"Welcome be thou, faire, fresshe May."

Chaucer may not have been so kind to May if he lived on the Great Plains rather than in Great Britain. May is the peak month for twisters to strike in the US. Though 16 people have lost their lives this year in tornadoes, thus far, tornadic activity is down compared to previous years, and the number of deaths due to tornadoes is below the long term average.

The first tornado death of 2001 occurred near New Durant, Mississippi in mid February. A woman spotted a tornado while driving her car, and she chose to abandon her vehicle and run to a ditch for cover. She took the course of action recommended by most storm safety experts, but the tornado didn't care. It tornado tossed her car on top of her. She simply didn't have time to get far enough away from her car. The most recent tornado death happened in on April 21 near Housington, Kansas. A man and his wife were watching TV and noticed that a storm was brewing to their west. The woman suggested that perhaps they should go to the storm cellar, but man assumed the Weather Bureau would issue a warning if a tornado was sighted and decided to stay put. When the tornado touched down near his home a short time later, he didn't have enough time to get to the cellar. At the same time, students attending a prom rushed into a basement of the Knights of Columbus Hall as this brut of a tornado (F4 on the Fujita Scale - devastating damage, over 200 mile per hour winds) roared past. No "Prom Night" movie could be any scarier than this. Fortunately, they were unhurt, but the town suffered a lot of damage.

Last year, there were nearly 900 reported tornadoes, the fewest since 1989, and 54 tornado-related deaths. The deadliest storms were in southwestern Georgia in, mid February, when a series of tornadoes blasted several locations late at night, claiming 18 lives. For any other country in the world this would have been considered a terrible year for twisters, but in the US it was regarded as a rather quiet year. In a normal year, approximately 1,200 tornadoes are sighted, and they're sometimes observed in every state. Last year, however, only 39 states recorded tornadoes. Per usual, most tornadoes occurred in May (231), but the number of twisters in May and April was 32% below average. Interestingly, there were considerably more twisters during the winter months and a lot fewer tornadoes during the summer months than usually occur.

Although the tornado count was down last year and so far this year, 1999 was a big tornado year - almost 1,400 twisters. The record year for tornadoes was in 1998) when 1,472 tornadoes racked the nation, killing 130 people - not close to the record of over 1,000 (1884). Actually, with an ever growing population and with more people spreading into places that were previously uninhabited, and bringing camcorders with them, it makes sense that more tornadoes are being seen and reported now than in prior decades.

Since there's no theory on tornado formation agreed to by the entire severe storm community, it's hard to know for sure why less tornadoes have formed so far this year and last year than in past years. In general, severe weather is associated with strong differences in temperature and moisture. While in 2000, there were some remarkable precipitation and temperature contrasts, these contrasts were spatially, at regional scales and temporally, at seasonal scales. For tornado formation, the contrasts have to be strong as smaller spatial and temporal scales. There must be extreme atmospheric instability and low-level boundary conditions present - clashes between air having differing temperatures and moisture conditions.

All of the ingredients, cold air, dry air, and warm moist air need to come together at the same time. Since in most years, the ingredients are available somewhere in the central US, it's the timing that's critical in determining whether or not they meet to form a storm system capable of forming tornadoes. If a ridge or dome of high pressure, for instance, takes up residence in the Mid West in late winter or early spring, the opportunity for all of the ingredients to come together simultaneously is greatly diminished - cold air may be directed to the east of the ridge and moisture may flow to the south of the ridge.

Typically, a tornado-producing thunderstorm has a lifetime of less than about 3 hours, often forming only one funnel. Remember, a funnel cloud is only designated as a tornado when it reaches the ground. Supercells are more rare, and more dangerous. These huge rotating storm systems are like giant heat engines capable of spawning families of twisters over the course of their lifetime, which may be several hours or longer. They form in the presence of strong vertical wind sheer (difference of wind speed with height) in the lower part of the atmosphere (beneath about 5 miles).

It's interesting that while sheer is the bane of hurricanes (wind sheer aloft deters hurricane development), it's an essential ingredient for tornadoes. For rotation to begin in a funnel, winds need to be moving in different directions at different heights in the atmosphere. Vertical sheer is often very forceful in the vicinity of fronts and jet streams. So far this year, the atmospheric conditions that lead to wind sheer haven't been as prevalent as in 1998 or 1999. Most of the cast of characters are around, but they're not on the stage at the same time, and they must be for a tornado to make a successful appearance.

For more about this see the Tornado Project - http://www.tornadoproject.com/faq/faq.htm#top 
Also see the science question for May 13, 1999.