During 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. The PNA is known to be one of the most influential on North America’s weather. Even though there is an active La Niña where a PNA tends to be uncommon, it will likely develop by the end of the first full week of December.
The graph below is the forecast PNA from the European model. It is currently analyzed at about zero, which means that the PNA is not really impacting weather patterns at the time. That will change, however, as we look further out in time. You will notice a brief dip in the forecast line for the very start of December, but it is then forecast to rise to high levels next week.
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 with there is a positive PNA. It shows above average heights, or in short, 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.
So far, based on the forecast for the MJO and PNA, we are able to conclude that a pattern change is likely to occur with warmer temperatures headed for the West and cooler temperatures for the East. The million dollar question is whether there will be any snowstorms combined with that cold air. The latest run of the European model weeklies shows relatively quiet weather for the month of December, but that forecast may certainly change. I definitely think there is a slightly above average risk for snow for the Eastern two-thirds of the country after December 7th, which is when the pattern is forecast to flip.
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).