Morrisburg-Long Sault and Winchester-Newington are under a severe thunderstorm warning as of 3:31 p.m. Tuesday, according to Environment Canada.
Severe thunderstorm warnings are issued when storms capable of producing hail, heavy rain, and winds up to 90 km/h are possible.
Environment Canada says the storm is located near Inkerman and moving east at 40 km/h. The areas impacted include Williamsburg, Dunbar and Hoasic.
People in affected areas should be prepared for large hail, damaging winds and torrential rainfall and to take cover immediately if threatening weather approaches, officials say.
There is also a severe thunderstorm watch in effect for Ottawa. While it has not yet announced a severe thunderstorm, Environment Canada says conditions are favourable for a storm to develop in Ottawa North, Kanata, Orléans, Ottawa South, Richmond and Metcalfe.