Last week, the Capital Weather Gang’s winter outlook was released, calling for slightly-below-normal snowfall for the Washington area. Much of the forecast was based on the idea that La Niña conditions would hold sway and limit how much snow we see. But not all La Niñas are created equal, and, especially if weather patterns in the Arctic and North Atlantic set up favorably to deliver cold air, the winter could turn out a lot snowier.
Temperatures across the area during La Niña winters tend to end up warmer than normal because the prevailing storm track is usually to our west and then north. Since the flow around storms is counterclockwise, such a track draws up mild air from the south.
On average, La Niña years are warmer, drier and less snowy than normal. Washington is four times as likely to receive less than 15 inches as it is to exceed 20 inches. It is almost twice as likely to have less than 10 inches.
The most pitiful La Niñas for snow tend to be the strong ones, which average the driest and warmest. But, this year, the La Niña event is on the weak side.
Weaker La Niñas are more variable and offer somewhat more snow potential. Notice, in the above maps, how the colors indicating above-normal temperatures (yellow) and below-normal precipitation (brown) fade during weak La Niña years compared with strong ones.
Especially when La Niña is weak, the phase of the Arctic Oscillation and its close cousin the North Atlantic Oscillation looms large and can overwhelm the La Niña signal. When both oscillations are negative, the winter can end up cold and snowy despite La Niña. This happened as recently as 1995-1996.
The negative phase of these oscillations feature above-normal pressures across the high latitudes and lower pressures across the mid-latitudes. Since air moves from higher to lower pressure, the negative phase of the two oscillations tends to help move colder air southward from Canada into the eastern United States.
Todd Crawford, a meteorologist at the Weather Co., assembled a terrific graphic showing the stark temperature differences during La Niña winter months depending on whether the North Atlantic Oscillation (NAO) is negative (top row in figure below) or positive (bottom row).
The La Niña months with a negative NAO tend colder than normal. When the NAO is positive, temperatures average well above normal.
Washington’s six warmest winters since 1950 were associated with a positive NAO and Arctic Oscillation (AO).
A comparison of the weather patterns for the six warmest La Niña winters since 1950 and three of the coldest illustrates how the AO and/or the NAO have profound influences on the character of the winter.
During the really warm winters, below-normal pressures stretch across Canada and Greenland (see above left). In such patterns, the cold air tends to become locked up in Canada and has a hard time spreading southward. The storm track remains situated well to our west, helping to pump warm air into the region. Once in a great while, even during warm winters, significant snowstorms (December 1973 and January 2000) may develop despite these unfavorable patterns. Still, if you hate snow and prefer milder temperatures, root for a positive AO and NAO this winter.
The weather pattern during cold and snowy winters (see above right) is markedly different than for the warm ones. During three of the four cold winters, we see areas of higher-than-normal pressure over Alaska and Greenland.
When high pressure is over Alaska, it is in a favorable position to help steer cold air southward. Similarly, high pressure over Greenland helps shove the storm track and jet stream southward. Snow lovers, root for these “blocking” areas of high pressure over Alaska and/or Greenland. If such patterns prevail, it would probably lead to more snow than the Capital Weather Gang forecast.
If someone put a gun to my head and said you have to pick a number for this year’s seasonal snowfall within three inches, I’d pick the median, which is 12 inches at Reagan National Airport. I’d pick this amount not because I think it will occur but because, in half the years during weak La Niñas, less snow than that fell and, in the other half, more.
This winter, I’ll be rooting for blocking areas of high pressure over Alaska and Greenland. A negative AO and NAO sure is helpful for getting snow during a La Niña year, and I like snow.