A May air-temperature model of Gulf of Mexico hypoxic area per nitrate load failed its 2024 test and the 1985-1999 back-cast; warmth had a weak link not specific to May

Question: does the 2024 LSU model, which makes Gulf of Mexico hypoxic area per May nitrate load a curved function of May air temperature at New Orleans, predict areas outside its 2000-2023 fit years? Approach: we used USGS daily WRTDS-K loads for the Mississippi and Atchafalaya rivers (ver. 3.0, 2026), LUMCON areas for 1985-2026, GHCN-Daily temperature, HURDAT2 storms and NDBC winds. We reproduced the model, froze it, applied it to 1985-1999 and 2024-2025, did month-placebo tests and made rolling one-year-ahead forecasts. Results: the first operational forecast (2024) was 5,107 km2 (95% interval 4,687-5,569). The measured area was 17,360 km2, and a load-only model gave 14,113 km2. A storm cannot explain an area that is larger than the forecast. In the 1985-1999 back-cast (12 years), the temperature model increased the mean absolute error from 4,493 to 6,606 km2 (difference CI +826 to +3,432). In the fit years, February and November temperatures fit the area-per-load ratio as well as May. Over 30 storm-free years, a 1 C warmer May gave 11% less area per load (p = 0.046), and a warmer June gave 23% less. River discharge did not remove this link. In rolling forecasts for 18 storm-free years (2000-2025), the curved May model decreased the error by 451 km2 (CI 50-851). May was the only one of 13 predictors with this result, and the authors selected it with the same data. A monotonic May term gave no clear gain. 15 of 18 published LSU forecasts from 2007 to 2026 were larger than the measured area. Limitations: one area value per year, three years after the fit with storms in two, and the exact model equation is not published.

Gulf of Mexico · hypoxia · dead zone · nitrate load · Mississippi River · forecast verification · ocean warming · WRTDS · out-of-sample test · LUMCON