What a week it was! Tornadoes dropped from the sky from Sunday, May 4 through Saturday, May 10 throughout the central and southern Plains. Images of houses destroyed and lives changed forever were all over the newspapers, Internet, and television. Tornado watches were common in many states.

Even our own state experienced tornadoes on Saturday, May 10, with twisters in Lafayette County. What was so different about the atmosphere above the United States during early May? Like reading from a recipe, conditions have to mix together in just the right way in order for nature to cook up such severity.

The meteorologists at the Storm Prediction Center (spc.noaa.gov) in Norman, Oklahoma are in charge of looking for the correct atmospheric ingredients to come together. When they see those conditions beginning to merge over one region of the country, they issue severe weather risk statements and draw out charts outlining the level of risk, i.e. low, moderate, or high. SPC also issues severe weather watches outlining a several hundred square mile area where tornadoes or severe thunderstorms are likely to occur. They were a busy bunch earlier this month.

The jet stream always plays an active role in any severe weather outbreak, and early May's upper air wind flow was no exception. Strong winds between 25,000 and 35,000 feet streamed across the central and southern United States, a bit farther south than normal for this time of year. That upper airflow was undercut by a low-level jet stream screaming northward from the Gulf of Mexico.

Winds at 4,500 feet were often in excess of 40 miles per hour from the south, while winds at 30,000 feet often exceeded 120 miles per hour from the west. The change in wind direction and speed with height, called wind shear, is a necessary ingredient when producing "supercell" thunderstorms. These supercells are rotating thunderstorms capable of producing tornadoes.

Other ingredients in the severe weather mix included cold and dry air aloft. The cold air occurs above 15,000 feet. Combining cold air above with warm air below leads to "overturning." This occurs when the cold air wants to sink and the warm air wants to rise, a natural tendency for both types of air masses. Cold air is heavier, or denser, then warm air. The overturning helps lift moisture from the ground up to higher altitudes.

The dry air moves in around 10,000 feet. It is "entrained" (or drawn) into the thunderstorm, helping to evaporate some of the precipitation aloft. The evaporation of water is a cooling process, so the dry air cools the atmosphere within the thunderstorm, causing that air to sink rapidly towards the ground. This sinking air can explode from beneath the thunderstorm and form a downburst of wind that can cause considerable damage.

Spring is the time mostly likely to see all the ingredients come together for severe weather. What made early May so unusual is that these conditions stayed around in one place for a week. It is estimated that close to 350 tornadoes occurred in one week, the most since tornado records began back in 1950.

A look at the preliminary tornado records for 2003 show how stormy this year has been. A possible 473 tornadoes have occurred since January 1, causing 53 deaths. Compare those numbers to previous years: 852 tornadoes, 55 deaths for all of 2002; 1213 tornadoes, 40 deaths in all of 2001; 1071 tornadoes, 44 deaths in all of 2000. Keep in mind that the numbers for this year are still preliminary because the National Weather Service has to survey damage sites to determine the exact number of tornadoes that have touched down.

One tornado that stays on the ground for many miles can cause a lot of damage. Many observations can come in about tornadoes and damage but the destruction can be done by just one twister. So getting the exact number of tornadoes still needs to be sorted out. Whatever the final number, this has been an incredibly active month.

Was there any atmospheric reason for severe weather conditions to be in place for a week? No, we can't blame global warming on this one. It is just one of those events in nature that make us stand back in awe. Remember that Wisconsin's peak tornado months are June, July, May, and August (in descending order). Let's hope the ingredients of the tornado recipe don't come together over the Badger state.