Results are reported from an improved measurement of ν_μ→ν_e transitions by the NOvA experiment. Using an exposure equivalent to 6.05×10^(20) protons on target, 33 ν_e candidates are observed with a background of 8.2±0.8 (syst.). Combined with the latest NOvA ν_μ disappearance data and external constraints from reactor experiments on sin^2 2θ_(13), the hypothesis of inverted mass hierarchy with θ_(23) in the lower octant is disfavored at greater than 93% C.L. for all values of δ_(CP)
