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
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
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
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
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.