Preliminary indications suggest the Envoy Air Embraer ERJ-145 which suffered a runway excursion at Chicago O'Hare had landed with a tailwind in gusting conditions.
Braking action before the touchdown on runway 10L was "medium to poor" up to the N3 taxiway – just over halfway along the runway – according to tower controller transmissions to the crew.
The ERJ-145, arriving from Greensboro on 11 November, had already executed a go-around about 25min earlier.
A SkyWest Airlines Bombardier CRJ700 immediately preceding had similarly gone around, telling the controller that it was due to braking action, and diverted to Green Bay. Braking action at that point was given as 'medium'.
But the ERJ-145 crew opted to make a second approach to runway 10L. The tower controller gave the runway visual range as 4,000ft according to communications archived by LiveATC.
Meteorological data indicates freezing conditions, light snow, and winds from about 350° at 17kt, gusting to 25kt, which would have presented a tailwind component.
The ERJ-145 slid off the runway, coming to rest with its right wing in contact with the snow.
Chicago tower declared the runway closed, informing personnel that they "had an aircraft go off the runway".
American Airlines, on whose behalf flight AA4125 was operating, states there were 38 passengers and three crew on board.