"They're the eyes and ears of Environment Canada", says federal warning preparedness meteorologist Geoff Coulson when he talks about the nearly 7,000 amateur storm watchers scattered across Ontario.

Severe weather training for these volunteers, who come from all walks of life, including firefighters, police officers, St. John ambulance workers, teachers and farmers, is happening Monday through Thursday in Thunder Bay, Fort Frances, Kenora and Dryden.

Participants learn about the four criteria forecasters use to determine whether a developing thunderstorm has the potential to be dangerous, explained Coulson.

Those four conditions are:

1. Hail

Could the storm produce hail as big as a nickel, or larger?

2. Downburst

Could the storm produce winds of 90 kmh or more?

That is the speed at which tangible damage occurs, such as "shingles are being torn off, branches are coming down," said Coulson. 

Downbursts leave a much broader trail of destruction than does a tornado, he said.

"Slow moving storms can do damage in a starburst pattern so where the winds hit the ground the damage spreads out in all directions from it."

If the storm is moving faster, the heaviest damage will be on the leading edge as the wind "acts almost like a plow, pushing things over as it goes."

Geoff Coulson

Geoff Coulson, a warning preparedness meteorologist with Environment Canada looks at the equipment, including an emergency weather alert radio, he'll be giving to some of the volunteer storm watchers he's training in northwestern Ontario. (Cathy Alex/CBC)

3.Heavy rain

Could the rain be heavy enough to cause flash flooding?

Forecasters look for the potential to have as much as one month's worth of rain fall in a single afternoon, said Coulson, citing the example of possible totals measuring 75 to 100 millimetres.

"Storm water systems can't handle that volume of rain at one time and we get huge impacts, like road washouts, basements flooding, things of that nature."

4. Tornadoes

Could the system produce one or more tornadoes?

"The rapidly rotating winds, the suction vortex drawing things in and that's why the damage from a tornado tends to be narrow, where that vortex is strongest. And the damage can be confused, laid off in different directions as the tornado draws it in and spits it out in a random fashion," said Coulson, describing the event`s destructive power.

Ontario records about 12 tornadoes each year, but the number could actually be much higher, he said, with some twisters going undetected because they touch down in remote areas.  

'Bolt from the blue'

The weather information Environment Canada receives from these volunteers can be crucial, said Coulson, explaining that even the information generated by Doppler radar, lightning detection systems and satellites is enhanced by what an eyewitness sees.

"If we get that timely report from a trained spotter on the ground saying 'toonie-sized hail', saying 'I've got tree damage' that can be a trigger for us to either update an existing warning with that new information or go from a watch to a warning now that we have strong evidence that storm is severe."

All parts of Ontario are susceptible to sudden and severe summer thunderstorms, said Coulson, offering the reminder "when thunder roars, goes indoors," and stay there for 30 minutes after the last rumble of thunder. "Lighting can occur all around the storm, up to distances of 17 kilometres away," said Coulson, adding that the "bolt from the blue" is a real weather event.