A messy Tuesday is ahead for much of Southern Ontario as a potent system moves through the region bringing the threat of prolonged freezing rain. This will all begin early Tuesday morning as we see the first bands of precipitation work into Southwestern Ontario just after midnight tonight. For Southwestern Ontario and the Golden Horseshoe, the story with this system will be heavy rain as temperatures will be above the freezing mark by the time the rain arrives. Between 10-25mm of rainfall accumulation is possible in those regions which could result in some localized flooding. There is even the outside chance of a non-severe thunderstorm for Deep Southwestern Ontario including Windsor and Sarnia!
This precipitation will encounter colder air further to the north along with the higher elevations of the Dundalk Highlands to the northwest of the GTA. This will allow for the risk of some freezing drizzle of rain for a few hours during the mid to late morning hours. It should transition over to rain by the early afternoon as temperatures slowly climb above the freezing mark. Expect an icy morning commute for locations like K/W, Guelph, Orangeville, Barrie and Peterborough.
The main concern with this storm will be through Central and Eastern Ontario where the cold air appears to be more entrenched and leading to a prolonged period of freezing rain. This begins late Tuesday morning which could start as some snow or sleet for the more northern regions and slowly mix in some freezing rain as we head into the afternoon.
We have fairly high confidence in the potential for 12+mm of ice accretion in a zone that includes Tobermory, Parry Sound, Sundridge and Algonquin Park. The Ottawa Valley and Northern Muskoka can expect around 6-12mm of ice accretion. However, there is lower confidence further south which includes Bracebridge, Kawartha Lakes and Smiths Falls as some models show a faster warmup while others are slower. If temperatures do indeed take longer to warm up, this would result in a bigger impact on that region. For now, we’re going with 2-6mm of ice accretion, but keep an eye on that local range of 6-8mm should the cold air be more stubborn than expected.
The freezing rain threat should come to end later in the evening on Tuesday. There are some indications that a few areas including Tobermory, Algonquin Park and the Ottawa Valley may not even see above freezing temperatures before colder air floods into the region late Tuesday. This means that the locations that are hardest hit by the freezing rain might continue to be impacted by the ice accretion for days to come. It’s not out of the question that this approaches ice storm levels for that 12-18mm zone so be prepared for extended power outages and disruption to travel that could linger into Wednesday.
There is also the risk of a flash freeze throughout Southern Ontario early Wednesday morning. All that rain from this storm plus melted snow will make for icy road conditions as the temperature drops. There is some uncertainty regarding the exact speed of this cooldown which would dictate how impactful the flash freeze could be.
