-PNA patterns represent a persistent cold upper trough over Southwest Canada and/or the Western U.S. Frequently, -PNA patterns are compensated for by an upper ridge across the East U.S. associated with positive North Atlantic oscillation (+NAO). The described pattern is fully developed in the 6-10 day period and likely to linger through the 11-15 day timeframe.

Tropical Storm Zeta formed in the western Caribbean Sea earlier today. Zeta is forecast to start moving northwestward tonight and reach the northern Yucatan Peninsula tomorrow night and gain hurricane intensity into the south-central Gulf of Mexico later Tuesday. The immediate environment improves and supports the forecast track and intensity. However, a shear axis and cooler northern Gulf waters plus an approaching upper trough from the west makes the central Gulf to landfall forecast very uncertain.

The most recent NCEP CFS V2 Nino34 SSTA forecast indicates a robust La Nina ahead including a January 2021 (cold) peak of -2.6C. If correct, the January Nino34 SSTA would be the coolest in the 1950-2020 climatology. Previously, the coolest Nino34 SSTA for January were in the -1.64 to -1.98 range with the 1989 La Nina the strongest on record (for January).

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").