Guest post by Paul Homewood
The tornado season remained a quiet one in September. According to data from NOAA’s Storm Prediction Center, during September, there were 43 preliminary tornado reports. This is less than the 1991-2010 September average of 74 tornadoes, and marks the least active September in terms of tornado activity since 2009, when eight tornadoes were confirmed. The majority of the tornadoes were weak and associated with the remnants of Hurricane Isaac as it moved through the Lower Mississippi River Valley and into the Midwest early in the month.
There were also no tornado-related fatalities during September. The below-average tornado activity was similar to the rest of 2012 to-date. The preliminary number of tornadoes during the January-September period was 843 with 119 tornado reports still pending for July, August, and September, marking the lowest January-September tornado count since 2002.
Preliminary reports suggest that 2012 will finish with the lowest number of tornadoes since at least 2005. Up to July, the running total of EF1+ events was 301, and I would anticipate a final year total of just under 400.
Just how unusual would this be? Well, it turns out not very! Since 1970, there have been six years when EF1+ tornadoes totalled less than 400.
| Year | No of EF1+ Tornadoes |
| 1978 | 352 |
| 1985 | 376 |
| 1987 | 316 |
| 1994 | 388 |
| 2000 | 352 |
| 2002 | 311 |
In other words, a quiet year like this one comes along about every six years.
Confirmed numbers for May/June/July
NOAA have now issued final numbers for May through to July, as shown below.
| Month | EF0 | EF1 | EF2 | EF3 | EF4 | EF5 | Total EF1+ |
| May | 88 | 16 | 6 | 3 | 25 | ||
| June | 85 | 23 | 5 | 28 | |||
| July | 25 | 7 | 7 | ||||
| May-July | 198 | 46 | 11 | 3 | 60 | ||
| 1970-2009 Average | 314 | 162 | 54 | 14 | 3 | 233 |
The analysis shows that, not only were total tornado numbers well down on the long term average, but also that, as a proportion, there was a sharper decline in the stronger categories. EF0 storms were 63% of normal, whereas EF1+ were much lower at 26%.
This brings YTD numbers for EF1+ up to 301, as at the end of July, compared to the long term average of 383.
Next month I am planning to take a closer look at a state by state breakdown.
Remember that all tornado reports can be found on their own separate tornado page here.
Paul, sincere apologies. It is indeed that link you provided at October 22, 2012 at 3:44 am
That is the figure 5 I was thinking of. The report was of similar layout and I was confusing the two.
Anyway, you have raised my interest in tornado count, which I think is probably an early indicator or the current cooling trend. This negative correlation does not seem to get much (any) official recognition so I think it’s worth digging into.
Sorry for not paying more attention.
My impression of complementarity of EF2 and EF3 in figure 5 was correct.
In fact this is not too surprising since, having eliminated EF0 and EF1 the vast majority of the remaining events will be EF 2 or3 . However, as I suggested , looking at the full range of data is more informative than 2002-2010.
http://i50.tinypic.com/2mhhqjd.png
Here I have used non detrended AMO as an indicator of NH surface temps to get the big picture, not to suggest AMO is a driving force of continental US tornadoes.
Several important climate characteristics can be noted.
1. EF2 and EF3 make up the majority of events, thus their variations on both long and short time scales are very similar.
2. During the warming period 1975-2000 the decadal scale variations in SST show a strong anti-correlation to the proportion of EF3 events ( hence +ve correlation to EF2 proportion). ie the HOTTEST part of these fluctuations shows considerably LESS of the stronger events.
3. On the multidecadal scale, during the same period, there is a lesser, opposing tendency towards a higher proportion of EF3 with increasing temperature. The total magnitude of this drift is about a quarter of the short term peak to peak: 0.03 vs 0.11
4. The smaller decadal variations outside this period tend to show the inverse relationship.
5. The pre-75 cooling period shows some down turn , clearer in EF2 plot, but data does not go back far enough to catch the beginning of this tendency.
6 The underlying , long term trend seems to peak around 1995 and drifts down since.
7. The coolest period in early to mid 70’s was the most extreme in all EF categories. The warm peak around 2002-2003 was the least active.
The complex and opposing response to short and long term trends in temperature may have confounded earlier attempts to draw a simplistic conclusion as to whether there were more tornadoes “due to global warming”.
In relation to 3. above the variation is 0.22 +/- 0.055 ie +/-25% , so there is a substantial change in the proportion of EF3 vs EF2 on the decadal scale that is _negatively_ correlated to temperature.
Here is the inverted EF2+ and EF3+ (inverted) tornado counts compared to HadCRUt v3 and v4
http://i45.tinypic.com/oavqea.png
This big picture is a very clear inverse correlation between tornado count and surface temps.
Again, as with shifts from EF2 to EF3 above, the apparently inverse effect during the oscillations of the warming period 1975-2000 , however, closer inspection suggests this is more likely a 2 to 3 year lag in the ocean dominated surface response.
Flatter periods around 1960 and post 2000 are quite tightly in phase. Larger swings starting with the 1970 dip show a lagged response.
This lag also suggests that the two physical phenomena are responding to the same driver rather than tornadoes being driven the globally averaged temperature.
Comparing tornadoes to hadCRUT3 we see the long term downturn in temps is again lagged by a few years. Tornadoes would appear to give an earlier indication of the change in direction.
Recent attempts to warm up the post 2000 period in hadCRUT4 tend to look spurious when compared to the independent physical climate signal.