Environment Canada says a major rainfall event will continue into Wednesday morning.
A rainfall warning remains in effect for virtually all of southern Ontario.
Meteorologist Geoff Coulson says there's been a dramatic shift in the weather pattern over the last couple of days.
"Instead of drawing down the air masses from the northwest and the north, this is a system that's drawing up moisture from the American south and southwest," says Coulson. "Strong flow in the low levels of the atmosphere bringing up this very mild and potentially record-setting air for some places in southwestern Ontario but also, unfortunately, bringing a lot of rain as well."
Coulson says about 25 mm (1 inch) of rain had been recorded at Sarnia's Chris Hadfield Airport as of 7am Tuesday.
He says parts of Sarnia-Lambton could get a total of 50 mm to 75 mm (2-3 inch) by noon Wednesday, creating a lot of ponding and localized flooding.
"Frozen sod and ground is affecting the ability to retain a lot of moisture so there could be ponding in these situations," says Coulson. "Some conservation authorities actually going up to flood watches or even flood warnings in some parts of southwestern Ontario, a combination of the warmth, the snowmelt, potential ice jams on some of the rivers in the area, so this is a real spring-like burst of weather."
Residents are advised to make sure their children stay away from swollen streams, rivers and lakes, and not venture out on any ice that still remains.
The temperature in Sarnia reached 15.7 C (60 F) at mid-afternoon Tuesday, setting a new record high for February 20. The previous record was 14.4 C (58 F) set in 2016.
Fog patches will redevelop Tuesday evening and dissipate overnight with more seasonal temperatures returning Wednesday.