Friday, September 21, 2018, 7:11 PM
The tornado threat diminished across Ontario Friday evening, after producing at least one damaging twister west of Ottawa.
Thousands of Hydro One customers lost power Friday as high winds moved through the area. While the threat for tornadoes has expired, we're continuing to watch the potential for damaging winds, which have prompted warnings for gusts close to 100 km/h. More on the impact and big cool down that follows this storm, below.
Friday marks the last full day of summer with the season set to go out with a BANG. A period of rain showers and thunderstorms were reported across southwestern regions on Thursday as the storm moved closer with parts of northern Ontario dealing with heavy, soaking rains. Special weather statements and rainfall warnings that were issued on Thursday remain in place for the north with an additional 20 to 30 mm of rain expected through Saturday.
"Heavy downpours can cause flash floods and water pooling on roads," warns Environment Canada. "Localized flooding in low-lying areas is possible."
The fall storm will quickly deepen over the Great Lakes region Friday as it races into northern Quebec. With the dynamic storm boasting both strong winds at the surface and aloft, there's a heightened threat for these two things:
First, the strong winds will increase wind shear (wind speeds increasing and changing with height in the atmosphere), supporting the risk for supercell thunderstorms capable of producing tornadoes in parts of central/eastern Ontario and Quebec.
Second, these powerful winds circling the storm may rush down to the surface, raising the risk for damaging wind gusts. This type of setup is expected as a squall line -- an organized line of thunderstorms -- develops along the cold front and blasts eastward. The strong winds will not be limited to the squall line, however. Friday will be quite windy ahead of the cold front and the wind will continue to be strong and gusty behind the cold front.
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Most thunderstorm activity will clear Ontario by 8-9 pm, leaving gusty northwesterly winds in its wake and a much cooler airmass.
Damage to buildings, such as to roof shingles and windows, may occur, warns Environment Canada. Residents are advised to take precaution as loose objects may be tossed by the wind and cause injury or damage.
Winds gusting to 60 or 70 km/h (locally higher) are expected after the cold front passes.
As the cold front sweeps through Friday afternoon and evening, there will be a noticeable cool down behind the system, dropping southern Ontario back to seasonal values and parts of the north well below normal.
"There's even the threat for a frost/freeze in rural areas well north and northeast of the GTA where the wind goes calm," says Weather Network meteorologist Dr. Doug Gillham.
Saturday will bring abundant sunshine along with crisply cool daytime highs below 20°C.
"Sunday will also be a pleasant early fall day with sunshine and high temperatures in the upper teens, but only low to mid teens for cottage country," Gillham adds.
Another fall storm is set to track into the northern Great Lakes early next week, bringing milder temperatures and a widespread soaking rain with the potential for thunderstorms Monday night through Tuesday.
"This will be followed by near seasonal temperatures for the end of the week, however we are watching the potential a more significant shot of chilly weather as we head into next weekend," says Gillham. "A more autumnal pattern is expected for the final weekend of September and into early October with cooler than seasonal temperatures."