ONTARIO: Thunderstorm Forecast for Thursday, August 17, 2023
/
NOTE: YOU CAN CLICK ON THE MAP TO OPEN A ZOOMABLE IMAGE WHICH WILL BE EASIER TO READ.
FORECAST UPDATE - Thursday Morning
Based on the latest data, it appears that the forecasted line of storms later today will be a lot weaker than expected. The environment just isn’t materializing as earlier data indicated which will mean the line currently over Michigan will likely fall apart as it crosses Lake Huron.
We will continue to hold with an ‘isolated’ risk across all of Southern Ontario as we can’t rule out a few marginally severe storms ahead of this line. It’s just very questionable how strong these storms will actually get at this point. Main storm hazards include 90 km/h wind gusts, hail up to the size of quarters and localized heavy rainfall. However, the ‘slight’ risk we had in our initial forecast has been replaced by the ‘isolated’ risk.
INITIAL FORECAST
As we venture into another Thursday in Southern Ontario, the familiar tune of severe thunderstorms is once again on the horizon. This time around, the risk is more extensive, featuring a line of potent thunderstorms that is set to cross Southern Ontario, starting Thursday afternoon and persisting throughout the day.
Though the overall severe threat is expected to max out at a 'slight' risk, it's important to note that the impact could be broader compared to prior occurrences. The main threats will include 100 km/h wind gusts, hail up to the size of ping pong balls and localized flash flooding.
This isn't an event with isolated, localized storms confined to specific areas. Instead, a linear arrangement of storms will extend from North Bay to Windsor, essentially encompassing a significant portion of Southern Ontario. While not everyone will encounter severe conditions, the line's potency will vary, so the degree of severity won't be uniform.
The timing of this event still bears a level of uncertainty. Initial model data hinted at the line's potential arrival during the late morning and early afternoon hours, intensifying throughout the day as the dynamic atmospheric conditions provide ample fuel to the line of storms. This line will start in the west across Northeastern Ontario and into Southwestern Ontario.
However, the most recent data nudges the line's onset closer to mid-afternoon. This shift has implications for the severity of the threat. With more daylight hours for atmospheric build-up, the storms could be more robust.
Additionally, we might see isolated storms forming ahead of the main line, particularly in Central and Eastern Ontario during the afternoon. These isolated storms could encompass all the classic severe threats: significant hail, potent wind gusts, and even tornadoes.
As the line of storms progresses, it could evolve into a 'squall line,' historically known for delivering widespread damaging winds and sporadic tornadoes across our region.
The hail risk seems relatively lower compared to recent events where we’ve seen hail as large as baseballs. However, isolated storms preceding the line may still pack the punch of large hail, possibly reaching the size of ping-pong balls. Inside the line itself, the hail sizes are anticipated to max out around quarter-sized.
The storm threat will carry it eastward, reaching the Golden Horseshoe during dinner hours and subsequently moving into Eastern Ontario by late evening. It's important to remember that this timeframe is tentative and may shift a few hours earlier or later.
Adding to the concern of damaging winds, flash flooding is becoming a significant issue we've identified in the model data. Certain areas might experience multiple waves of precipitation, particularly in the Ottawa Valley where persistent storm activity ahead of the main line could transpire. In such scenarios, local rainfall totals could exceed 100mm by the end of the day.
