The U.S. shifts into an extraordinarily warm September pattern as indicated in today's medium-range forecast which trends warmer in the 11-15-day period as indicated by ECM ENS. Almost the entire U.S. is warmer than normal as mid-September arrives.

El Nino is strengthening. Oceanic El Nino warmed to +1.5C in the Nino34 region last week. Warming to >+1.0C finally reached the Dateline. The southern oscillation index (SOI) for August is projected near -1.00 which is the second monthly stronger El Nino-style -SOI of the past 4 months.

Given the warmer oceans and constricting ice cap of recent decades, there is a tendency for warmer climate vs. the 30-year normal and September in the U.S. is an example. During the past 10 years, September has averaged 1-3F warmer than the 30-year normal, making summer feel longer.

The latest U.S. population weight CDD forecast was adjusted slightly warmer (321 to 329) for August which is a moderately hotter than normal signature (compared to the 30-year normal). August 2023 will average slightly less hot than the past 3 years.