With neither an El Niño nor a La Niña in place, the winter forecast is unusually difficult. In our winter outlook, we touch on some of the other weather patterns that might impact our weather this year. In this article, I’ll discuss two of the most important and how they typically affect our temperatures while also playing a major role in determining how much snow we get. These patterns are the Pacific North American Pattern (PNA) and the Arctic Oscillation (AO). Both can sometimes lead to a southern displacement of the jet stream that forces storms to track to our south – in a favorable position for snow – depending on which phase of the patterns are present.
The Pacific North American Pattern (PNA)
This pattern has two general phases.
The positive phase (on the left below) has the ridge or northward bulge in the jet stream over western North America. That keeps the eastern half of the U.S. in a trough or dip in the jet stream. This dip in the jet usually holds the storm track to our south which pulls cold air to the East while also keeping a storm track near the coast. Snow lovers need to pull for this phase of the PNA pattern this winter.
The negative or reverse phase of the PNA pattern (on right above) has the ridge or northward bulge in the jet stream well off the Pacific coast which helps force a trough in the western U.S. and a ridge of high pressure over the East. This ridge helps nudge the storm track to our north. The flow around storms (low pressure systems) is counterclockwise so any storm passing to our north produces a period of southerly winds out ahead it which ends up warming us up and usually leads to a period of above normal temperatures. The negative PNA pattern generally keeps us warmer than normal (sometimes well above normal) and sometimes drier than normal. It is not a good pattern for snow lovers.
The Arctic Oscillation (AO)
The difference in pressure and winds at higher latitudes (the Arctic) versus mid-latitudes, indicated by the AO, plays a huge role in winter temperature and precipitation distribution across the U.S.
The AO is defined to be in its positive phase when the belt of winds around the Arctic is circulating in a counterclockwise direction and the pressures are lower than normal. Over the mid-latitudes, conversely, pressures are above normal. On the figure below (top left), note how the 500mb heights (or pressures) are below normal (blue) across Canada, Greenland, and Iceland and above normal (orange) to the south of the Great Lakes revealing a strong positive AO. Such a configuration enhances the upper level winds across Canada and the northern U.S. while also bottling up the cold air. When the AO is positive, temperatures east of the Rocky Mountains are typically above normal (see figure top right). If the PNA pattern is negative at the same time (causing a bulge in the jet stream in the East), temperatures can really torch and end up being well above normal. That is a true snow lovers’ nightmare.
When the AO is in its negative phase, pressures and heights are higher across the north and lower to the south (bottom left). Since air moves from areas of higher to lower pressure, that pressure differential helps force cold air southward into out region. Plus, the storm track is usually forced to our south. The National Weather Service’s Climate Prediction Center provides an index of both the Arctic Oscillation and its close cousin the North Atlantic Oscillation (NAO). Snow lovers want both indices to be negative. Personally, I tend to track the AO more closely since it is more predictive of four inch or greater snowfall events in the D.C. area.
Questions you may have about these patterns
Can we get snow when the AO is positive and the PNA is negative?
Yes, but such a combination is not very favorable for getting even a modest snowstorm in the District. Below is a scatter diagram showing all four inch or greater events at Reagan National Airport between 1950 and 2010. Only 11 percent were observed when both the AO and PNA were in this unfavorable combination. 89 percent occurred when the PNA was in the positive phase or the AO was negative.
What is the best combination of AO and PNA for snow lovers?
By far the most common combination for substantial snow events has been a positive PNA and negative AO. That combination is most likely to place the storm track to our south while also providing us with enough cold air for snow.
What about the really big storms? The scatter diagram below illustrates the importance of having either a negative AO or positive PNA for getting an 8 inch or greater snowstorm in the District. All but one of these events occurred with either a positive PNA pattern or a negative AO. The only exception occurred when the AO and PNA were essentially neutral. No major snowstorms have occurred in the District when the PNA was strongly negative at the same time that the AO was positive. The vast majority of snowstorms were associated with a positive PNA index and a negative AO. All our big snowstorms in 2009-2010 had that combination.
As a snow lover I’ll be rooting for the Pacific North American pattern to flip to positive and stay there. However, with the Pacific Decadal Oscillation (PDO) currently locked into a negative phase (see Matt Ross’ winter outlook for additional discussion of the PDO), a negative PNA pattern is probably more likely to be prevalent. I’ll also be rooting for a negative Arctic Oscillation: lots of high latitude blocking indicated by above normal surface pressures and 500 mb heights across Greenland, Baffin Island and into northeastern Canada with below normal heights (pressures) to the south. Most meteorologists feel the AO is only predictable out to a week or two so we can always hope it flips from its current very positive mode.
One caveat is that even if the PNA index averages positive and AO index averages negative for the winter, there are no guarantees we will get a ton of snow. The winter of 1952-1953 had that “golden” combination but only 8.3 inches of snow was observed for the season.
What does the lack of an El Niño or La Niña mean for snow prospects?
In short, given “neutral” conditions in the tropical Pacific – neither an El Niño or a La Niña – it is very important for the AO to be predominantly negative for the District to receive significant snow.
The scatter diagram below indicates whether each of our heavy snow years occurred during an El Niño, La Niña, or neither (neutral conditions).
The good news for snow lovers is that five of the 17 “big” (at least 20 inches) snow years were associated with neutral winters. However, during all 5 of those winters the AO was predominately negative. In fact, all the big snow years but one fall below the red line on the figure indicating that they occurred then the AO was predominately negative. The lone exception was 1982-1983. That year, even though the AO balanced out positive for the winter overall, almost all the snow fell in a February (most of it in a single snowstorm, one of D.C.’s heaviest) when the AO index was extremely negative. Bottom line: unless the AO averages negative this winter, our chances of having a big snow season are very small.