In North America, the late summer 2025 weather pattern forecasts were most effective during the 6-10-day utilizing ECM ENS, traditionally the best operational model. In second place, AIFS ENS continued to score well since the July 1st debut. ECM ENS and AIFS ENS have routinely scored no. 1 and 2 for medium-range forecasts during mid-to-late calendar summer 2025.

Emergence of the marine heatwave in the Northeast Pacific potentially increases the risk of El Nino in 2026 as the anomalous warm water spreads to the tropics. The NCEP CFS V2 Nino34 SSTA forecast continues to edge into El Nino territory Q2/2026. The change in ENSO will be applied to new season 1-4 ahead forecasts. The upcoming La Nina is a relatively short duration episode.

Merging between marine heatwave (MHW) NEP25A and NEP25B occurred during late AUG/early SEP as exceptionally warm sea surface temperature anomalies (SSTA) reached the West Coast of North America in the Northeast Pacific. The warming water surface episode is similar with the previously strongest warming event observed during 2013-16, commonly referred to as the “warm blob”.

August 2025 brought a cool surprise to the East U.S. including the 7th coolest in the 131-year record for South Carolina. Meanwhile the West, Southwest, and far South U.S. were MUCH ABOVE normal during August 2025. The dryness in the Missouri Valley to New England was profound featuring record dry for Kentucky, Ohio, and Vermont. Georgia was excessive wet during August. The Arizona heat observed in August was the second hottest on record.