WINNIPEG — Lessons learned from Canada’s strongest documented tornado in Elie, Man., three years ago — and research from like-sized twisters in the U.S. — should give forecasters a head’s up in time for this year’s tornado season.
Wind shear is believed to have played a major role in the 2007 tornado at Elie and major twisters that have been studied in the U.S., said John Hanesiak, a University of Manitoba associate professor at the Centre for Earth Observation Science.
Wind shear, the change in wind speed and direction within a slight change in altitude, happens frequently in tornadoes.
But research is showing rapid changes in wind direction in the lower part of the atmosphere — which happened the day of the Elie tornado — may be a warning sign, said Hanesiak.
A paper in the Electronic Journal of Severe Storms Meteorology in the U.S. shows a “kink” separating speed shear from directional shear in the environments of some supercells producing significant tornadoes. The big ones tended to be characterized by angles near 90 degrees, whereas the “non-tornadic storms” were not.
A careful look at the low-level “hodograph” — a graph showing the vertical pattern or profile of winds measured from a specific location during a certain time —in both time and space in relation to the storm motion can help to forecast supercell tornadoes, said Hanesiak.
The F-5 Elie tornado topped the Fujita severity scale. An F-5 rating is for winds of 418 to 509 km/hr that cause tremendous damage to large structures and can tear off exterior walls and roofs. Tornadoes of this magnitude account for less than one per cent of all tornadoes, Environment Canada says.
In Elie, the suppertime twister picked up houses and vehicles whole. It cut a swath 300 metres wide and 5.5 kilometres through the town about 40 kilometres west of Winnipeg.
Fortunately, no one was seriously injured.
“The Elie tornado event — spawned from a supercell storm — had this very strong low-level wind shear and the critical angle of 90 degrees as the paper outlined,” said Hanesiak.
“We know this from the weather balloon data — the weather balloon was launched from downtown Winnipeg at the local weather office. Without this critical balloon data, we would be somewhat guessing what the actual environment was like on this day.”
In the meantime, a “change of season” workshop is held every spring with university researchers and Environment Canada meteorologists sharing information, said Hanesiak. Tornado and wind shear research from the U.S. that gibes with the Elie experience will be on the radar at the workshop the first week in May, he said.
“The forecasters at the Winnipeg weather office launch weather balloons every day at 1 p.m. …We can use that information…It gives some indication of how likely a storm might be (a tornado) if it does form.”
Tornado season in Manitoba extends from April to September with peak months in June and July, according to Environment Canada.