A special weather statement has been issued, calling for rain for most of Eastern Ontario, including the Kingston, Brockville and Belleville regions.
Environment Canada says the significant rainfall will hit Friday.
The inclement weather is expected to move into Central and Eastern Ontario overnight on Thursday and continue into Friday evening.
The weather agency is calling for a total rainfall of 25 to 40 millimetres by late Friday evening and is warning of the possibilities of floods.
The City of Ottawa has officially called a state of emergency due to serious flooding in the Ottawa Valley regions.
WATCH: Provincial governments grapple with how to deal with aftermath of flooding
According to the Cataraqui Region Conservation Authority, this increased rain from a major storm in the Ottawa region has contributed to flooding in the lower St. Lawrence River, as well as higher water levels in Lake Ontario.
Over the weekend, Lake Ontario’s water level rose about 10 cm, which is 28 cm above average.
The conservation authority says forecast suggests a 50 per cent chance that Lake Ontario’s levels will rise at least 20 cm over the next
two weeks, with the potential for greater rise under the wettest scenarios.