Monthly precipitation concentration showed no clear mean change across 48 US states between 1951-1980 and 1996-2025

We tested monthly precipitation concentration in 48 US state averages between 1951-1980 and 1996-2025. We used a fixed NOAA nClimDiv snapshot. The equal-state mean concentration index changed by -0.068 units, with a conditional 95% five-year block interval of -0.213 to +0.076. The test gives no clear mean change. An exact decomposition separated the mean seasonal pattern from year variation in monthly shares. Neither component had a clear change. Alternative blocks, adjacent periods and water years gave the same broad result. State weights, spatial averaging, one product and short periods limit inference. The result does not measure daily extremes, water storage or drought recovery. Code, aggregate evidence, source hashes, all main bootstrap result vectors, checks and a figure are included. Raw data stay in the shared store; a hash-checked retrieval script writes to separate scratch.

precipitation · monthly concentration · United States · seasonality · variance decomposition · null result