A momentum jet injected into a confined container breaks up to “diffusive turbulence” after traveling a critical distance. It has been argued that an adverse pressure gradient developing within the container, acting against the jet momentum flux, is responsible for this break up (Risso and Fabre, 1997,“Diffusive Turbulence in a Confined Jet Experiment,” J. Fluid Mech., 337, pp. 233–261; Voropayev et al., 2011, “Evolution of a Confined Turbulent Jet in a Long Cylindrical Cavity: Homogeneous Fluids,” Phys. Fluids, 23, 115106). Experimental evidence for this adverse pressure gradient is presented in this paper, supplemented by a control-volume analysis to explain the results. The rise of pressure from the jet-injection level to a location beyond the jet break up x_b is shown to be proportional to the jet momentum flux. The overall (integrated) sidewall friction on a control volume is negligible, compared to the increase of pressure, if the flow control volume extends beyond x_b. For smaller lengths of the control volume, the side wall drag is not negligible compared to the pressure rise. The Reynolds number similarity was evident for jet Reynolds numbers above 6000. This work was motivated by its applications to degassing of crude oil stored in the U.S. Strategic Petroleum Reserves, which are slender salt caverns. To improve its quality, periodically oil is cycled through a degassing plant and injected back to the cavern as a jet, and the degassing time is critically dependent on jet dynamics.