The Great Lakes are running warm for this time of year. Water temperatures on all five lakes are the warmest we’ve seen in mid-October since 1995, and Lake Superior has run above-normal all year. Warm lake waters will play a large role in upcoming fall weather, and we’ll get a taste of that influence in southern Ontario to close out the weekend.
The lakes will retain much of their heat even as air temperatures cool off in the coming days and weeks. An approaching cold air mass above the surface will set up a large temperature contrast over the bodies of water, creating an unstable environment overhead.
COLD AIR OVER WARM LAKES LEADS TO LAKE-EFFECT SHOWERS
Adding on the northeasterly wind direction off of Lake Huron and Georgian Bay, all ingredients are present for lake-effect showers throughout Sunday.
These narrow bands of heavy rain could tick up rainfall accumulations upwards of 10-20 mm locally, whereas other areas across southern Ontario may only see trace amounts.
WATERSPOUTS ARE ALSO POSSIBLE
A bonus ingredient often present, and likely to continue into November, is wind shear, or the difference in wind direction from the surface to higher levels of the atmosphere.
Passing fronts disturb the wind directions with altitude, creating pockets of rotation. This could lead to waterspouts over the warm, unstable Great Lakes. Boaters should continue to monitor forecasts for this hazard.
THE WARM LAKES WILL ASSERT THEIR INFLUENCE INTO NOVEMBER
Fall-like temperatures and weather patterns are just getting started in the GTA. Temperatures have run above seasonal for the first half of the month in Toronto, even notching a week-long streak of days above 20°C.
Saturday’s high temperature of 15°C was the closest we’ve seen to average so far this month, but the day still came in above the seasonal temperature of 13°C.
While a break from the precipitation is in the forecast for the GTA’s work week, Ontarians should be on the lookout for these weather systems heading through October and into November. Water takes more time to change its temperature than air, so these atmospheric set ups are likely to persist for some time.