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Why might forecast models be diverging with the long range pattern?

Good morning! The past several days have been quite active in the meteorological community.We began last week on the tail end of a pattern featuring stagnant warmth, with ridging in the Eastern United States and cold air bottled up in Western Canada and the Pacific Northwest. A well advertised change has occurred since that time, however, with cold air surging southeastward into the Great Lakes and Northeast States.

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Winter Forecast 2017-2018

Seasonal forecasting is quite the challenge, but one we always look forward to. It seems every year we learn something new about the atmosphere. It is critical to have a fundamental understanding of how the atmosphere works, and the implications that changes from its base state can have on the weather pattern.

Instead of focusing on individual numerical indexes and their verbatim values, we are going to try to paint a picture of the atmosphere and what it will be doing over the next few months based on these phenomena:

  1. ENSO (El Nino Southern Oscillation) and tropical forcing
  2. High latitude behavior and blocking
  3. Analog years based on global pattern evolution

Read more

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High latitude blocking increasingly likely through November

Good afternoon! Autumn like weather is in place as we speak across much of the Northeastern United States, with relatively seasonable temperatures. Enjoy it while it lasts. It will feel much more like winter very soon, as an arctic blast is set to arrive from Friday into Saturday. Very quickly, temperatures will fall 15 to 25 degrees below normal for this time of year, with record low temperatures possible over reporting climatology sites in the Northeast states. While this intensity of cold air will be short lived, colder than normal air will remain persistent.

A vigorous polar disturbance inside a shortwave trough will be swinging through the Great Lakes and Northeast tonight and Friday. This feature will amplify into a small closed low over Quebec later on Friday Night. The result will be a frigid airmass from Northern Canada swinging southward with a vengeance and reaching the Northeast USA from Friday into Friday Night. This type of southward movement of polar/arctic air is unusual this early in the season.

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(Tropical Tidbits)

Afternoon Video Update: NAO Blocking Signal Grows

 

Good afternoon! The well-advertised signal of high-latitude blocking developing in the 2nd half of November continues to grow. It all starts with an amplification and slowing down of a Pacific wave train, which helps to pump latent heat towards the Poles, further leading to ridging extending towards the Poles. Subsequently, this changes the background state of the Arctic to the point where North Atlantic blocking becomes more feasible, and thus the entire Arctic is able to see blocking. This has big implications as we head into late November and December. 

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