amnh_nao.jpgBlow up a long balloon half-way. If you leave the balloon alone the air pressure is the same everywhere within. Squeeze one end of the balloon and the air pressure will decrease on the squeezed end and increase on the unsqueezed end. A process analogous to squeezing and releasing a balloon occurs in the atmosphere over the North Atlantic and our weather is greatly influenced by whether the atmosphere is being squeezed or not squeezed (the atmosphere isn't literally being squeezed but Gothamist likes to think of it in that way).

When the atmosphere is squeezed over Iceland, a giant low pressure system forms over that island while the excess air forms a giant high pressure system over the Azores. When the atmosphere isn't squeezed the pressure difference between Iceland and the Azores is much less, with weak low pressure over Iceland and weak high pressure over the Azores. We call the change between these two atmospheric states the North Atlantic Oscillation, or NAO. When the NAO is in its "squeezed", or positive, state we typically get mild, wet winter conditions. When the NAO is negative the eastern U.S. gets more cold air outbreaks and more snow.

Guess what happened early in February? The NAO went extremely negative on us. As long as the NAO stays negative we'll stay cold. Unfortunately, just like Gothamist can't go to a bar for just one beer, the atmosphere doesn't stay in this pattern for just one storm. The forecast shows the NAO staying negative for the next two weeks (yes, we know the forecast just looks like a bunch of squiggly lines, but all those lines are negative). Unfortunately, forecasting changes in the NAO is extremely difficult. At best we can say that until the NAO switches we can expect colder than normal temperatures.

Image from the American Museum of Natural History