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ONTARIO | First taste of fall

'Unusually STRONG' August storm hits Ontario, severe risks


Digital writers
theweathernetwork.com

Tuesday, August 21, 2018, 12:52 PM - Widespread statements and warnings are in place as a powerful fall-like storm targets Ontario with multiple shots at miserable weather on Tuesday. This system packs a triple-threat for the region with thunderstorms, strong winds, and locally heavy rain on tap. Flooding has already been reported (see below). We also can't rule out the chance for an isolated tornado.

"This is an unusually strong system for August," says Weather Network meteorologist Dr. Doug Gillham. "It is unusual (but not unheard of) for the pressure to drop this low with a non-tropical system during August." More on the timing and impact below. 

(SEE ALSO: Don Valley Parkway flooding. Here's why it happens so often )


WEATHER HIGHLIGHTS:

  • Potential for significant severe weather on Tuesday; hinges on morning atmosphere
  • Soaking rain possible for wildfire-stricken regions; rainfall warnings in effect for parts of northeast
  • Brief cool down followed by renewed surge of warmth for late month

WATCH BELOW: TIMING TUESDAY'S STORMS



THE SET UP: A BRIEF SHIFT INTO FALL

The atmosphere is starting to show signs of gearing up for its seasonal shift, as we see the return this week of a broad upper level trough over the Great Lakes, and a strong storm that goes along with it. While ridges of stagnant high pressure mark the summertime jet stream, these dips of low pressure are more a signature of the colder months, serving as support for the stronger, larger storm systems of the fall and winter. Tuesday's storm is one of the first of this transitional season, when we see the return of widespread rain along with the risk for thunderstorms.

HOW SEVERE WILL IT GET? CONDITIONS MUST ALIGN

The extent and severity of Tuesday's thunderstorms will depend almost entirely on how much clearing we see through the earlier part of the day, ahead of the main pulse of energy with the system.

If we do see that clearing -- and the heating from the sun that goes with it -- we could see some very strong thunderstorms over parts of southern Ontario, as there will be ample upper level support to fuel long-lived storms. Torrential rain, damaging wind gusts, and even isolated tornadoes will all be possible on Tuesday if morning conditions work out in favour of added heating. 

In the event we see mainly cloudy skies persist through the morning hours, the severe risk will be greatly diminished, however heavy rain is still anticipated with the passage of this system.

TIMING: MULTIPLE ROUNDS

Severe or not, expect thunderstorms ahead of the cold front to move into southwestern Ontario through the morning hours on Tuesday, sweeping through the Niagara Peninsula and GTA through toward noon, and moving into eastern Ontario by late afternoon. The second round of storms -- the one expected to bring the bulk of the rain, along with the severe threat -- accompanying the cold front is likely to develop through southwestern Ontario through the mid-afternoon to evening hours. 

"This is the time frame when the threat for severe storms appears to be the highest," Gillham says. 

SOAKING RAIN COULD BE GOOD NEWS FOR NORTHEAST FIRE ZONES

One benefit from this round of wet weather will be some much-needed soaking rain for those still dealing with an active wildfire season across parts of northern and northeastern Ontario. Given the expected track of the low, some of the heaviest rain -- up to 70+ mm -- is likely to fall east of Lake Huron and Georgian Bay, including the region impacted by the so-called Parry Sound 33 fire, which started on July 18 and still sits at over 11,000 hectares in size. Rainfall warnings were issued for parts of the Nickel Belt by Monday night, in anticipation of potential flash flooding and water pooling on roads.

"Localized flooding in low-lying areas is possible. Watch for possible washouts near rivers, creeks and culverts," warns Environment Canada. 

FLOOD REPORTS FROM SOUTHERN REGIONS

Further south, the heaviest rain is likely to be more hit and miss, falling where thunderstorms enhance rainfall intensity. Still, we do have the threat for localized flooding if thunderstorms train over the same areas. Before 10 a.m., there were already reports of flooding in parts of Toronto and the on ramp from Dundas Street to the northbound Don Valley Parkway was closed shortly after 9 a.m. due to flooding.




The City of Toronto and Metrolinx has added extra staff at Union Station in case of flooding.



(MUST READ: Think you can drive through a flood? Think again)

WINDS

Gusty winds will also be a feature of this system, with the strongest gusts sweeping down from the north-northwest as the cold front moves through Tuesday night into Wednesday.

DON'T WORRY, WE HAVEN'T SEEN THE LAST OF SUMMER JUST YET

While we're looking at a short break from the heat in the wake of the cold front, summer isn't abandoning Ontario just yet. 

"Wednesday will give us just a brief taste of early fall weather," says Weather Network meteorologist Dr. Doug Gillham. "With a mix of sun and clouds and a brisk northwest wind, temperatures will struggle to climb into the lower 20s across the GTA, and parts of cottage country will remain in the upper teens." 

The higher terrain of Huron and Perth counties will also be on the chilly side, with highs in the mid to upper teens for some, especially where the brisk breeze blows in off Lake Huron. The 401 corridor from Windsor to Cornwall should make it safely into the low 20s, however, with comfortable humidex readings in the mid-20s.

Our fall preview won't be long-lived, however, as heat surges back into southern Ontario as we head toward the weekend. By Friday, we'll be looking at widespread highs in the upper 20s once again, as another system approaches from the southwest for the weekend.

"We have the threat for showers and thunderstorms (but not a washout) on Saturday," says Gillham. "The heat and humidity will return for Sunday as the humidex soars into the upper 30s. We will see more sunshine on Sunday, but we will still have the risk for scattered showers and thunderstorms during the afternoon and evening."

WATCH BELOW: UNLIKELY FOOD SOURCE SAVES MISSING HIKER


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