When tropical cyclones recurve (turn north and northeast after departing the tropics) they do so by losing their steering currents provided by the subtropical ridge (“Bermuda High”) and increase forward movement (accelerate) if an approaching upper trough is to the west of the storm. These conditions were present Friday evening as Delta went ashore setting the stage for a broad and strong wind (and intense rainfall) field north and west of the storm.

One of the most dramatic signatures is the North America drought which encompasses the western U.S. and eastern Canada. Regional warm SSTA caused the dryness. The “warm blob” in the northeast Pacific is linked to an upper ridge pattern turning Alaska drier, intensifying the western North America drought and allowing that drought to extend eastward reaching Iowa. Warm SSTA in the western North Atlantic basin once again enabled a summertime upper ridge pattern worsening East Canada to New England drought.

Delta is strengthening steadily due to presence over 85-86F water, light shear and excellent upper ventilation. The improving environment allowing Delta to intensify is in-part due to the heat release of weakening Gamma. The heat release strengthened the Florida subtropical ridge which is now pushing Delta more quickly west-northwestward. The warmer mid-to-upper atmosphere has also caused the upper shear near Delta to ease. The position of the jet stream and upper shear axis across the western Gulf of Mexico coupled with the anti-cyclone over Florida has provided Delta with excellent upper ventilation to allow Delta to fully express itself as a category-2 hurricane.

Possibly the greatest influence on North America climate during the past 7 years is the evolution of the persistent very warm waters of the northeast Pacific Ocean referred to by climate scientists as the “warm blob”. The “warm blob” caused the 2013-14/2014-15 “polar vortex” winter seasons and the persistent Western U.S. drought of recent years plus many other high impact climate regimes most perilous to North America and particularly the U.S.