New to the forecast is a developing heavy rainfall regime developing in the East/Southeast centered on the 6-10-day period according to ECM. Most of the rainfall is mid-to-late period as a Pacific trough arrives and regains strength while entraining Gulf of Mexico moisture. Heavy rain causing flash flooding is possible from the Tennessee Valley to the Mid-Atlantic States and Southeast also featuring severe thunderstorms during October 27-29.

The Gulf of Alaska has cooled off considerably this autumn season and is cooler than normal for the first time since 2012. More recently, the cool waters have evolved off the U.S. West Coast. The California Ocean Current is up-welling subsurface cool waters causing a horseshoe cool anomaly to form from the Gulf of Alaska to the California Coast and southwestward toward Hawaii.

JUL/AUG/SEP 2021 was the 3rd hottest on record given the 127-year climatology and 15th wettest in that time period. Most impressive was the record wet climate during that time for New York, Massachusetts and Rhode Island.

A wet fetch out of the deep tropics interacts with a low pressure trough causing excessive rainfall forecast by GFS in the 8-14-day period across Southeast Brazil, Uruguay and northeast Argentina.