Southern Ontario: Thunderstorm Outlook for Saturday, July 24, 2021

MAP UPDATED - July 24 @ 2 PM

UPDATE (July 24 - 2:00 PM)

We haven't changed much from our forecast that we posted late last night. Although we have expanded the orange (severe risk) zone further to the northeast to include the Owen Sound and Meaford area. The environment is stronger than we expected in that area so that was enough to merit an upgrade.

Another minor change is expanding the tornado risk zone around the Georgian Bay shoreline so it extends further inland through parts of Muskoka and Simcoe County. The tornado risk isn't super strong here, but it can't be ruled out if a few storms pop up.

The main concern with regards to tornadoes is around the Lake Huron shoreline from Kincardine to Grand Bend. We're constantly seeing this area highlighted in the data for some quite intense storms and the environment is very favourable for one or two tornadoes. If you live near Lake Huron, you should have a plan in place to seek shelter if the tornado potential actually materializes over the next few hours.

Already seeing some storm development over Michigan and this will likely further intensify as it tracks to the west and towards Southwestern Ontario later this afternoon. It looks like the late afternoon and early evening will be when we should see the strongest severe threat although the storm risk may linger well after sunset - a nocturnal tornado can't be ruled out so it's extremely important you stay weather aware today.

Outside of the tornado threat, these storms could bring the potential for large hail and especially widespread wind damage. Another concern will be the heavy rainfall associated with all the storm activity. Some regions particularly around the northeast and eastern shoreline of Georgian Bay could see between 50-100mm by the end of the day. So flash flooding could be a huge issue considering many of these locations have already seen a lot of rain over the past few weeks.

Old Forecast Discussion

A rainy and stormy start to the weekend is expected across parts of Southern Ontario with the risk of some severe thunderstorms during the afternoon and evening on Saturday. Exactly how strong this risk will be is still a little questionable as timing could make or break the storm threat. Current model data show a line of precipitation with embedded thunderstorms sweeping through Southern Ontario during the afternoon on Saturday which could pose an isolated severe threat, but the main risk will come later in the day with a potential second round of storms.

Where this second round of storms initiates will be key in what threat they will pose to our region. If that first line of precipitation is earlier than expected this will allow enough time for the environment to build and support storm development later in the day. On the other hand, if the line moves through later in the day it’ll mean that storms initiate over Michigan and will be quite weak by the time to cross over Lake Huron late in the evening.

IF (and that’s a big if) the second round of storms develop and reach our region early enough we could see the potential for large hail, damaging wind gusts and maybe even one or two tornadoes with the highest chance along the Lake Huron shoreline and down into Deep Southwestern Ontario. If these storms hold off until later in the evening then the focus will shift towards mainly a damaging wind risk as the cluster of mature storms track to the east during the overnight hours. This risk may stretch into areas to the east of Georgian Bay as well with some indication of a few isolated cells developing over Georgian Bay.

The further east you go the lower the storm threat as the strongest activity will be mostly focused on the western part of our region. The Ottawa Valley will likely not see much activity until maybe Sunday morning as the storms from Saturday evening reaches that area, but they’ll be much weaker and well below severe levels

We’ll continue to monitor the latest data and issue a final forecast by Saturday early afternoon if required.