The 2021 northern hemisphere accumulated cyclone energy index (ACE) is about 75% of normal so far in 2021. Only the North Atlantic basin ACE index is above normal at 79.2 (29% above normal). Interestingly (and rare), both the northeast and northwest Pacific basins have observed below normal ACE index for the tropical cyclone season so far in 2021.

During the past 7-10 days the subsurface east-central equatorial Pacific Ocean has turned sharply cooler. Compared to last year, the amplitude of the cool signature this year is stronger than last including a less warm subsurface West Pacific in 2021. The NCEP CFS V2 identifies the cooler subsurface and applies the cooler regime to the forecast and the result is a stronger La Nina developing during quarter 4 of 2021.

Extratropical storms in stronger (positive anomaly) flows tend to be stronger and move more slowly. This pattern is responsible for most instances of extreme weather including storm intensity and warmth ahead/cold behind the storm. In an effort to project where extreme storm risk is highest for each month of quarter 4 of 2021 for the U.S. and Europe the Climate Impact Company constructed analog forecast used to generate sensible temperature and precipitation anomalies is used.

In a surprise, the GFS Ensemble has equaled or exceeded ECM Ensemble in medium-range skill scores (of 2-meter temperature forecasts) during the past 30 days. Regions of the high demand energy East U.S. produced better GFS 9than ECM) ENS skill scores - a rare event!