How often do strong/violent with low probabilities

[STurner]

There a few dates I can think of when the SPC issued like a 2-5% area for tornadoes and even though strong/violent tornadoes did occur. These would include May 12, 2004, July 18, 2004, and August 26, 2007. How often does this occur and how many other days have been just like the ones I mentioned above? I was also wondering if it is possible to get an F5/EF5 tornado associated with squall lines or if it has ever occured on days with low tornado probabilities.

 

[Connor McCrorey]

If I remember correctly, May 3, 1999 started as a slight risk in the morning, was upgraded to moderate later, and was eventually upgraded to a high risk that afternoon.

 

[Matthew Harding]

I was also wondering if it is possible to get an F5/EF5 tornado associated with squall lines.

The odds of getting an EF5 tornado embedded in a squall line are quite low. To get the major tornadoes, the storm needs to be in an environment where it can get all the moisture and CAPE that it can. Further, when storms line out, the chances of significant tornadoes (EF3-5) decrease because usually the squall line is either on a dryline or on a cold front. Usually the main threats from squall lines are damaging winds and large hail, however, thats not to say that there hasn't been tornadoes that have formed at the edge of squall lines, they just haven't been the major (EF3-5) tornadoes.

Hope that helps!

 

[Skip Talbot]

I think the instances in which a strong or violent tornado occurs when they were otherwise not forecasted is when there are extreme values for some of the necessary ingredients, but lacking amounts of other ingredients. I noticed you mentioned some summer dates. Without actually looking at the setups, I'd guess that there was extreme instability but components of the shear were missing (typical in summer). If for some reason a storm can find a local source of directional shear (outflow boundary) that was otherwise missed, then you can get a violent tornadoes because of the updraft strength created by the extreme instability. Also if the directional shear is weak, or the speed shear is unidirectional, the storm mode is typically forecasted to be linear. Early in the storm's life, however, there is a window for tornado development. July 13, 2004 comes to mind in which dewpoints over 80 degrees in central Illinois created extreme instability. Strong northwest flow aloft but otherwise weak directional flow, created an environment favorable for a derecho. Indeed a massive derecho did form, but before it took off one of the first cells dropped an F4 on Roanoke. The day 1 tornado forecast for that event was 5% (which was probably correct given that this was one of the only tornadoes that day). The Plainfield, IL F5 formed in a similar environment as well and was also underforecasted.

Environments with excessive shear and limited instability can produce strong or violent tornadoes as well. If there is some instability that was missed in the forecast either because of added heat or moisture or because the instability was hiding in the low levels then you can get a tornado outbreak with violent tornadoes. Without instability generally you'll see 0% tornado probabilities. April 20, 2004 comes to mind in which central IL had an outbreak with an F3 because of some strong heating behind a warm front that wasn't forecasted. The initial day 1 tornado forecast was 0% over the area.

 

[Andrew Butler]

05/25/08 Day 1 was 5% tornado up until the 2000z Outlook for northern Iowa. (i.e. Parkersburg)

http://www.spc.noaa.gov/products/outlook/archive/2008/day1otlk_20080525_1630.html

Granted, the last outlook included northern Iowa in the 10% hatched.

This is kind of grasping at straws because personally I don't remember a lot of days with low probs that had violent tornadoes.

 

[Dean Baron]

The Parkersburg tornado is a good example. I read the assessment for the NWS-DMX about how things went during the event, and they placed a lot of emphasis on gravity waves playing a big role creating the EF5. Without the gravity waves, atleast what I got from the assessment, the storm and tornado probably would not have been as strong. The gravity waves that day were confined to a relatively small area in north central Iowa, and places where there wasn't that extra localized source of lift there weren't really any storm. The tornado that hit Hugo, MN the same day rode an outflow boundary and the Parkersburg storm was associated with the gravity waves. If you look at the storm reports from that day there went many (if any) reports between the Hugo storm and the Parkersburg storm, which if I remember correctly was an area without an additional lifting mechanism.

I think if the stronger/more violent tornadoes occur on days with the lower TOR risks it is because the threat for the stronger tornadoes is too localized for the SPC to really say in their outlook, or they may not even notice the localized threat. Maybe I'm wrong but this is the way I've always thought about it...

 

[Connor McCrorey]

Speaking of gravity waves, there are several excellent articles on the Jarrell, TX, F5 and the conditions under which it formed (extreme instability, shear enhanced by a nearby boundary).

 

[Greg McLaughlin]

The only case I can think of where a violent tornado occured in a squall line was April 24, 1993 in Catoosa, OK. That supercell was the "tail-end charlie" of a larger squall line and became tornadic as it crossed the Tulsa metro. The F4 that hit Catoosa killed 7 people as it tracked along I-44 from east Tulsa through Catoosa. A second tornado formed immediately east of Catoosa and I believe it received an F3 rating.

Anyway, the most likely scenario for a strong or violent tornado would be with a "tail-end charlie" supercell or when a discrete supercell merges with an advancing squall line.

The odds of getting an EF5 tornado embedded in a squall line are quite low. To get the major tornadoes, the storm needs to be in an environment where it can get all the moisture and CAPE that it can. Further, when storms line out, the chances of significant tornadoes (EF3-5) decrease because usually the squall line is either on a dryline or on a cold front. Usually the main threats from squall lines are damaging winds and large hail, however, thats not to say that there hasn't been tornadoes that have formed at the edge of squall lines, they just haven't been the major (EF3-5) tornadoes.

Hope that helps!

 

[Terrence Cook]

the most likely scenario for a strong or violent tornado would be with a "tail-end charlie" supercell

Indeed, this was the scenario for the Evansville, IN F-3 tornado (Nov 6, 2005). It formed in the tail end supercell of a line that stretched 200+ miles north. It's worth mentioning that the squall line was one of the more violent I have ever experienced, probably due to the fact that it hit at 2:00AM or so.

http://www.crh.noaa.gov/pah/?n=evansvilletornado-nov.6,2005

 

[Gabe Garfield]

I think the answer is: not often. I think that, generally, violent tornadoes form in environments characterized by strong low-level shear and strong low-level instability. Since this concomitance of parameters is generally quite evident -- not to mention rare -- the odds of missing such an event are quite low.

That said, I think that the embedded supercell structure is probably the hardest to detect. Very rarely, as Greg M. mentioned, a violent tornado will form in close proximity to a squall line. Squall lines aren't generally associated with strong tornadoes -- I think this is due primarily to CAPE being used too quickly, or more likely, water loading simply overcomes positive buoyancy. At any rate, a rare storm within a semi-continuous squall line will produce a violent tornado. Other than that, I think the SPC generally can forecast violent tornado probabilities rather well.

 

[Shane Adams]

May 11, 2000 in Iowa rings a bell.

 

[Greg McLaughlin]

I think the answer is: not often. I think that, generally, violent tornadoes form in environments characterized by strong low-level shear and strong low-level instability. Since this concomitance of parameters is generally quite evident -- not to mention rare -- the odds of missing such an event are quite low.

That said, I think that the embedded supercell structure is probably the hardest to detect. Very rarely, as Greg M. mentioned, a violent tornado will form in close proximity to a squall line. Squall lines aren't generally associated with strong tornadoes -- I think this is due primarily to CAPE being used too quickly, or more likely, water loading simply overcomes positive buoyancy. At any rate, a rare storm within a semi-continuous squall line will produce a violent tornado. Other than that, I think the SPC generally can forecast violent tornado probabilities rather well.

Also, strong tornadoes are rare in squall lines because a squall line is a line of outflow-dominant convection where the updraft is along the leading edge of the storm and the warm moist inflow flows up and over the cold pool like a plow. As we all know strong tornadoes typically form with discrete mesocyclones where the updraft is isolated and dominant.

 

[afischer]

The twice-mentioned Parkersburg EF-5 is one of the most classic examples I've seen with regard to a somewhat "surprise" violent and deadly tornado. The NAM-WRF and then RUC performed very, very poorly in advance of that event. In particular the low-level shear to the cool side of the warm front was grossly underforecast... while the observed LLJ wasn't super impressive (~35 kt), strong low-level shear was still present due to classically "kinked" hodographs helping to maximized the SRH. CAPE was also way underforecast by the NAM-WRF, though apparently the GFS did a better job with the CAPE per a write-up from DMX. IMO warm sector "surprises" are a lot more rare than those associated with fickle warm fronts (Parkersburg) or OFBs (e.g. 06-23-02).