowing how strong a LL wind field of a supercell will be modified by topography etc.

I hope that I wrote a little bit more understandable Otherwise I think we have to discuss this in another room, because this drifts off too much from what there should be dicussed in this room.

Regards, Helge

PS: The subject with issuing watches. I know that a forecaster has to look
at many more things for issuing one. I only wanted to empahsize my
topography topic

 

[Rich Thompson]

Helge,

The terrain in AR had little impact on the watch decisions yesterday. Perhaps the most common influence of the high terrain in AR is to somewhat impede the richest moisture return toward SW MO, though timing had as much to do with limiting moisture farther N as the terrain did yesterday.

I don't believe I've encountered an environment where I was willing to use the terrain as the final deciding factor between watch-no watch, or "normal" - PDS. As Greg S mentioned, we just don't how much influence 1-2 thousand foot mountains have on the minute-by-minute evolution of a tornadic supercell.

Rich T.

 

[HAltschule]

Yet, there is ample evidence that topography cannot *completely* supress tornadoes (there are many other factors way up high in those storms).

Hoorah, Hoorah. Case and point. As the SPC forecasters know, we have some really neat local topography here in the Albany, NY area. The winds that are turned by the North/South Hudson Valley have been known to influence low level shear dramtically. In fact, when storms hit the Hudson River Valley and then the hills directly to the East, they frequently blow up in intensity. DEFINITELY some localize terrain effects here.

Check out this great study on this very topic and the F3 tornado that ripped through the Berkshire Mountains in 1995!!

http://www.erh.noaa.gov/er/aly/Research/terrain529.pdf (by Lance Bosart, Ken LaPenta, Mike Dickinson and Anton Seimon).

Enjoy

 

[Helge Tuschy]

"The terrain in AR had little impact on the watch decisions yesterday."

Hi Mr. Thompson,

thanks for your answer.

Regards, Helge