Over the weekend, I discussed how changes in the phase of the MJO will lead to a change in the U.S. weather pattern by mid-December. There is another factor called the PNA, or Pacific/North American Pattern, which will also instigate a pattern change. There are different teleconnections that take place on Earth, and the PNA is one of them. However, the PNA is one of the most influential on North America’s weather. Even though we have an active La Niña (which usually limits the development of the PNA), it will likely still fully form by the end of the first full week of December.

The graph below is the forecasted PNA from the European model. It is currently shown to be at about zero, which means that it is not really impacting weather patterns at the time. That will change, however, as we enter December. You will notice a brief dip in the forecast line for the very start of December, but then a quick and steep rise to higher levels in the next week.

ECMWF Forecast PNA

When the PNA rises above zero, it is recognized as to being in positive territory. Below, you’ll see the typical pattern that is formed over North America when there is a positive PNA. It shows above-average heights, or above-average temperatures, across the western U.S. and western Canada. The saying, “what goes up must come down,” definitely applies to meteorology. When there is a ridge or above-average heights over the West, there must be a trough or below-average heights and temperatures across the East.

500mb Height Anomalies During a Positive PNA
(Source: State Climate Office of North Carolina)

So far, based on the forecast for the MJO and PNA, we are able to conclude that a pattern change is likely to occur, bringing warmer temperatures to the West and cooler temperatures for the East. The million dollar question is whether there will be any snowstorms combined with that cold air for the eastern U.S. The latest run of the European model weeklies shows relatively quiet weather for the month of December, but that forecast may certainly change at any time. I definitely think there is a slightly above-average risk for snow for the eastern two-thirds of the country after December 7th, when the pattern is forecasted to flip.

EPS Geopotential Height Anomaly Forecast for Dec. 7th

The cold air and the northerly component to the wind will also make the environment favorable for lake-effect snow. That risk is definitely higher than normal, starting next weekend (Dec. 9-10).


Jackson is Head of Content and Social Media at WeatherOptics. He is currently a student at the University of Miami, studying Meteorology and Broadcast Journalism. Dill produces forecast articles for the website and helps to manage the content schedule. He has also led the growth of WeatherOptics’ social media accounts, working to keep them aligned with the company’s evolving vision.

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