Toronto was pummelled Tuesday by a severe storm that toppled trees, caused flash flooding and left many hydro customers in the GTA without power.

Drivers on Highway 401 were forced to pull over after driving conditions became too dangerous.

At the south end of Bayview Avenue, traffic was down to a single lane at 6 p.m. after a massive tree was uprooted by powerful winds.

At least two trees crashed into homes in the GTA, leaving one resident shaken after a tree landed in her disabled father's room near Coxwell and Danforth Avenues.

"I found out that my father was actually in the house when (the tree) fell and a neighbour ran over to see if he was ok," the homeowner told CTV News.

"My father suffers from major disability and a heart condition, this could have easily have killed him."

Residents were left without power from Finch Avenue to Lawrence Avenue West, Leslie Street to Victoria Park Avenue, Sheppard Avenue to Wilson Avenue and Dufferin Street to Bathurst Street.

Hydro crews were dispatched throughout the city to fix the mangled mess of power lines.

A red alert, which signals crews to leave the tarmac because of potential lightning strikes, was issued at Pearson International Airport late Tuesday afternoon.

Flights were delayed and others cancelled from Toronto to Ottawa and Montreal as the storm headed east.

Environment Canada issued a severe thunderstorm warning for the GTA around 3:30 p.m.

A cold front travelling from Michigan is to blame for the storm.

Two weeks ago a severe thunderstorm ripped through the GTA causing similar damage.

With a report from CTV's Austin Delaney and Tom Hayes