Three cars - a red BMW X3, blue Audi S4, and white Toyota Yaris – collided on the A483 at Belan at around 6.15pm.
The warning - from 7pm on Monday until 10am on Tuesday - covers parts of Gwynedd, Conwy, Denbighshire, Flintshire and Wrexham.
Police told motorists to stay away from the area.
The tractor run from Llanidloes to Bettws Cedewain will be held on Saturday, December 14.
Residents can now have their say about Bute Energy's plans for overhead lines crossing Powys.
One of the major competitions of its kind in the UK returns this Saturday (November 16) to a treasured historic house near Newtown.
