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THUNDERSTORM RISK

Wednesday: Wind, rain, isolated tornado threat over Ontario


Digital writers
theweathernetwork.com

Wednesday, June 13, 2018, 4:20 PM - Right on cue, strong thunderstorms began their march across southern Ontario on Wednesday afternoon, leaving about 40,000 customers without power through 4 p.m. The main threats these storms have been producing are heavy rain, small hail, and strong winds, but the slight risk for isolated tornadoes remains with us into the evening.

A warm, moist, and unstable air mass is place across northeastern, central, and southern Ontario with humidex values into the mid to high 20s across central and northeastern Ontario and into the high 20s to low 30s across southern and eastern portions. A strong upper level trough, its associated surface low pressure system, and a cold front are slicing through this unstable airmass Wednesday afternoon, bringing a wide swath of thunderstorms - some of them reaching severe levels. More details on the impact and timing, below.

(Visit our COMPLETE GUIDE TO SUMMER 2018 for an in depth look at the Summer Forecast, tips to plan for it and much more) 


WEATHER HIGHLIGHTS:

  • Severe thunderstorm watches and warnings in effect for parts of southern Ontario (click to visit our alerts page)
  • Heavy rain, damaging wind gusts, hail, and even an isolated tornado are possible
  • Initial line of storms cutting through southern Ontario from London to the GTA through Niagara through about 5 p.m.
  • Secondary line trying to fill in behind that, with strong storms from the Quebec border through cottage country to the northern GTA
  • Watching to see if this secondary line holds together to bring another round through GTA, Niagara through next few hours
  • Reports of pea to dime-sized hail from Georgian Bay to Stouffville

TIMING

The main line of storms along and ahead of the cold front will bring the risk for severe storms as well. This will likely have impacts on the GTA commute, with slowed traffic reported on most of the 400-series highways around the Golden Horseshoe by 4 p.m., as torrential rain continues is waves across the region. The main time of concern around the Golden Horseshoe and GTA lingers through the early evening - through about 6 p.m. - after which the band of activity should pass south of Lake Ontario on the western edge.

Storms will surge east along the 401 corridor up towards the Kawarthas and eastern Ontario through the evening hours, with the key period of concern falling between about 5 and 7 p.m. for the National Capital Region. With storms coalescing into a squall line on the eastern edge of the cold front, the primary risk for severe weather for eastern Ontario and southwestern Quebec looks like strong-to-damaging winds, torrential rain, and small hail.

(SEE ALSO: Ontario's upcoming heat rivals ALL of July, August 2017)

CLICK TO PLAY: ROUGH TIMING OF THUNDERSTORMS IN ONTARIO


IMPACT: MAIN THREATS

The main threats with these storms are the potential for heavy rain (given the abundant moisture that will be in place), hail, and strong winds, which could be in excess of 90 km/h. Some forecast guidance is also showing wind shear (winds changing direction with height) could become favourable across the severe threat area for a few supercells to develop. Note that a supercell is a storm with a rotating updraft. A tornado threat could develop, again mainly across the severe risk area if lower level wind shear and instability remain strong enough, although the tornado threat is quite low.

Thumbnail image courtesy: Courtesy: Korey Rideout/Facebook London, Ontario

 

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