The day before the event

[STurner]

I have a few things to ask about April 7, 2006. The day SPC issued a high risk for severe weather the day before the event. Now this event by no means was a bust but seemed to underperform somewhat compared to what was anticipated. dont get me wrong it was a pretty large tornado outbreak and unfortunately quite a few people lost their lives. I believe this tornado outbreak produced around 60-70 tornadoes and several of them were F2's and a couple of them were F3's. There was an unprecedented 60% hatched area for strong tornadoes and some of the other hatched areas were also rather large. It made me think there would be at least 100+ tornadoes with multiple F3 and F4 tornadoes and maybe even a couple F5's. It seems like it would have been somewhat similar to the Superoutbreak of 1974. Now there may have been some factors that were somewhat limited for an outbreak of that magnitude and some of the tornadoes may have been stronger but did not hit much, or the homes that were hit were poorly attatched. Does anybody have any answers for what prevented this from being an extremely large tornado outbreak with multiple strong/violent tornadoes.

 

[Skip Talbot]

I think its a mistake to judge the scale of an event by how often SPC issues higher probabilities. Remember that the percentages on the outlook are the probability that an event will occur within 25 miles of a point. You'd expect there to be more tornadoes in a 60% hatched outlook than a 30% hatched outlook, but there are other factors than density of events playing into that probability such as uncertainty in the forecast. If we knew ahead where the tornadoes would be, you'd see very thin strips of 100% tornado probability. Also remember, that 60% is the maximum probability that SPC issues, so if they are almost certain that tornadoes are going to occur over a large enough region, it makes sense to go with a 60% outlook. That doesn't mean its going to be the largest outbreak of all time simply because SPC rarely issues a 60% probability.

The April 7, 2006 event was already underway by the the time the 2000z 60% hatched outlook came out, with numerous tornado reports received. If you look at the reports map there was a big red smear of reports right over the 60%, and yes there were also some F2-F5 reports. So the forecast totally verified. SPC has taken some flak over busted high risks, but this is one they nailed in my opinion.

 

[Chris Hayes]

I'm no expert and I dont pretend to be, but I somewhat remembered this event and looked it up. There were 55 tornadoes this day including 2 F3s and 4 F2s. The rest were F0 and F1. I'm an instability hawk. I'll take instability with low shear over high shear lowe CAPE any day. Instability this day was not extreme and CAPE got no higher than 1000-1500 over much of the high risk area. Had the instability been better there could have been a more extreme outbreak. It certainly wasnt for lack of trying as the storms across much of the area remained scattered supercells for quite some time.

 

[Rich Thompson]

I was working at the time the day 2 forecast was issued for 7 April 2006. The following is my summary of the forecast scenario:

Pros - 1) broad warm sector with at least moderate instability and clearly sufficient vertical shear for supercells, 2) likelihood of multiple discrete storms in that warm sector for a period of many hours, and 3) a forecast of sufficient low-level shear and moisture to support multiple tornadoes, some strong.

Cons - 1) The only question in my mind at the time was the magnitude of the low-level moisture. The NAM (maybe it was still the ETA at the time) showed upper 60s surface dewpoints into nrn AL, while observations suggested that model forecast could be a few degrees on the high side.

As the event unfolded, the dewpoints were more like 58-64 F instead of 68 F, which clearly impacted the CAPE and LCL heights. Given the large number of training supercells across AL that evening, it's not a stretch to see several strong/violent tornadoes given a little increase in the low-level moisture. The 60% tornado area did verify with almost exactly 60% coverage of tornadoes, though I don't think any SIG tornadoes occurred in the hatched area after 20z. Again, it's my opinion that the moisture was a little bit of a limiting factor in an otherwise dangerous setup.

The event was enough to justify a HIGH risk, but it was not quite in the league of the other upper tier tornado outbreaks. Maybe it wouldn't have stood out so much if it hadn't been the first day 2 HIGH risk.

 

[Wes Carter]

Here was the outlook:

And here is what we got:

As far as I'm concerned, it verified. I can remember this day vividly because it was the first time I chased, and I saw my first two tornadoes on this day. But it was a rare fairly easy chase. The supercells were LP/Classic giving very good views even in our hilly terrain with trees. The dewpoints did not get as high as expected, allowing the cloud bases to be fairly high for the Southeast yet still produced tornadoes. This event might have been a bit less remarkable had it occurred in the Plains, but in Tennessee it was a very rare event.

One thing I found fascinating is that the supercell that spawned the first tornado I ever saw formed in Arkansas and dissipated in Virginina almost 13 hours later, producing 9 tornadoes along it's route.

Quoting Elizabeth Konop:
"A single cell (hereafter referred to as “Cell Aâ€￾) initiated at 15:10Z the 7th in eastern Arkansas, propagated across the entire state of Tennessee, and finally dissipated at 04:47Z the 8th in western Virginia. Cell A remained intact for 13
hours, constantly fed by the aforemenioned synoptic scale features along the leading edge of a 300 mile long line of thunderstorms, many supercellular."
Source

 

[STurner]

Yeah thank you everyone I did think it was a pretty big event. It is when SPC had that 60% hatched area you would automatically think several violent F4 and F5 tornadoes. Had the CAPE had been around 3000 J/kg and dewpoints in the mid to upper 60s would it have been like the Superoutbreak. Also if their was to be another Superoutbreak at this present time like there was in 1974, would SPC issue a much larger 60% hatched area than the one on April 7, 2006.

 

[MatthewCarman]

They can't go higher than 60% because they just don't know how bad an event will be untill after it is over and making the 60% hatched area larger would mean they have to be that more certain that a large outbreak is under way.

Edit: I am no expert and this is just my oppinion.

 

[Brian Stertz]

Cap...moisture quality....too much shear...weak LL winds. All of these factors seem to cloud pre-chase forecasts. Heck have seen things change in 6 hrs. before. It's crazy to watch what look to be locks fall apart...and then some more seemingly marginal episodes go bananas...like that wild Utica IL event. Best thing is to have your butt prepared for anything....especially in May & June !!

 

[Wes Carter]

They can't go higher than 60% because they just don't know how bad an event will be untill after it is over and making the 60% hatched area larger would mean they have to be that more certain that a large outbreak is under way.

Edit: I am no expert and this is just my oppinion.

He was saying a 60% area that covered more real estate, not a probability higher than 60%. But if they could accurately give predictions above 60%, I'm sure that chasers could save a lot of money on gas.

 

[Chris Nuttall]

It is when SPC had that 60% hatched area you would automatically think several violent F4 and F5 tornadoes.

As someone has already pointed out, you're misinterpreting the product. Those are probabilities of a tornado occurring within 25 miles of a point. A hatched area means that there is at least at 10% probability of a significant tornado (EF2-EF5) occurring within 25 miles of a point. That 60% hatched area simply meant that the forecaster had high confidence that a tornado would occur in that area AND that it would be rated EF2-EF5. There could be 10 tornadoes in that hatched area all rated EF2, and that probability is verified.

Had the CAPE had been around 3000 J/kg and dewpoints in the mid to upper 60s would it have been like the Superoutbreak.

There is no way to know for sure. You could take the conditions for the day and tweak them (adjust the CAPE, dewpoint, and shear) and plug them into a high-resolution model and see what the output would be. However, the model won't show you tornadoes or what they would be rated. It could produce a simulated reflectivity image, but that's just going to show storm coverage and storm mode. A high number of storms does not necessarily equal lots of tornadoes nor does it equal any tornadoes (assuming they occur) being any particular strength.

 

[STurner]

It did meet the criteria of a high risk when you think of like a 30% hatched area for sure because two of the tornadoes were rated (E)F3+. I just thought with an unprecedented 60% hatched area for strong tornadoes and SPC mentioned a pretty high likelihood of violent in their forecast, that it most likely would happen. No, it is not 100% guaranteed but rarely does SPC mention violent tornadoes in their forecast. I said it was a pretty large outbreak and it did unfortunately kill quite a few people.