The July 8 flooding that resulted from a severe thunderstorm in the GTA region was the most expensive natural disaster in Ontario history, the Insurance Bureau of Canada says.

In a report today, it issued a preliminary estimate of insured property damage, pegging it at more than $850 million. It warned consumers their insurance bills may rise as a result.

Toronto floods leave power system 'hanging by a thread'

The storm dumped 126 millimetres of rain on Toronto in a two-hour period, flooding basements, downing trees and halting both subway and airline service. The downpour was more than the amount of precipitation Toronto would get in an average July.

The resulting power outages affected at least 300,000 Toronto residents, and about 1,400 passengers were stranded for hours on a commuter train filled with water.

Ralph Palumbo, IBC vice-president for Ontario, said the industry is moving forward quickly with claims and is generally well prepared for such an event.

But he said the preliminary estimate may be low-balling the damage caused July 8.

"While these preliminary estimates are staggering, we do expect them to go even higher," he said in a press statement.

IBC does not yet have a preliminary estimate of insured damages for Alberta’s July flooding, it said in the same report.

Previously, the most severe weather events in Ontario were an Aug 19, 2005 wind and rain storm that did $671 million of damage, and a July 24-29, 2009 storm that did $228 million of damage.

Palumbo warned that the insurance industry has to be prepared for mere frequent severe weather events and cautioned the Ontario government over its plans to restrict increases in insurance premiums.

A Liberal budget in April mandated a reduction in auto insurance premiums of 15 per cent.

"Damage caused by more frequent severe weather is just another situation our members must prepare for,” Palumbo said.

“The Ontario government is urged to acknowledge this and to exercise caution when it comes to imposing auto insurance premium and return-on-equity reductions without first introducing reforms that will reduce costs in the product.”

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  • In this photo provided by Michael Li, water rises in a GO Train stranded on flooded tracks in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/Micheal Li via The Canadian Press)

  • Stranded passengers await rescue on a flooded GO Train in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/The Canadian Press, Frank Gunn)

  • A GO Train is stranded on flooded tracks in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/The Canadian Press, Winston Neutel)

  • A GO Train is stranded on flooded tracks in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/The Canadian Press, Winston Neutel)

  • A tow truck driver floats a car out of the Don Valley Parkway in Toronto on Monday, July 8 2013. (AP Photo/The Canadian Press, Frank Gunn)

  • A woman gets back into her flooded car on the Toronto Indy course on Lakeshore Boulevard in Toronto on Monday, July 8 2013. (AP Photo/The Canadian Press, Frank Gunn)

  • A woman gets back in her car in flood water on Lakeshore West during a storm in Toronto on Monday, July 8, 2013. (AP Photo/The Canadian Press, Frank Gunn)

  • Stranded passengers are rescued from a flooded GO Train in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/The Canadian Press, Frank Gunn)

  • Stranded passengers are rescued from a flooded GO Train in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/The Canadian Press, Frank Gunn)

  • Stranded passengers are rescued from a flooded GO Train in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/The Canadian Press, Frank Gunn)

  • Stranded passengers are rescued from a flooded GO Train in Toronto on Monday, July 8, 2013. A severe thunderstorm forced the shutdown of Toronto's subways, cut power to 300,000 in Canada's largest city and caused Porter Airlines to cancel all flights out of the downtown airport Monday evening. (AP Photo/The Canadian Press, Frank Gunn)

  • Toronto Apocalypse 2013

  • After the storm cleared, traffic heads out west of the downtown core.

  • Toronto Apocalypse 2013

  • Toronto Apocalypse 2013

  • Toronto Apocalypse 2013

  • <a href="https://twitter.com/meltait/status/354380494454001664" target="_blank">Tweet from @meltait</a>

  • <a href="https://twitter.com/hungrypo/status/354382745381789696" target="_blank">Tweet from @hungrypo</a>

  • <a href="https://twitter.com/TeenainToronto/status/354399592109993985" target="_blank">Tweet from @teennainToronto</a>

  • <a href="https://twitter.com/Nock4Six/status/354362607794270209" target="_blank">Tweet from Nock4Six</a>

  • <a href="https://twitter.com/weathernetwork/status/354416176895500290" target="_blank">Twitter: Tweet from @weathernetwork</a>

  • <a href="https://twitter.com/joyandwoe/status/354376407238971394" target="_blank">Tweet from @joyandwoe</a>

  • <a href="https://twitter.com/mathewi/status/354377501700669441" target="_blank">Twitter</a>

  • <a href="https://twitter.com/dorkkly/status/354384348629323776" target="_blank">Tweet by @dorkkly</a>

  • <a href="https://twitter.com/robcer/status/354366213209600003" target="_blank">Tweet by @robcer</a>

  • <a href="https://twitter.com/sabrina_scott/status/354358458629369858" target="_blank">Tweet by @sabrina_scott</a>

  • <a href="https://twitter.com/astroboy/status/354390796469944320" target="_blank">Tweet by @sukminc</a>

  • Flood

    Sent to us by <a href="https://twitter.com/MsNikkiThomas" target="_blank">@MsNikkiThomas</a>

  • <a href="https://twitter.com/Lucypalm/status/354369513317351427" target="_blank">Tweet by @lucypalm</a>

  • <a href="https://twitter.com/jumpnhigh/status/354390010704822272" target="_blank">Tweet from @jumpnhigh</a>

  • <a href="https://twitter.com/vickilaszlo/status/354381014333808640" target="_blank">Tweet from @vickilaszio</a>

  • <a href="https://twitter.com/DSCanCreations/status/354410559065706496" target="_blank">Twitter</a>

  • Dave Pankew

  • Dave Pankew

  • Lakeshore and Spadina.

  • Union Station Tuesday

    <a href="http://www.huffingtonpost.com/social/hp_blogger_Kenny Yum"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/contributors/kenny-yum/headshot.jpg" /></a><a href="http://www.huffingtonpost.com/social/hp_blogger_Kenny Yum">HuffPost Blogger Kenny Yum</a>:<br />A little wet on Tuesday morning.

  • Flood

    <a href="http://www.huffingtonpost.com/social/hp_blogger_Kenny Yum"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/contributors/kenny-yum/headshot.jpg" /></a><a href="http://www.huffingtonpost.com/social/hp_blogger_Kenny Yum">HuffPost Blogger Kenny Yum</a>:<br />Near Kiping Station Monday after 6 p.m.

  • Flood

    Sent to us by <a href="https://twitter.com/MsNikkiThomas" target="_blank">@MsNikkiThomas</a>

  • Flood

    Sent to us by <a href="https://twitter.com/MsNikkiThomas" target="_blank">@MsNikkiThomas</a>

  • The Storm

    <a href="http://www.huffingtonpost.com/social/MaryHynes"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/MaryHynes">MaryHynes</a>:<br />Another shot from the subway

  • After the storm @ Union Station

    It's like the rain never happened

  • The Storm

    <a href="http://www.huffingtonpost.com/social/MaryHynes"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/MaryHynes">MaryHynes</a>:<br />From the Viaduct

  • NEXT: Toronto wants Calgary Mayor Nenshi

    As the City of Toronto dealt with flooding on Monday, some Twitter users wondered about Calgary Mayor Naheed Nenshi's availability. <a href="http://www.huffingtonpost.ca/2013/07/08/toronto-flooding-2013-naheed-nenshi_n_3564081.html" target="_blank">READ THE STORY HERE</a>