Tornado Forecast Busted 9-14-05 - Let's Discuss Why

[Nic Wilson]

I made the decision not to chase in the morning after noting two things which I have learned do not equal chasing success in the NW TX/Oklahoma areas.

Factor #1 was definitely the synoptic set-up with bad timing of the ejecting jet max from the southwest (too late in the day) and the fact that the forcing mechanism was a Cold Front -- I'll take a dryline over that anyday. The SW OK area recovered nicely after the morning elevated garbage, so the added CAPE and significant convergence along the front didn't help sustain the cap across a broad area.

Factor #2 was the presence of that early morning elevated convection. This shifted the extreme instability pretty far south of the Red River in TX, this area was disjointed from the best shear as others have noted. 500 mb winds were much weaker (~30 kts) and low-level flow much more uni-directional.

On second thought, I don't know of many scenarios that are good for chasing in my first 12 months in this state haha. Everyone's luck is bound to change around here -- in 2006 sometime! I love days like yesterday where the shear is off the charts, but something just didn't feel right. Another $50 in my pocket for when I totally bust my next chase...have a great weekend everyone!

 

[Jeff Snyder]

In regards to instability... Below is the 00z 15 Sept WRF initialization of CAPE...

The NAM and RUC initializations looked similar as well (as the NAM should, since I think the WRF takes its initialization off of the NAM, though I could be wrong). In fact, the RUC (from SPC Mesoanalysis, which I've learned to take with a grain of salt) showed the max CAPE to be in southern OK, decreasing with southward extent into TX.

 

[Nic Wilson]

Thanks for that graphic Jeff, the best CAPE axis set-up just to the west of the morning elevated convection and it aligns well with where the storms probably had the best chance of moving off the boundary and become surface based. Another thing that hasn't been discussed is LCL heights were disappointingly in the 1200-1500 m range across this area. If Tds could've been a bit higher that would've helped matters.

I'd also like to give props to the GFS for this event. I gave weather briefing on Wed. Sept. 7th and all the signals were there in that 12z run that Tues/Wed. would be very active across the central and southern Plains (a full 6-7 days in advance!). It truly is amazing the advancement of our numerical weather prediction models on the synoptic scale. Errr mesoscale, yea thats what we need to work on.

 

[Scott Olson]

I don't think it was a lack of instability or shear. Even with southward extent into Texas shear was sufficent for organized severe cells and in SW OK at least moderate instability was juxtaposed on 60-70 kts Effective Shear and 200-400 m2/s2 0-3km SRH. I think the lapse rates/subsidence and a large spatial area reaching convective tempature played a key role as well as the storm motion generally being parallel to the boundary.

 

[Bill Tabor]

Originally posted by Jeff Snyder
I'm not sure what you mean by 500mb negative buoyancy.

Right. I suppose that is a bad use of terms. I mean't suppressed by the 500mb NVA which was showing on earlier NAM model runs for that entire area and that I posted under FCSTS. There was supposedly a shortwave coming into west Tx past the Tucamcari profiler but I don't think it made it presence known to help assist during the daylight event. Maybe that forecast NVA never materialized completely - not sure. Anyway generally more of a subsidence issue.

So, you were thinking it was under the 250mb right entrance? Hmm, would have to check on that as it would be interesting, but I was pretty sure it was under the 300mb right exit.

 

[Joe Nield]

I see several mentions of LCL here, but lately I've become of the mind that the big players, given what overall appears to be a relatively supportive synoptic environment, are down low, such as LFC heights, low level CAPE, and 0-1 km shear. Jon Davies has done fairly significant research into these topics, and we here at IND have spent a lot of time, especially over the last year to 16 months since the May 30, 2004 outbreak, discussing these parameters and watching to see how these parameters come into play and evolve during tornado events in our area.

LCL, while it has been shown to have some correlation with tornadic supercells (lower being preferable, of course), really only tells you something about the boundary layer relative humidity, and nothing about the quality of low level parcel acceleration that may be critical in promoting tornadogenesis. The parameters that do have something to say about the location and strength of this good low level parcel acceleration/stretching appear to perhaps hold value operationally in determining an environment conducive to tornadogenesis, with a low LFC generally implying little to no low level CIN, etc, and thus strong vertical motion in the low levels.

Check out Jon Davies' excellent website at: http://members.cox.net/jdavies1/

 

[afischer]

Another problem--basically relating to the "instability and shear not coincident" problem noted by others--was the narrow nature of the favorable corridor for tornadoes immediately north of the front. I think the tight gradient of CINH (south to north) may have been a problem for storms crossing the boundary. If isolated updrafts could have moved parallel near/north of the boundary I think they could have utilized both boundary layer instability and the decent boundary layer SRH values in place and maybe popped out a tor or two. I don't think LFC heights were a problem, since moisture was deep and rich and 700mb temps do-able at 11C or so.

[This is ignoring the "mess of too many updrafts" problem, which of course was a biggie.]

 

[Joe Nield]

Originally posted by afischer
Another problem--basically relating to the \"instability and shear not coincident\" problem noted by others--was the narrow nature of the favorable corridor for tornadoes immediately north of the front. I think the tight gradient of CINH (south to north) may have been a problem for storms crossing the boundary.

Absolutely. The best boundaries for efficient tornado production are those which have modified enough that there is only a weak instability gradient across the boundary itself (read: temps and dewpoints hardly drop), allowing storms to remain essentially surface-based while drawing on the significant increase in SRH that the backed winds north of the boundary provide.