Rainfall warnings are now in effect for Grande Prairie, Beaverlodge, Valleyview, Hinton and Grande Cache.
Environment Canada says there could be 50 to 60 millimetres by late Friday, with up to 80 millimetres in places.
Localized flooding is possible in some areas.
People are advised to watch for flash floods, washouts and water pooling on roads.
(Photos of heavy rain in May courtesy Joanne Hotte near Beaverlodge)