The structure, variability, and mean-flow interactions of the quasi-2-day wave (Q2DW) in the mesosphere and lower thermosphere during January 2015 were studied employing meteor and medium-frequency radar winds at eight sites from 23 degrees S to 76 degrees S and Microwave Limb Sounder (MLS) temperature and geopotential height measurements from 30 degrees S to 80 degrees S. The event had a duration of 20-25 days, dominant periods of 44-52 hr, temperature amplitudes as large as 16 K, and zonal and meridional wind amplitudes as high as 40 and 80 m/s, respectively, at middle and lower latitudes. MLS measurements enabled definition of balance winds that agreed well with radar wind amplitudes and phases at middle latitudes where amplitudes were large and quantification of the various Q2DW modes contributing to the full wave field. The Q2DW event was composed primarily of the westward zonal wavenumber 3 (W3) mode but also had measurable amplitudes in other westward modes W1, W2, and W4; eastward modes E1 and E2; and stationary mode S0. Of the secondary modes, W1, W2, and E2 had the larger amplitudes. Inferred MLS balance winds enabled estimates of the Eliassen-Palm fluxes for each mode, and cumulative zonal accelerations that were found to be in reasonable agreement with radar estimates from 35 degrees S to 70 degrees S at the lower altitudes at which radar winds were available.