I often cringe when people say that the weather is “wacky” or “unnaturally wild.” I’ve heard that quite a bit this year; and, I suppose, for understandable reasons. To me, “typical” weather is simply just the average of all the “wacky” weather that’s happened over the course of meteorological observations. If catastrophic weather happens, though rare by its definition, it surely has happened before and will again.
This spring and early summer have been rather active in terms of tornadic activity and, of course, flooding. The following graphic from the Storm Prediction Center (NOAA) shows this year’s tornadic activity as compared to the last few years and the 10-year average:
http://www.spc.noaa.gov/wcm/torgraph-big.png
Some observations:
First, both the actual (January-April) and preliminary (January-present) counts for 2008 put it ahead of all recent years and the 10-year average. The preliminary count is a notorious over-estimate, as hindsight shows that certain reports were non-tornadic, or different reports were actually for the same tornado. Nonetheless, we’re on pace for a very active year.
Second, take the long-term average with a grain of salt. With the advent of a better radar network, increased knowledge of thunderstorms, and a population that is covering more and more land, we’ve been able to detect and report more tornadoes than in the past. Perhaps this has little affect on the 10-year average, but it certainly does for climatologies of any greater length of time.
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An interesting analog is tropical storm detection. With better satellites and other detection equipment, we’ve actually gotten better at measuring if and when a tropical wave becomes a tropical depression, tropical storm, and/or hurricane. This advance in technology has to be included when discussing if the number of hurricanes per season and severity of these storms is increasing due to global warming. To date, no scientific study has demonstrated that there is a solid link between global warming and both tornadic and tropical storm activity. In fact, most have found that there’s really not much in the way of a correlation. There are just too many factors that affect the development of these storms.
A quick bit about the flooding. Being in Wisconsin, you’ve experienced it and heard about it ad nauseam, so I won’t rehash details. One thing I’d like to clear up is what a “500-year flood” means. I’ve heard a lot of people ask, “How can two 500-year floods happen within 15 years of each other?” A 500-year flood is simply a level of flooding, based on river crest levels, that has a 1 in 500, or 0.2%, chance of happening in a given year. It does not mean that it only happens every 500 years. The odds are the same each and every year. If I were to win the lottery at 50,000,000 to 1 odds, it doesn’t mean I’d necessarily have to play the lottery 50,000,000 more times to win again, or that by playing it 50,000,000 more times I’d even necessarily win again.
The following article provides a nice view on the recent flooding in Iowa, and how in many ways, man was just as much to blame as Mother Nature:
http://www.washingtonpost.com/wp-dyn/content/story/2008/06/19/ST2008061901432.html
