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Updated: Tue, 29 Apr 2014 10:15:01 GMT | By CBC News, cbc.ca

U.S. tornadoes: Why twisters are so unpredictable



Meteorologists can forecast where supercell thunderstorms are most likely to develop but they can’t predict if they will result in tornadoes. James Bryant/Associated Press

Meteorologists can forecast where supercell thunderstorms are most likely to develop but they can’t predict if they will result in tornadoes. James Bryant/Associated Press

Just days before a series of deadly tornadoes cut a swath of damage through the U.S., the National Oceanic and Atmospheric Administration’s Storm Prediction Center had said that "it is fair to say that 2014 is now the least active tornado year, to date, in at least the past 60 years."

But as Bill Bunting, operations chief of the NOAA's Storm Prediction Center, told CBC News, what they've learned over the past years is that a slow start doesn't have much predictive capability in terms of how the remainder of the season unfolds.

“[Tornadoes] are a very localized phenomenon and it’s really difficult to say more than a few days out what might be possible," Bunting said. "It’s that time of the year. It’s getting into late April and May – it's the peak of the tornado season, and the atmosphere apparently knows that.

Over the past days, a rash of deadly tornadoes and heavy storms have rumbled across the centre and south of the U.S. 

But unlike hurricanes, which meteorologists have some success in forecasting, predicting tornadoes still poses significant challenges. 

"We can't offer long range tornado outlooks because we honestly still do not fully understand how tornadoes form,” CBC meteorologist Jay Scotland said.

Hard to predict

"We know that all tornadoes are spawned by strong thunderstorms but not all strong thunderstorms end up producing tornadoes. Predicting which storms will produce tornadoes with any certainty is still impossible."

Meteorologists can forecast where supercell thunderstorms are most likely to develop but they can’t predict if they will result in tornadoes, he said.

“This is why we tend to highlight areas of high risk. We look for the ingredients required for tornado activity, which include heat, humidity, wind shear and a trigger to fire up strong storms like a cold front for example," Scotland said. "This is how we produce our severe weather risk maps you see on TV."

As strong storms fire up, Environment Canada and the U.S. National Weather Service will issue tornado warnings usually as a tornado is imminent or occurring thanks to tell tale radar signatures or reports from storm chasers and spotters, Scotland said.

"This is why it is so important to heed all official warnings and seek cover immediately."

The tornado season, which usually begins in March in the U.S. got off to a slow start this year because of the unusually cold weather patterns across the U.S., with much of the nation below normal temperatures.

“That environment doesn’t favour the northward return of Gulf of Mexico moisture like we’re seeing happening this week and so for that reason even the typical occasional wintertime severe weather events that we would see in a normal year didn’t occur," Bunting said.

“But now that we’re getting into late April, the Gulf of Mexico is such we’re getting moist air masses into the southern and central U.S., combine that with storm systems aloft and we’re seeing the result.”

With a few exceptions, tornado season in Canada generally takes place from May until September and generally peaks in June and July, Scotland said. The risk for tornadoes increases during the summer months as the jet stream heads north allowing humid and unstable air from the Gulf of Mexico to make its way into Canada which is the fuel for strong thunderstorms, he added.

For Tuesday, more severe storms are forecast in the U.S in Mississippi, parts of northwest Alabama, Tennessee and will shift eastward into southeast U.S. On Wednesday, the southeast and mid-Atlantic states "will be under the gun once again," Bunting said.

"This is a multi-day event. We’ve been sort of forecasting this multi-day event now for five or six days and it’s unfortunately playing out as we feared it might," he said.

“It’s not uncommon when you have these strong upper level storm systems with unstable air in place to have a multiple day event.”

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