The US tornado season has got off to a strong start. Last week saw dozens of destructive tornadoes touching down across 11 states. In the worst hit areas in the Ohio Valley and central plains areas, houses were uprooted, power lines torn down and cars thrown around like toys. Thirty-nine people have been confirmed killed in the outbreak.
Risk modeller Eqecat estimates the recent tornadoes caused between $1bn and $2bn of insured losses.
Strong temperature differences between the north and south meant that the conditions for severe thunderstorms were set up just right, specialists say.
These thunderstorms interacted with a strong jetstream which set them rotating, forming the deadly tornadoes. And, because the jetstream was unusually strong, tornadoes formed with very long tracks and moved across the country very quickly.
In Kentucky, there was a report of a tornado that had a track of 34 miles.
“The outbreak was caused by a strong, spring-like low pressure system that moved out of the southern plains and over the Great Lakes on Friday, March 2,” according to Dr Tim Doggett, principal scientist at AIR Worldwide.
“However, this storm system was accompanied by a strong cold front that swept eastward from Kansas to the Appalachian Mountains, before finally moving offshore on March 3. Ahead of the front, warm humid air was drawn northward from the Gulf of Mexico, providing the fuel for the widespread outbreak of severe thunderstorms.”
The recent devastation comes after a particularly active tornado season last year when two large outbreaks left 593 fatalities and a combined insured loss of around $20bn. In April 2011 Alabama was badly hit and then in May another outbreak swept through Missouri; a single twister flattened parts of Joplin City.
If the tornado losses last year were counted as a single event they would have constituted the fourth most expensive disaster in US history.
This year’s early season openers came through similar geographic areas but avoided high population density locations. Also, hail wasn’t quite so prominent a feature this time around, Doggett points out, because the atmosphere is colder at this time of year and not so conducive to hail formation as it is in April and May.
While the tornado activity last week is higher than normal for time of year it is not necessarily a sign that the whole season will be above average: a few quiet weeks would return activity to an average year.
“It is not a harbinger for the rest of the season, though January and February are more usually associated with activity in Florida and the Gulf coast,” Doggett told me. “The activity usually shifts in March and April towards Texas and also northwards into Kentucky and Tennessee. This transition is caused by the movement of the jetstream and the temperature of the air below.”
Doggett says weather specialists are a long way off being able to make accurate long-range predictions about future tornado seasons, in the way that forecasts are made about the hurricane season.
“You can make a forecast of anything – but how good it will be is another thing. The variability from year to year in tornadoes is huge and the activity within a season is too,” Doggett says. “Even looking at El Nino Southern Oscillation (ENSO) is not that helpful. The tornado peril has a strong natural variability. Forecasts can be made - but we haven’t seen a lot of success so far.”
Storm watchers at Risk Management Solutions (RMS) agree that there is great uncertainty around the 2012 outlook. But they add that there are some influential factors to consider. For example the temperature in the Gulf of Mexico is a potential indicator of tornado activity levels because warm moist air moving north is a precursor for thunderstorm development.
Sure enough, according to NOAA, temperatures in the Gulf are running at around 22-24 deg C which is 1 deg C above average, increasing the potential for an active season.
It’s also thought in some quarters that the El Niño Southern Oscillation (ENSO) can influence tornado activity, with El Niño years having stronger tornadoes in the winter months, December through February. This has not always been the case: we’re currently experiencing La Niña conditions but the forecast is for neutral conditions to take over in the coming months.
ENSO does have the potential to affect the distribution of tornadoes, however, RMS points out, with La Niña years experiencing a shift in the Tornado Alley to the southeast – though as with intensity, it isn’t always the case.