Rainfall warnings are in effect for approximately 40-60 millimeters across much of the region.
Heavy rain forecast for Thursday and Friday in the Ottawa area may require changes to Halloween costumes and plans.
Rain warnings have now been issued for most of eastern Ontario and the Gatineau region west of the Laurentian mountain ranges.
In eastern Ontario, except for the westernmost Deep River and Algonquin Park areas, 40 to 70 millimeters of rain could fall from Thursday morning to Friday evening. According to Environment Canada, the heaviest rainfall will occur Thursday afternoon and night.
Communities in western Quebec from Quonset to Grenville-sur-la-Rouge and north to approximately Gracefield could receive approximately 50 millimeters of rain.
In general, but especially when visibility and stopping distance may be affected by rain, Ottawa police advise trick-or-treaters to ensure they can be seen by using reflective tape or flashlights and to cross the street only at intersections.
Drivers should slow down and be alert for anyone crossing intersections or parked vehicles.
Ottawa is about to experience its wettest month of the year, with its international airport receiving 93.5 millimeters of rain so far this month.
The airport's weather station currently reports 226 millimeters, or 27 percent, of below average rainfall, leading to a severe drought that is affecting farmers and drying up rivers and wells.