Unseasonably warm water temperatures may be lovely for autumn swims at Cherry Beach, but they're anything less than ideal once cooler weather hits, as Toronto experienced this past weekend when a super-soaker rainstorm battered the city.

Warm water + cold air = an unstable atmosphere capable of producing water spouts and plenty of heavy, lake effect precipitation.

That's the basic gist of what meteorologists are saying in articles this week warning that the Great Lakes are warmer right now than they have been in mid-October since 1995.

A large chunk of eastern Lake Ontario with a surface temperature of 66 F ( 19 C) on October 12th. Looks like the warmest the lake has been this late into Fall since 1995. #nywx pic.twitter.com/l5VKejgc7h

— Jim Teske (@JimTeskeNC9) October 13, 2021

All five Great Lakes (including Ontario, which borders the City of Toronto) are running warmer-than-normal for this time of year, according to The Weather Network, threatening to play a major role in the weeks and months ahead.

"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," explains Weather Network meteorologist Rachel Modestino.

"Adding on a favourable wind direction off of lakes Huron, Erie or Ontario provides the ingredients for lake-effect showers."

With the warm front now over Lake Ontario, we'll pause for a quick break while we wait on the cold front to catch up. The cold front arrives around daybreak, kicking off another burst of heavy rain and gusty thunderstorms. Severe weather unlikely, but not impossible. pic.twitter.com/2MJlZyMMQj

— Eric Snitil (@EricSnitilWx) October 16, 2021

Depending on which direction the cold air comes from, narrow bands of very heavy rain could "tick up rainfall accumulations locally," says Modestino, or barely hit southern Ontario at all.

But it's not only rain we need to worry about — the difference in wind direction from the water's surface to higher levels of the atmosphere (called wind shear) could create "pockets of rotation," leading to the formation of water spouts on Lake Ontario and other Great Lakes.

In fact, it already has.

Cool water spout over Lake O. pic.twitter.com/XLOjDTyWG7

— Curtis Fielding (@FieldingWinery) October 18, 2021

One local storm chaser says they saw "numerous funnels from the 401" over Lake Ontario on Sunday. At least two more Twitter users spotted the same thing on Monday, one of them from Bluffer's Park in Scarborough.

Marine weather warnings are currently in place for all five of the Great Lakes, according to Environment Canada, which currently lists "strong wind" as a hazard for boaters on Lake Ontario.

"Ontarians should be on the lookout for these weather systems heading through October and into November," warns the Weather Network. "Water takes more time to change its temperature than air, so these atmospheric setups are likely to persist for some time."