Question about NAO

[Anonymous]

I thought that a negative NAO meant snow and cold for the east. And when the PNA went positive east had an even better chance. Whats happening? Why doesnt the east have any snowstorms in the forecast if the NAO is going negative and the PNA is going positive?

[donsutherland1]

Jrodd321,

If you build it [NAO-/PNA+ pattern], they [snowstorms] may come. The following are historic averages for NAO-/PNA+ and NAO+/PNA- winters (1950-51 through 2003-04) for select cities:

NAO-/PNA+
Boston: 49.0"
New York: 30.3"
Philadelphia: 26.6"
Washington, DC: 21.3"

NAO+/PNA-
Boston: 35.4"
New York: 18.4"
Philadelphia: 15.6"
Washington, DC: 12.8"

1950-51 through 2003-04 Averages:
Boston: 42.6"
New York: 24.4"
Philadelphia: 21.5"
Washington, DC: 16.6"

Of course, there are other variables, too. For example, the overpowering El Niño of 1997-98 overwhelmed the NAO-/PNA+ pattern.

Currently, there is growing ensemble support for a negative NAO/positive PNA. However, such a setup has not yet evolved but may be starting to evolve. With a pattern change likely, the January 15-22 period could see some accumulating snow even in such snow-starved places as Philadelphia and Washington, DC. A significant snowfall will need to wait until later but as the pattern matures or moves toward change, the threat of a significant snowstorm could be growing (best timeline: late January through mid-February). Of course, such a storm is never guaranteed. Some such setups have gone quietly and uneventfully.