The Return of the -NAO: Significant High Latitude Blocking Pattern to begin 2021

Good Thursday evening to everyone, it’s New Year’s Eve and in the weather center we are paying close attention to an evolving synoptic pattern that will influence the weather across the United States in the first days and weeks of 2021.

You may remember back in November when we released the Winter Outlook, the anticipation was that a blocking pattern could develop at some point in the winter across the Northern Latitudes. The teleconnection known as the -NAO or “Greenland Block” occurs when the height pattern rises across Greenland, parts of Canada and the Northern Atlantic. The image below is a depiction of what the jet stream pattern looks like during a negative phase of the NAO (North Atlantic Oscillation). With the blocking comes a slow down in the overall pattern.

All global model guidance and teleconnections indicate that the -NAO will begin to evolve over the next seven days and could become quite strong as we get toward the January 10th-15th timeframe.

The global model ensembles are in strong agreement with this as well. Here in the short term, the US will remain in a zonal flow which for here in the Southeast means we get a wild swing in airmasses between warm, humid and cold, dry.

The images below is a look at the height pattern and 5000 ft temperatures for days 5-10 in the current forecast cycle. The red colors on the first image indicates where heights are rising and you can plainly see that is occurring between Greenland and Hudson Bay. However if you look across the Pacific, zonal flow will continue as the heights remain low off the West Coast of North America. So, while the process of blocking begins, its actually locking in the milder air which is flooding in from the west.

However by the end of that Day 5-10 range some significant changes are happening. First off the axis of blocking has moved west closer to Hudson Bay and Northern North America instead of Greenland. The significance of that is two-fold…

  1. The block centering near Hudson Bay will shift the jet stream further south meaning more storms riding through the southern tier and not as many storms cutting up through the Great Lakes.


2. The height changes will shift the winds in high latitudes from westerly to more WNW and that will begin to shut off some of the Pacific Maritime influence. (i.e. colder air source)

So, if you look at the evolution of things potentially, blocking will begin starting next week and then by the following week the block will retrograde and help force the storm track and cold air further south into the US.

We have seen the -NAO show up in recent years but it would be in the months of March and April. In the spring time that means rainy and cold but not particularly wintry in the southern states. But when it happens in the heart of winter, history shows that significant winter weather usually pans out for the Eastern United States.

Below is the precipitation anomaly forecast for January 7th – 14th. As you can see the map indicates that storminess could be happening in the Eastern US. This is a pattern that will need to be watched for specific storm threats. Right now this is well outside the current 7-Day forecast so there is a lot of time to watch this evolve.

It’s been several years since we have had a strong -NAO in January so its definitely piqued our interest…stay tuned!

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