7/13/04 REPORTS: Great Lakes/Ohio Valley

Reviewing the synoptic setup, I am still astounded that such a strong tornado formed. Based on the CAPS analysis from 2 pm, there really is nothing on the synoptic scale that would suggest that strong tornadic supercells would form. I think it is very likely that this event was driven by the all-too-familiar extreme CAPE/low level boundary combo. The storms moved very far to the right of the mean flow, muck like the Jarrell, Texas storm of 1997. I am sure someone will do a case study...should be interesting.

The damage done to that factory looks like F4 to me...it really tore it up. From the prelim. damage survey, this tornado might have been even stronger (considering the two houses that were completely wiped). It's pretty unlikely that they will rate it an F5, but it is certainly a possibility. It sure is great that no one was killed. 8)

Gabe
 
It wasn't a synoptically obvious event, but you don't ignore >6000 CAPE w/ 100kt@H250, 50kt@H500, 40kt@H700 events, even if it is unidirectional. Surface winds were progged to be southwesterly but ended up being more southerly.

No, that picture is not mine. It was allegedly taken with a Sprint camera phone. What an incredible year.

Scott
 
It wasn't a synoptically obvious event, but you don't ignore >6000 CAPE w/ 100kt@H250, 50kt@H500, 40kt@H700 events, even if it is unidirectional. Surface winds were progged to be southwesterly but ended up being more southerly.
Click to expand...

This is true. I suppose the more amazing thing was how long the storms stayed relatively isolated...the merger into an MCS took a long time, allowing the storms to stay tornadic for quite a while.

Gabe
 
I don't think the environment was ever uni-directional - except perhaps for the early elevated convection. Flow aloft was definitely NWerly, so with SW flow at the surface you have a quarter turn hodograph. At times, the flow did appear closer to 200 instead of 235, and may have been more like 180 near the storm updrafts resulting in motion opposite the surface flow (streamwise vorticity). There was a strong thermal boundary nearby (64F at Pontiac at 1943Z), but this was well east of the main tornadic cell, which occurred in a region where a number of new cells were generating and the southwesternmost storm produced (in an amazing 40 minutes from first echo) the large tornado. While not a textbook severe weather event, such events do occur several times a year. With extreme instability, amazing things can happen with marginal low-level shear - sort of the opposite of the IL event in April were incredible shear was combined with marginal instability - both yeiding significant tornado events.

BTW, I have a new appreciation for cell phone cameras - not a bad pic!

Glen
 
These tornadoes formed in an incredibly humid environment. Dewpoints of near and even ABOVE 80 degrees were observed in northern IL. I cannot recall another tornadic event with dewpoints this high before. Although I'm sure it has happened.
 
Reviewing the synoptic setup, I am still astounded that such a strong tornado formed. Based on the CAPS analysis from 2 pm, there really is nothing on the synoptic scale that would suggest that strong tornadic supercells would form. I think it is very likely that this event was driven by the all-too-familiar extreme CAPE/low level boundary combo. The storms moved very far to the right of the mean flow, muck like the Jarrell, Texas storm of 1997. I am sure someone will do a case study...should be interesting.
Gabe
Click to expand...

While an outflow boundary could very well have played a role, I don't think it was necessary. Based on the 18Z ILX sounding, about 2 hours before the tornado, there was 25-30 knots of speed shear in the lowest 0-3km. There is almost 90 degress of directional shear over 0-6km AGL, and 30-35 knots of speed shear over 0-6km. The 700mb winds were 35 knots, and I've rarely seen flow at this level less than 35 knots when violent tornadoes were produced. And don't discount 20 knots at 850mb, especiall with the direction shear present. With the amount of CAPE present, 20 knots at 850mb is fine. Also, this storm did not more very far to the right of the mean flow. The hodograph has an estimated storm motion to the southeast, which is pretty close to the direction the supercell moved. I do agree about the extreme CAPE, with the actual CAPE being nearly 4000 j/kg, that's a lot to stretch a vortex.

Keep in mind this sounding represents the atmosphere about 2 hours before the supercell produced the tornado. The midlevel winds strengthened throughout the day as the shortwave approached. I'm willing to bet the 0-3km shear was even stronger when the tornado was produced. Based on this sounding, I'm confident the environmental shear alone was conducive for violent tornadoes.

ILX 18Z Sounding

Cheers,

Jim Bishop
http://stormgasm.com
 
I'd have to agree with Jim here for the most part. I don't think it was THAT synoptically non-evident. I mean, you still had about 90 degree directional shear in the 3-4 km, which makes this no different that the typical Plain's southwest-flow aloft with se surface winds, which yields 90 degrees of veering. Wind profiles did strengthen through the day, yielding a more favorable environment than the 18z ILX shows. Yes, 0-3km SRH wasn't too impressive, but the extreme instability made than made up for it. I'm not entirely sure why NW flow events catch people "off-guard', since I've had plenty of success in these types of events. It's really no different than the more textbook setups, and there really isn't any more reason for storms to go MCS instead of discrete supercells as long as the shear vector is appropriately oriented to the convergence-inducing boundary; shear vectors perpendicular or 45 degree to the boundary tends to favor discrete activity, while shear vectors parallel to a boundary tend to favor MCS development... The 'perpendicular shear vector - boundary orientation' may be more common in the plains in the spring given the usual orientation of the dryline, but I've seen many northwest flow events away from the plains still with favorable orientation...

I don't feel the environment would have supported strong+ tornadoes if there was only 2000 CAPE available, as the low-level shear isn't that strong. However, increasing the CAPE to 4000-6500 changes the story signficantly...
 
TWC is showing new video of the Roanoke tornado, from a similar angle taken by the cell phone camera. Looks like one of the largest tornadoes of the 2004 season, impressive.
 
It wasn't 'obvious' that a 6hr discrete supercell with a violent tornado would occur synoptically, at least the night before. The first storm initiated at about 9 am. Sfc winds were progged to be almost WSWrly (they ended up being SWrly almost Srly) with a fairly unidirectional profile, though speed shear was progged to be extreme with extreme instability/steep lapse rates. Yes, I understand SR flow (hodos) and can adjust for NW flow days, but I don't recall the directional shear as transpired as being progged. Any factor of boundaries is not synoptics. Was it obvious that it COULD happen? Yes, you *never* ignore (you treat it very seriously) that kind of instability with that kind of speed shear, but it was not obvious (at least the night before, I never had a chance to do a proper fcst the day of) that it would be a day with a discrete classic supercell lasting for many hours. Possible, sure, but how likely? If it was, where were all the forecasts of that preceding it? I saw nothing from anyone. Hindsight is convenient. There is some culpability in models and overreliance on them of course; and it also again illustrates how fairly subtle differences can be of paramount importance. Additionally, the ETA showed precip not firing till late afternoon/early evening, a far cry from mid-morning supercells with a peak in the early afternoon.

Anyway, as recently announced, the tornado has been rated F4. I just got back from doing a survey, and have pics uploaded to a raw directory here: http://www.skyodyssey.com/chases/2004/13Jul04/Damage/
I'll get a real page up eventually and will also add a chase account with pictures when I recuperate. I saw 'mesos' and some nice updraft structure, several rotating wall clouds, some funnels, some nice towers in the distance, some decent mammatus, golfball hail (missed the softballs) , and severe winds (and a LOT of damage), but missed the big tornado. Long story, I'll get around to sharing it sometime, maybe ;-).

Scott
 
I agree that the situation WAS very synoptically evident. SPC did a good job with the event, although they were playing it safe with their 5 % given the non-Oklahoma classic textbook setup. I didn't look at the radar data in detail, but I ask those whom were: was this storm playing along an outflow boundary from an earlier supercell? Nonetheless, the hodograph supported decent helicity, definitely contained adequate supercell shear, and the CAPE was tremendous, making this an obvious eye-opening day for the forecaster.

Here's the official damage report. -- F4 rating

http://www.crh.noaa.gov/ilx/events/jul132004/jul13.php

With the right boundary in the right place at the right time, I wouldn't have been suprised to have seen an even more historic vortex.

Chris
Click to expand...

I also agree, though it did catch me off guard just a little...

Here are some composites: http://www.waveformpc.com/roanoke/

RUC shows 0-3km SRH at 175M2/S2, which sounds kind of low, until you mention CAPE in excess of 5000J/KG. Also, SFC obs nearest to Roanoke, IL indicated SFC winds almost due south, with a northwesterly 500mb jet of 55KNTS sliding into the region... Thats more than 90 degrees of rotation from SFC to 500MB.
 
It wasn't 'obvious' that a 6hr discrete supercell with a violent tornado would occur synoptically, at least the night before.
Click to expand...

Thats true...

Here is the 12Z ETA from July 13 (12Z Init) with forecast 10m SFC winds -

Blue = ETA 500mb winds
Green = ETA SFC winds
Yello = Actual SFC obs

Looks like even the 12Z ETA was off by about 70-80 degrees in SFC wind direction...

I guess I have to change my "opinion" on things, since even the 12Z ETA from July 13 didn't even pick up the SFC winds. This was more mesoscale than synoptic scale, since synoptic scale SFC winds (over a broader area) were more southwesterly... Only one or two stations reported southerly, which IMO would be mesoscale...

[Broken External Image]:http://www.waveformpc.com/roanoke/actual_vs_eta.gif
 
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