Last Wednesday, a severe weather system moved through the Capital Region, producing heavy rains, hail, and frightening clouds. The system was also tornadic in nature, prompting Environment Canada to issue a tornado warning for the City of Edmonton and surrounding areas. While the storm was powerful and resulting in some localized roadway flooding, it failed to produce a tornado in the City, causing many on social media to discredit Environment Canada’s warnings after the fact.
These criticisms are unwarranted, but highlight the lack of understanding of severe weather that some have. Nonetheless, these are legitimate concerns, so I will do my best to show why the watches and warnings were warranted.
It was too cold for a tornado.
This is an important observation – the weather on Wednesday hardly seemed conducive to tornadic activity. The morning was rainy, and the temperature hardly scrapped 14 degrees – far from the 30 degree temperatures that many associate with the biggest storms.
Temperature is only one part of the equation, however. I’m hardly an expert on such things (I’m in Political Science, not Atmospheric Science), but there were a number of factors in play that did make conditions favorable for tornadic development.
On Wednesday, there was plenty of moisture in the atmosphere from previous rain. When the clouds cleared from that system, the clearing provided some heating for storms to develop, triggered by the jet stream sinking over Central Alberta. The key factor, however, was the wind shear and helicity values. Wind shear is the change in wind direction at different heights, which can cause a ‘spin’ in the atmosphere, and can create a corkscrew motion. Helicity measures that spin, and on Wednesday, the highest helicity values were right over Edmonton. When the storms fired and moved into that wind shear environment, they began to rotate. At this point, the thunderstorms evolved into supercells – those powerful storms that are defined by a rotating updraft, and fully capable of producing tornadoes. In this situation, a tornado watch was issued, noting the potential for tornadoes to develop.
Okay, so they issued a watch? Why did they issue a tornado warning?
First, it’s important to understand the distinction between watches and warnings. A watch indicates the potential for a severe weather situation to develop, while a warning indicates that the event is imminent or occurring.
Even though many felt it was too cold for tornadoes, they did touchdown. There was at least one confirmed touchdown on Pigeon Lake, and a possible touchdown near Thorsby. This was the same storm that rolled through Edmonton about an hour later.
On Dopplar radar, the storm was continuing to show strong signs of rotation, demonstrating a vortex signature. With a history of producing tornadoes and the radar signature, it would be irresponsible of Environment Canada to not issue a warning. Thankfully, the storm did not produce a tornado. However, it had the potential to do so at any moment, and the fact that it did not happen should not discredit the warning itself.
But they ALWAYS issue these warnings and nothing ever happens.
The last time a tornado warning was issued for the City of Edmonton was 1996. They are exceedingly rare, and only issued by Environment Canada if absolutely necessary. Tornado watches are more common but still fairly rare, though severe thunderstorm warnings and watches are reasonably common in the summer months. When a severe thunderstorm warning is issued, we almost always see the impact of the storm – though due to the size of Edmonton, some neighbourhoods might not see the same severe impact as others.
Tornadoes are very rare in Edmonton.
Tornadoes are pretty rare in Edmonton, but they still occur on a surprisingly frequent basis. The Black Friday tornado of July 31st, 1987, is the first thing people think of when they hear “Edmonton” and “tornado”, with good reason. That F4 tornado devastated the east side of the city and Sherwood Park, leaving 27 dead, over 300 injured and hundreds of millions in damage, one of the worst natural disasters in Canadian history.
A lesser known fact is that prior to 1987, seven tornadoes had been reported within Edmonton. In 1989, a tornado touched down in West Edmonton, causing minor injuries and property damage. We have been lucky in Edmonton since; however, multiple tornadoes have been reported in the metropolitan area.
We should be thankful that no tornado formed on Wednesday, and that the tornadoes that did touch down outside the city caused no damage. The fact that something didn’t happen doesn’t discredit the warning that Environment Canada issued, a warning that they take very seriously. The staff of Environment Canada are trained meteorologists who know what to look for, and all warnings issued by them should be taken seriously.
Wednesday’s storm should serve as a reminder to all Edmontonians of the natural hazards we do face, as rare as they may be. It is important to know what to do when severe weather strikes, and to take shelter when tornadoes are possible. It’s just the beginning of severe weather season in Central Alberta, so be prepared.
Graeme Archibald is a recent Political Science Honours graduate who went storm chasing in Oklahoma once; he is a scientist at heart.
Image courtesy of mccan934 on flickr