Tornado Update–May 2013
By Paul Homewood
http://www.spc.noaa.gov/wcm/torngraph-big.png
Tornado numbers so far this year continue to run at the bottom end of the range. However, the real news last month were the two EF-5 tornadoes that hit Oklahoma, in which, sadly, at least 32 people have died.
There have now been 60 EF-5 (or F5, prior to 2007) tornadoes since 1950, approximately one a year. It is a fact, however, that they tend to come in clusters. In the 64 years since 1950, EF-5’s have struck in 30 of them. Of these 30 years, 12 have had more than a single EF-5.
1974 remains the year with most EF-5’s, totalling 7, as Figure 1 illustrates.
Climatological Factors
NOAA have this to say about the circumstances leading to the two tornadoes that hit Oklahoma.
A massive, deadly, and historic tornado outbreak impacted the Central and Southern Plains between May 18th–20th. The severe weather was associated with a cut-off low pressure system and strong cold front that moved from the Colorado Front Range into the Great Plains. Ahead of the front, warm, moist, and unstable conditions filtered into the region. The front provided the necessary wind shear in the lower atmosphere for the storms that did spawn to begin rotating, with several of these super cell thunderstorms producing devastating tornadoes.
On May 31st, a cold front moving through the Central and Southern Plains spawned severe weather from Oklahoma to Indiana. There were 30 preliminary reports of tornadoes in eight states. The hardest hit areas included the Oklahoma City Metro area, the St. Louis metro area, and locations north of Tulsa, Oklahoma.
A look at regional temperatures during May give a good idea of just how cold the month was. More significantly though, this cold extended throughout the south and east, as the map below shows. A change of wind direction can , at any time, introduce warmer and moister air, but it certainly is not the case that temperatures to the south and east of Oklahoma were higher than normal.
http://gis.ncdc.noaa.gov/map/cag/#app=cdo
A look at the wider GISS map for May not only backs this up, but also shows that sea temperatures in the Gulf and off the coast of Florida were also well below normal.
Are Tornadoes Getting Bigger?
According to NOAA, the El Reno tornado, the second EF-5 event, was the widest ever recorded in the US. This follows other recent attempts to suggest that tornadoes are getting bigger, even though the number of severe tornadoes has been reducing in recent decades.
http://www.ncdc.noaa.gov/sotc/tornadoes/
But is this actually true? As is often the case in these matters, things are not quite as simple as they seem. We know that changes in observation practices have made comparison of tornado trends highly problematic, even over the last 20 years. Similarly, satellite technology allows us to spot and measure hurricanes out over the ocean, that would have been either missed completely, or underrated a few decades ago.
Can we therefore rely fully on past tornado records? A couple of examples of Texas tornadoes suggests not.
1) 31st May 1968 – Hale County
From NOAA’s Storm Data Publication for the month, the damage from this F5 tornado was described as being “two to three miles wide”. In the official database, the width is listed as 3000 yards. (Sorry, there is no direct link for this, but the data is held in the CSV files here.)
Below is a screenprint of the entry in the Storm Data Publication.
http://www1.ncdc.noaa.gov/pub/orders/IPS-3E60EA0D-5145-447E-A2C3-B795F0A3A5B4.pdf
2) 27th May 1997 – Jarrell, Williamson County
From the Storm Events Database, this F5 tornado is listed as having a width of 650 yards.
Yet the narrative tells us :-
“This time is based on a clock found at a destroyed residence in the extreme northwest corner of the subdivision and the home believed to be the first struck by the tornado. Here, it widened to it maximum width of three-quarters of a mile.”
These are just two random events, but in both cases the listed width has been grossly understated. It therefore seems extremely dangerous to rely on the historical record, when making comparisons with current events.
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NOT A LOT OF PEOPLE KNOW THAT 
Thanks for this update, I was hoping you would! Posting a link to this back to the front I fight upon.
Reblogged this on CraigM350.
The NOAA crowd loves to throw out statments like “biggest ever”, “most ever” or “worst ever”, but upon closer examination, these claims usually are highly questionable. Goes to show that their agenda and their funding is far more important to them than historical accuracy.
Before the mid-1990s, tornado widths in SPC represent the mean over the full path. In the mid-1990s, it was altered to record the maximum width of a tornado. This change in observation practice was considered to be gradually phased in over a ten year period. This fact explains why tornadoes appear wider today than the relatively recent past.
Thanks for this Stephen.
Is this actually official? Have you got any links for it?
Paul – my source was a leading authority on tornadoes, I’ve never looked for anything on the web. Basically, your analysis above is completely consistent with the knowledge of changes in observation practices over the years.
I was in the US when Piers Morgan and Bill Nye discussed the El Reno tornado (CNN, 3rd or 4th June?). Nye associated it with climate change, and this prompted my questions on the large trend in observed tornado width from the late-80s to mid-90s. It’s not a secret, and I can try to get an official source if you like.
Thanks, Stephen
i just found a peer-reviewed reference:
Click to access lengthwidth.pdf
“…there was a policy change in the NWS to report maximum path width, instead of mean path width, that did not occur until 1994 (McCarthy 2003) .”
The McCarthy reference might be difficult to get:
McCarthy, D. W., 2003: NWS tornado surveys and the impact on the national tornado database. Preprints, First Symp. on F-Scale and Severe-Weather Damage Assessment,
Long Beach, CA, Amer. Meteor. Soc., CD-ROM, 3.2.
Thanks Stephen.
NOAA’s claim referred to Doppler radar-measured max width. The standard practice is to measure the width of the damage at the surface. The quote below indicates that radar-derived estimates are larger than those based on surface damage. Might be worth investigating the use of radar-derived widths – when did it start, how much is it used, is there any calibration of such a measure to the surface damage, etc. Quite a lot of work, but worth it to establish climate-trend-quality historical records.
“I note that a publication exists regarding the Spencer, SD tornado of 30 May 1998 and the relationship between radar-observed winds and damage. It’s only one study, but among the conclusions was that the radar-measured winds converted to F-scale (not EF-scale!) ratings typically exceeded the actual damage at the ground. A number of hypotheses were offered to explain the discrepancies. Clearly, much more such work needs to be done and perhaps a consensus may emerge on how to convert Doppler-measured winds to EF-scale ratings.”