During the first 10 days of May, more tornadoes occurred in the U.S. than during any other 10-day period. Why has this month's weather been so violent?
Tornadoes have come thick and fast this month! Since the beginning of May, over 500 tornadoes have been sighted, and during the first 10 days of the month, more than 300 twisters were reported, which is about 100 more than the previous 10-day record in 1999! This barrage left 50 people dead and hundreds injured.
On average, approximately 1,200 tornadoes occur in the U.S. each year, and they're sometimes observed in every state. However, the number of tornadoes can vary sharply from one year to the next. While this year we're on pace to break the all-time record for the number of tornadoes (the most ever recorded was in 1998 when 1,472 twisters steam-rolled across the country), last year, there were only about 850 tornadoes (the fewest recorded since 1989).
Although this month's weather has resulted in a record number of twisters, it's not unusual for tornadoes to come in clusters. For example, according to the National Weather Service, the average number of tornadoes in the U.S. during the month of October is 29, however, in October 2001, 116 twisters touched down.
It's hard to know exactly why the number of tornadoes varies so much from month to month or from year to year, because it's not known precisely how they form. Severe weather is almost always associated with strong differences in temperature and moisture, but for tornadoes to form, the contrasts have to be strong at small spatial and temporal scales. There must be extreme atmospheric instability -- clashes between air having differing temperatures and moisture conditions. These conditions can be met in every month of the year somewhere in the US, regardless of whether or not it's an El Nino year, a La Nina year or a Leap year.
One of the essential ingredients for tornado formation is wind shear, which is associated with "spin" in the air. For rotation to begin in a funnel, winds need to be moving in different directions and or at different speeds at different heights in the atmosphere. Whenever and wherever strong wind shear occurs, the potential for tornadic development exists. A cold front propagating across the southern plains or Mid-West, for example, can act as a triggering mechanism for tornado formation, if a large area of moisture is surging northward from the Gulf of Mexico and if a vigorous jet stream is aloft. Wind shear is always present in the vicinity of strong cold fronts. In the spring, the above conditions are more likely to be occur than at any other time of the year.
While, most tornado-producing thunderstorms have short lifetimes, often forming only one funnel, supercells are like giant heat engines, capable of spawning families of twisters. These huge rotating storm systems are more rare, bigger, stronger and meaner than run-of-the-mill thunderstorms, and they've played a major role in the number of twisters observed this month.
One of the things that separates supercells from the "ordinary" thunderstorms is that violent updrafts and down-drafts reinforce each other, which allows a supercell to be sustained for hours at a time. In a mature supercell, the entire sky seems to be dominated by a mushrooming expanse of clouds. If you're far enough away, you can see that the top of the storm is flattened like an anvil. Actually, the anvil description is somewhat outdated. I'm guessing that more people have seen a tornado than a blacksmith's anvil. The horizontal spreading of the top of the storm indicates that the supercell is butting up against the tropopause. If a tornado is going to form, it's likely to descend from the southwestern flank of the storm, where the interplay between updrafts and down-drafts is most lively. When rotation occurs around an updraft area, it's called a mesocyclone, and this is where the storm-chasers head.
Another factor that may have contributed to this month's terrible tornadic outbreaks is the speed, or rather the lack of it, of weather systems as they've moved across the country. Those of us who live along the eastern seaboard have had to deal with prolonged bouts of cool, wet weather this month as systems approaching from the west have slowed-down or stalled. Even though the jet stream, which guides storms eastward in the middle latitudes, has been particularly energetic this spring, this doesn't necessarily mean that storms are fast-moving.
The jet stream is a relatively narrow band of winds embedded in the prevailing westerlies (in the middle latitudes). These current of air gallop along at velocities greater than 50 knots per hour and are typically located in the upper troposphere over regions of strong horizontal temperature contrast. Usually, significant regions of wind shear are found in the vicinity of the maximum jet winds. If the jet stream had traveled across the U.S. in more or less a straight line, then storm systems would have zipped eastward with little deliberation along the way. However, this spring, the jet stream has taken a few dips and meanders as it has snaked its way over the mid section of our nation. This has resulted in systems hanging around longer than they otherwise might. Therefore, those conditions conducive to forming supercells have persisted for several days. It should be kept in mind that there isn't just one "jet stream." The polar jet stream and the subtropical jet are most responsible for pushing weather systems along, but in addition, there may be lower level "jets" as well as the upper troposphere variety.
So to a large degree, this month's tornadic hyperactivity is simply due to the vagaries of the weather itself. In some years, there's a more favorable milieu for tornado development than in others. As was previously mentioned, last year, was a down year for the number of twisters in the U.S., and some of the more powerful ones occurred in places that aren't always associated with cyclones and storm shelters. For instance, LaPlata, MD was the focus of a vicious F4 twister that clawed through the southern part of the Free State. This year, tornadoes are more geographically behaved -- they're menacing their preferred stomping grounds. The cast of characters hasn't been content to just wait in the wings, they're all on stage at the same time, and have come back for unappreciated curtain calls. Please, no more encores!
Last month, a tropical storm, Tropical Storm Anna, formed in the Atlantic Ocean north of the Bahamas. Although this storm had extratropical as well as tropical characteristics, it has been classified as the earliest known tropical storm in the western Atlantic Basin. Could this storm have been one indicator that the coming tornado season would be especially severe? The short answer is no. It's interesting that while wind shear is an essential ingredient for tornadoes, it's the bane of hurricanes (wind shear aloft deters hurricane development). One of the reasons that no hurricanes have hit the Atlantic Seaboard the past several years is that there was sufficient wind shear over the tropical Atlantic waters to discourage the strengthening of tropical storms into hurricanes. By the way, the Hurricane Prediction Center has just issue their outlook for the coming autumn, and according to their computer models, it looks like this could be a rather potent hurricane season -- baton down the hatches.
See also the science question for May 16, 2002 and the following tornado-related web sites.
http://www.spc.noaa.gov/climo/torn/monthlytornstats.html
http://www.tornadoproject.com/alltorns/ustorns.htm#top
http://www.tornadoproject.com/fscale/fscale.htm#top
http://www.spc.noaa.gov/climo/reports/030504_rpts.html
This week's question is provided by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! This year Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.