Mark in CA;
Would mean some pretty powerful downward force, no?
Yes, it is a crack. No it does not necessarily mean "downward" - the crack extends from the right side of the fuselage, across the top but does not appear on the left side of the fuselage. I don't know the 737 but that may be a joint where the rear fuselage section joins the over-wing section. That would be more consistent with stronger lateral forces than vertical forces although rough terrain will present a complex mixture of such forces, of course.
Note that the track of the aircraft curves very slightly left, and that the heading of the aircraft is pointing slightly left (about 10 to 15 degrees) of the track taken.