Flood watches have been issued by the Maitland Valley and Ausable Bayfield Conservation authorities.
Rainfalls could reach between 50 and 80 millimetres between Monday and Tuesday.
Conservation officials say the current snowpack has no more room to absorb this week's rain, before running off.
Large creeks and rivers are expected to exceed their banks, with riverline flooding beyond traditional low-lying floodplain areas.
Officials say there is also a risk of ice jams in the Lower Maitland River Tuesday, between Wingham and Goderich.
You are reminded all ice should be considered unsafe, and to be watchful of kids and pets around strong flowing waterways.