A large area of much warmer-than-normal ocean water off the North America West Coast persistent since 2013-14 and commonly referred to as the "warm blob" has a tendency to correlate to high pressure in the upper atmosphere across the northeast Pacific sometimes "bridging" northward across Alaska to the polar region. The 15-day upper air forecast by the GFS ensemble indicates this pattern emerges for late October. The upper ridge is compensated for by a downstream deep cold upper trough in central North America ("polar vortex").

La Nina is strengthening and a robust episode is forecast to peak in December. The first half of 2021 La Nina slowly fades. Primary climate impacts will be developing dryness to drought across the Southern U.S. and a very wet regime in Eastern Australia.

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.