2011-04-27 MISC: AL,TN,MS,KY,OH,IN,WV,GA

Jun 17, 2007
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Woburn, MA
Of course it will happen again, given enough time. From the limited good data period we have, it does not seem to be a common event.
We went 37 years between Superoutbreaks 1974 to 2011, but that's only two data points. Since tornado documentation declines
quickly prior to the 1950s, it's hard to really know the real frequency of such large and intense outbreaks. Prior to 1950, there is
evidence of outbreaks on the 1974 and 2011 scale, but on paper they do not look that impressive, at least in terms of the
number of tornadoes in each outbreak. Many violent tornadoes do show up well in this period, but even some of those were very
likely missed. Most focus in those days were on tornadoes that hit populated areas.

If another Superoutbreak occurs say in the next 10 years, that still does not give us any real pattern, nor is it a "sign" of anything.
When dealing with events that only occur once every several decades or more, you need a much longer period of solid, reliable records
to get any real sense of how often these occur. In this case, at least 150 years of good tornado records is likely needed.
 
Feb 22, 2015
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Norman, OK
Reading through this for the second time this month makes me wonder where and when this could happen again and if it could be worse. We haven't really seen a tornado outbreak nearly as severe as this since it happened, and lots of areas are definitely long overdue, especially in the Kansas/Nebraska/MO/IA areas, living in these areas (basically the heart of tornado alley) and being 15, I still haven't seen a significant outbreak in my area and I'm wondering if it could happen soon.
There are several issues with getting outbreaks of this size W of the Mississippi River. One is that any sort of significant westerly component to the low level winds tends to advect dry air from Mexico rather than the Gulf of Mexico given the geography of the US (4/11/65 occurred with a SW/WSW LLJ, which wouldn't work in the Plains). These bigger events often occur with large scale upper level systems that tend to be going through their mature phases at the times of the outbreak. Often troughs that produce events in the Plains are still in their developmental stages (unless they're already swinging out negatively tilted, in which case their strong forcing for ascent often leads to what I'll elaborate on below).

You also need a rather broad area of moderate large scale ascent and moderate capping present to initiate multiple storms in the warm sector/along confluence axes, keep prior convection to a relative minimum to allow significant destabilization and not initiating too many storms to have a quick transition to linear (which is why more broad cyclonic flow like 4/3/1974 is favored vs highly amplified/sharp configurations). Plains events often only have one or two initiation sources (usually the dryline being one of them). Because of the proximity of the Plains to the EML source region, you often have quite strong capping during the afternoon prior to initiation with Plains events, and thus you need quite strong forcing for ascent to initiate storms. This often leads to storm modes becoming messy after the first few hours (see 5/24/11 or 5/10/10 for prime examples). If you don't have that strong large scale ascent, you'll wind up with only isolated cells or a cap bust.

There aren't many cases I can think of where you had that "sweet spot" of moderate ascent and a moderate cap in the Plains. Ones I can think of right off the bat would be 5/3/1999, 3/13/1990 and perhaps the outbreaks of April 22 and April 27, 1912 for the Southern Plains. For closer to your region, March 23rd, 1913 is probably your benchmark for NE especially. But even these events aren't on the same level as some of the monsters further east like 4/27/2011, 4/3/1974, 4/11/1965 and 3/21/1932.

I guess what I'm saying is, it's not impossible, but it's more unlikely over your region than it may be over the Great Lakes/Ohio Valley/Southeast.
 
Oct 10, 2004
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Madison, WI
I'm curious, has anyone done a comprehensive storm-by-storm analysis of each supercell in this outbreak and the tornadoes it produced (a bit like the one done for 5/3/99, "Storm A," "Storm B," etc.)? The Wikipedia tornado list is an excellent resource but doesn't break them down by storm:

https://en.wikipedia.org/wiki/List_of_tornadoes_in_the_2011_Super_Outbreak#April_27_event

You have to infer, for example, from the times and locations that the storm that produced the first significant tornado of the afternoon outbreak, an EF3 that began at 1836 UTC (1:36 PM CDT) near Oxford, MS, later produced an EF0 tornado at 1928 UTC in Union County, MS before either dissipating or being absorbed into another storm.