Fig. 1-3: The 6-10/11-15 day most likely upper air patterns forecast by the mega-cluster ensemble identify a -PNA pattern characteristic of a La Nina climate.
Discussion: New Englanders’ ponder a cold winter ahead foreshadowed by unusual late October snow and snow cover plus following cold temperatures. Television meteorologists cite analogs supporting the early snow/following cold winter theory. Years ago this relationship had some credibility. But not in the climate change era as the oceans are much warmer and correlate to warmer mid-latitude climate during the cold season.
The latest example of what can happen following a cold and snowy brief extreme in the Northeast U.S. during a fortified La Nina climate pattern is about to evolve: Evolution and intensification of a negative phase Pacific North America (-PNA) pattern.
-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.
-PNA patterns are common during La Nina. A wintertime La Nina climate favors an upper rough over Western Canada which can easily stretch southward into the Western U.S. propelling a strong -PNA pattern. -PNA patterns cause cold and snow and the widening snow cover makes breaking the -PNA pattern difficult.
Upstream the compensating +NAO pattern eats away at any snow cover and south of snow cover in a +NAO pattern during winter temperatures are very warm. This is the pattern developing the next 2 weeks and will be semi-permanent through the La Nina winter of 2020-21.
Fig. 4: The 15-day Pacific North America index forecast indicates a strong negative phase “locking-in”.