The airmass was quite warm and humid and there was enough convective energy and windshear (winds turning with height) to sustain thunderstorms across the region.
Subsequently, Environment Canada issued a Severe Thunderstorm Watch for a large portion of Southern Ontario in the morning and extended it to other areas early in the afternoon.
A Severe Thunderstorm Watch means that conditions are favourable for the development of severe weather. It's not a guarantee that you will see severe weather, but that you should have a heads up that the possibility exists.
A Severe Thunderstorm is one that has any one of the following....
- Wind gusts of 90km/h+
- Rainfall expected of 50mm+ in one hour or less OR 75mm+ in 3hours
- Hail of 2cm or greater diameter
Even if it's currently sunny and beautiful over your region... a severe thunderstorm watch can still be issued, which was the case for many in Southern Ontario on Wednesday. As I mentioned above, a cold front was the trigger... so the longer the sunny periods last over your region before the trigger moves in, the more heating of the ground and more unstable the atmosphere becomes.
Severe thunderstorm watches can be issued hours in advance if conditions are ripe for the development of such storms.... but again, it's NOT a guarantee that severe weather will occur.
When a WARNING is issued.... that means that severe weather is imminent or occurring.
Now that all the ingredients are in place for thunderstorms... we have to wait for them to fire up. Where they fire up becomes more of an educated guess. Certain atmospheric parameters exist in particular areas that will be conducive to rapid development as compare to others. Locations that are closer to the Lakes often have what's called convective inhibition (CIN), as modified air from the lakes in the lower levels alters the convective process as compared to locations just a bit further inland.
Storms were expected to be scattered across the region rather than linear (lined up along the front). Therefore it was necessary to have the watch cover a larger region rather than a focussed area.
Below is a snapshot of the watches (Yellow) and warnings (Red) that were issued yesterday afternoon/evening. The red areas had many tornado warnings issued within them.
On Wednesday... a particular area northwest of the Golden Horseshoe seemed to be a bullseye for strong storms and possible tornadic activity. In the graphic below from the Storm Predicition Centre in the USA, the area in red had a higher probability of a tornado forming somewhere in that area. But you can also see that the area of higher probability of severe weather extended quite a bit around it, encompassing the Golden Horseshoe region and much of Southern Ontario.
Tornado warnings were issued in that red area and a probable tornado did occur with many witness photos being taken. Environment Canada is on the scene today investigating the damage.
Severe thunderstorms continued to pop up all across Southern Ontario into the evening hours and late night. Some areas saw them, some did not. But the Watch was issued for the entire region due to the sporadic nature of the storms. One such cluster of storms moved just to the south of Hamilton in the early morning hours around midnight to 1am. It missed Hamilton by about 20km.
So remember... the next time a Severe Thunderstorm Watch is issued for your region, it gives you the heads up of the possibility of severe weather. Don't ignore it... keep it in the back of your head that you may need to put into action a safety plan should the weather start to become severe. After all... it's better to be safe than sorry.
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