Get Your Shovels Ready: Ontario's Snowbelt Could Be Buried Again This Winter
/
Satellite image from GOES-16 shows lake effect snow coming off the Great Lakes on February 29th, 2024. Courtesy of CSU/CIRA & NOAA.
The summer of 2025 was a hot one in Southern Ontario, in which we saw high temperature records consistently broken throughout the region. This trend has continued into October, with temperatures surpassing 30°C in parts of Southwestern and Eastern Ontario during the first weekend of the month.
Despite the fact that we’re gradually moving into the coldest part of the year, the heat from the summer and fall will continue to impact local weather conditions this winter. This will be particularly evident as lake effect snow, a phenomenon that occurs every year in the late fall and early winter in Southern Ontario.
The two main ingredients necessary for the development of lake effect snow are a large unfrozen body of water and a very cold air mass. The air mass needs to be at least 13°C colder at the 850mb pressure level of the atmosphere (this is typically found at around 1500 metres) than the temperature of the surface of the water. Once this threshold is reached, it’s like a switch being flipped and the lake effect snow machine starts.
There is more to the development of lake effect snow than just cold air and an open body of water. Another key component in lake effect snow development is moisture. Ideally, the relative humidity at the surface needs to be at least 80% for lake effect snow to form and levels below 70% could actually inhibit development. It can usually be assumed that the lake itself can provide enough moisture, but this is not always true. There also needs to be limited wind shear with height between the surface and the 700mb pressure level so that the moisture is more focused, sort of like a hose. The strongest, most organized bands of lake effect snow develop when the wind shear is less than 30°.
Finally is the concept known as “fetch”, which is the distance that the air mass travels over the lake. Fetch needs to be at least 100km in order for lake effect snow to develop and the greater the fetch, the more snow is produced. When considering prevailing wind directions, the traditional snowbelts are found in areas that are downwind of the greatest possible fetch over the Great Lakes, i.e. Buffalo and the entire length of Lake Erie.
The creation of lake Effect Snow. Courtesy of Environment Canada.
As the cold air mass travels over the much warmer surface of the lake, the warmth and moisture from the surface is transferred into the lower atmosphere. The warmer, moister air rises and it eventually cools and condenses, forming narrow bands of clouds. These clouds continue to travel over the open lake, gathering even more moisture, until they eventually reach land and the snow starts to fall at rates that can easily exceed 5cm per hour and could even be as high as 20cm per hour! It’s important to note that the hardest hit areas are actually not found immediately at the shoreline, but rather 30-50km inland from the lake.
The direction of the winds dictates which areas are hit by the lake effect snow so as long as the ideal conditions continue, so too will the development and subsequent falling of lake effect snow. This could lead to several days of heavy snow hitting the same area while there are sunny skies less than 20km away. A slight shift in wind direction can quickly change which area gets hit and that makes lake effect snow notoriously tough to forecast. In Southern Ontario, lake effect snow typically hits areas to the east of Lake Huron and Georgian Bay, but it can also impact communities to the north of both Lake Erie and Lake Ontario.
Surface Temperature of Lake Huron throughout 2025 plotted with the 30 year average. Courtesy of NOAA CoastWatch.
So how does a hotter-than-average summer impact lake effect snow? To put it simply, a warmer summer leads to warmer lakes. Looking at Lake Huron, the main source of lake effect snow in Southern Ontario, the surface water temperature is approximately 2°C warmer than the 30 year average.
One of the important properties of water is its high heat capacity, which means that lakes heat and cool slower than the surrounding land. The warmer lakes will take longer to freeze, meaning that the lake effect snow machine can run even longer.
The current surface temperatures across the Great Lakes are very similar to last year at the same time. Many will remember last fall when the Muskoka Region was buried after several days of intense lake effect snow, trapping people in their homes and cars after the highways were closed.
This trend continued through most of the winter due to large expanses of open water remaining present in the Lakes. This was particularly the case in Bruce, Grey, and Huron Counties, where consistent lake effect snow resulted in massive snow piles and drifts that were up to 12 feet tall! The amount of snow also had an effect on local schools, with more than 30 snow days announced for students of some school boards.
Surface temperatures of the Great Lakes as of October 7th, 2025. Courtesy of Noaa Coastwatch.
Surface temperatures of the Great Lakes as of October 6th, 2024. Courtesy of NOAA Coastwatch.
While it is still too early to predict exactly how much snow will fall and where over the coming months, it is looking likely that we can expect a considerable amount of lake effect snow like last year.
