A strong weather system carrying heavy rain is moving into southern Ontario, bringing large amounts of precipitation to Toronto and surrounding areas, according to Environment Canada.

The weather service issued a special weather announcement Tuesday to warn that parts of southern Ontario could get as much as 45 mm of rainfall overnight -- just shy of the amount that would trigger an official warning.

Environment Canada said a low pressure system hovering over Mississippi Tuesday morning has deepened as it tracks north toward the Great Lakes.

"For southern Ontario, at least, it looks like a good, soaking, fall rainfall," Environment Canada meteorologist Geoff Coulson told The Canadian Press.

By 5 p.m., parts of Toronto were getting soggy as the system moved into the area. The core of the rainfall is expected to continue overnight.

Rain will be heavy at times, accompanied by brisk easterly winds. Isolated thunderstorms are not out of the question.

The majority of southern Ontario can expect between 25 and 30 mm of rainfall through the night. However, an area stretching from Toronto and Hamilton to Ottawa can expect more.

"Some areas may even get a little bit more than that, especially in portions of eastern Ontario where local amounts up to about 45 millimetres could be possible," said Coulson.

Environment Canada issues a rainfall warning when it anticipates rainfall amounts to reach 50 mm or more within a 24-hour period. It continues to monitor the situation, but no warning has been issued.

"We won't be dealing necessarily with any widespread flooding issues. It has been relatively dry recently," said Coulson.

Rain will continue to fall in eastern Ontario on Wednesday morning.