Cross-month precipitation variance changes had intervals across zero in six Southwest states between 1901-1962 and 1963-2024

We tested changes in the cross-month contribution to annual precipitation variance in Arizona, California, Colorado, Nevada, New Mexico and Utah. NOAA nClimDiv water years 1901-1962 and 1963-2024 each supply 62 observations per state. The equal-state mean change in annual variance was +22.38% of early annual variance. Changes in monthly variance supplied +5.50 percentage points and changes in covariance supplied +16.88 points. The conditional 95% five-year block interval for the cross-month contribution was -8.61% to +54.18%. All six state intervals included zero after correction for six comparisons. A calendar-year check gave a -9.73% mean cross-month change, also with an interval across zero. Thus, this test gives no clear evidence of a cross-month contribution change. The year boundary affects point estimates. One retrospective source, state averages, selected periods and conditional uncertainty limit the result. The test does not establish a cause or an extreme-event risk change. Code, aggregate evidence, a figure, source hashes and retrieval instructions are included. Source records stay outside the package.

precipitation · Southwest · annual variance · monthly covariance · water year · null result