For quite some time now, we have been discussing the critical pieces that go into our Winter Forecast. The puzzle starts with current conditions – from the tropical Pacific to the Atlantic and Indian Oceans and everywhere in between. The current ENSO state has a huge impact on the forecast for any season, so understanding how these processes are impacting the global circulations is key.
There are secondary processes, however, that have a huge impact on the sensible weather experienced throughout the United States on any given day, week, or throughout a month. Understanding how these work are the meat and bones behind any seasonal forecast, and can make or break a forecast – even one that was founded on a correct forecast of larger scale processes (ENSO, etc).
But why, exactly, does high latitude blocking have such a huge impact on the weather in the Northeast States? Why is it such a critical piece of any medium or long range forecast? The answer lies in what exactly “high latitude blocking” means, and how the blocking itself disrupts the atmospheric circulations around it.
High Latitude Blocking is a fancy term for ridging in the higher latitudes of the northern hemisphere, essentially from the far Northeast Pacific through the Canadian and Arctic regions and towards Greenland and the North Atlantic. Blocking refers to ridges of high pressure that “Block” the normal atmospheric circulation of wind, and result in totally different circulations and progressions throughout the atmosphere when compared to normal.
You can imagine, then, how a high latitude block on the Atlantic side of Canada plays a critical role in the forecast for the East Coast. If a huge ridge of high pressure forms over Greenland, all of the cold air typically residing there will be dislodged further south into Southeastern Canada and New England. If that high latitude block didn’t exist, the cold air would never make it that far south and the atmosphere would behave more “normally”.
We can look to the current medium range forecast for clues as to how high latitude blocking impacts the forecast. For the past several days, cold air has been dislodged southward from Western Canada into British Columbia by a large high latitude block on the Pacific side of Canada. Temperatures have averaged 10 or more degrees below normal over a large area. But this cold air has not made a movement eastward with any vigor.
Why? Because high latitude blocking is not present in Central or Eastern Canada. So cold air modifies and slides north/northeastward into Canada, away from the United States and back into the Arctic regions. What is happening with the pattern over the next few days is critical, because forecast models suggest high latitude blocking will develop and strengthen. This will allow cold air to move eastward with more vigor, reaching the Great Lakes and New England.
Medium and long range forecast models and ensembles also suggest that a large ridge of high pressure will develop in Greenland and towards the Davis Strait in Central Canada, further exacerbating the situation and dislodging cold air into the Eastern 1/3 of the United States. We illustrate the key flow of air around these disturbances in our corresponding graphics. Furthermore – our top analog years to this year, with similar forcing and sea surface temperatures in the Tropical Pacific Ocean, all featured a period of high latitude blocking in Late November and December.
Finally – and to put the cherry on top of this – high latitude blocking slows down the atmosphere (think of the word “blocking” in that sense). This allows a much greater opportunity for stronger storm systems to form as troughs can become more amplified and phasing is much easier when atmospheric disturbances are moving slower and are blocked off in certain directions. It would not be surprising, therefore, to see a large storm system in the Central and Eastern United States in later November, possibly a wintry storm system for the Northern 1/3 of the USA including New England.
Stay tuned over the next few days as we continue to refine the forecast. Hopefully this brief explainer on high latitude blocking helps some of the terminology and forecast details make more sense moving forward. Have a great Wednesday!