Tornado season is just about upon us. Across the country, 2011, was the 4th deadliest year on record with 500 casualties (the average is 60). There is never just one factor as to why a particular tornado season is so deadly. Part of the reason is just pure chance. One major tornado goes across a corn field one year and another tornado rips through a heavily populated area the next, each with entirely different results.
Personally, no other type of storm scares me as much as the tornado. I can keep relatively safe from lightning, shovel out after a big blizzard, floods aren't much of a problem where I live, hurricanes are fairly predictable and I don't live on the coast. Tornadoes are quick, random and can be incredibly destructive - something the folks across Western Massachusetts needn't be reminded of. Last year, marked the first time since the Worcester tornado back in 1953, that a major twister tore through our state. You can clearly see the path the June 2011 tornado took on this satellite photo.
Tornadoes are caused when warm, and more importantly very moist, air collides with cold and dry air. Of course, cold and warm air are often colliding across the country and there is more to a tornado than just two air masses. Tornadoes also occur in other parts of the world. However, the United States, because of the way all the air masses clash, sees more twisters than anywhere else on the planet.
Another key ingredient to tornadic activity is a strong jet stream. Last spring we saw a very strong La Nina occur off the coast of South America. The colder Pacific waters associated with La Nina tend to shift the jet stream and bulk of the tornadoes a bit further east and south than would normally occur. The strong La Nina last spring was certainly part of the reason areas that normally don't see such destructive tornadoes did. (El Nino is the opposite:warm waters in the same region)
Springs with bad tornadoes, winters with less snow?
While there are very few, if any, one to one correlations in weather, there are some interesting possible connections between seeming unrelated events. For example, let's look back to the spring of 1936. That year was the second deadliest on record with 552 deaths from tornadoes. The winter of 1936-1937 ranks as the least snowy in Boston with only 9" of snow. Other active tornado years like the spring of 1974 were followed by low snow years in our part of the country. The great Worcester tornado occurred in 1953 and during the following winter Boston saw only about half the normal snowfall. Again last spring, lots of tornadoes, this winter very little snow. Meteorologically, the patterns that set up to create deadly tornadoes in the spring, may be related to the same patterns that cause less snow during the subsequent winter. I suspect over the coming years meteorologists will better understand some of these relationships.
What does it all mean?
If you look at the entire 2011 tornado season you will see a very high number of tornado touchdowns counted at 1709, which caused 550 casualties. However, this doesn't tell the entire story. April was a record breaking month with 753 of the seasons 1709 twisters touching down. In April alone, 364 people were killed by tornadoes, two-thirds of the casualties for the year. May, a normally active tornado month was much quieter than usual for the first three weeks then the deadly Joplin, Mo. F5 tornado hit on the 22nd . June saw the destructive tornado outbreak in Massachusetts but was less severe in other places. So, while 2011 was the forth deadliest year for tornadoes, the activity was clustered rather than spread out evenly over the entire spring. Perhaps the most striking example of the clustering of fatalities was 1925. That tornado season, 794 people died from twisters, more than any other year. However, all but 99 of those fatalities came from one super storm. The Tri-State tornado remains the longest-tracking, deadliest tornado on record.
The twister killed 695 people along its 219 mile path through Missouri, Illinois and Indiana. If you removed that one thunderstorm, your record year becomes nothing special. This is why I am always telling people not to make make sweeping generalizations about our weather and climate from these extreme events.
The upcoming spring.
No one can accurately predict the numbers of tornadoes or casualties storms may cause this spring. We have already had more severe weather than we did last year. Take a look at the map below which shows the locations of reported tornadoes (red), wind damage (blue) and hail (green).
There are some differences in the global patterns heading into spring this year as compared to last. One of the most notable changes is that the La Nina of last year is much weaker and there are even signs of a possible El Nino developing. These changing conditions make it unlikely that the exact same areas will have the worst of the upcoming tornado season.
Average number of tornadoes each year
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