With Autumn officially underway and the leaves starting to turn, many are beginning to wonder about the upcoming winter, and if the Autumn pattern will foreshadow it. Unfortunately, it is not as easy as “cold October yields cold winter” or “warm October yields warm Winter”. If that were the case, we’d have long range forecasting figured out by now, and there would be no need for winter outlooks. The best we have for now are correlations, analogs, physical pattern drivers, and pattern persistence. When added together, the summation of these various factors provide a higher probability of success in medium to long range forecasting, but these methods are far from perfect. The evolution of different atmospheric oscillations during the autumn season can give us a substantial clue as to the ensuing winter’s dominant index modalities.
In this particular case, let’s examine the autumn north pacific pattern of years since 1990, giving us a sample size of 22+ years. The North Pacific pattern is often represented as the East Pacific Oscillation (EPO). Like the North Atlantic Oscillation the EPO has certain, generally predictable outcomes when it oscillates from positive to negative phases. The positive phase of the EPO is usually associated with lower than normal heights across the northeastern Pacific, much of Alaska, and northwestern Canada. Conversely, the negative phase of the EPO generally produces higher than normal heights over the region just described. The positive (+EPO) tends to flood the United States with maritime pacific air due to the low pressure around Alaska, while the negative (-EPO) acts as a block to maritime pacific air due to high pressure over the NE Pacific/Alaska. The -EPO often sets up a cross polar flow with high pressure centered over Alaska and the flow oriented across the north pole from Siberia into Canada. The -EPO pattern has yielded some of most impressive, severe arctic outbreaks on record.
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