Thursday, December 08, 2011

Checking in on the AO and the NAO

A lot of times in winter, we'll hear much said about the NAO, PNA, and the AO...three of the more useful indices to help figure out general weather patterns in the medium range of forecasting.

Specifically today, I want to touch on the Arctic Oscillation, or the AO and the North Atlantic Oscillation, the NAO. Most of you understand the NAO (Cliff's Notes: Ridging in the upper atmosphere in the vicinity of Greenland generally creates an atmospheric block, allowing for more cold to deposit out of Canada and into the East and sit for awhile). For a thorough definition of the AO, here's the Wiki article on it. Essentially, we're looking at atmospheric blocking in the Arctic.

You've probably heard much abut this because the last two winters have featured incredibly negative AO indices (as well as negative NAOs), which many have linked to the stronger cold intrusions and snowier winters we've had of late. Stu Ostro of The Weather Channel posted a cool image the other day on Facebook, which showed the last couple winters compared to this one. It's clear that we're in another round of extreme AO behavior, but this time it's to the other side of the coin. If you're looking to blame the extremely mild start of winter on something, the AO is one thing you can point your finger at.

But what about going forward. Well, let's take a look at this morning's AO composite from NCEP at the left. You may want to open this in a new window while you're reading.

The black line in the top chart is the "observed AO," what it's actually verified at. Ignore the sharp dip that occurs today (bad data)! Seeing the AO peak at +4 is, simply put, ugly for winter lovers. This certainly won't guarantee it can't get cold, but couple that with a (raging) +NAO, and any cold that does move through is completely transient. You won't sustain it for more than a couple days.

Of more interest, I want you to look at the bottom three panels, which show the forecasts for each day at a 7-day, 10-day, and 14-day lead time. The AO "spread" is indicated by the upper and lower red lines. The black line, again, is the verified, actual AO value (again, ignore the dip centered on today). For the better part of two months, we've seen the AO consistently come in higher than forecast at almost every time scale. So, fluky, right?

Let's take a look at the same chart, but for the NAO at the right. Again, it's been primarily neutral to positive (especially of late). But more importantly, notice the forecast verification, especially at a 14-day lead time. This is a cold biased model right now...a very cold biased model.

So in the coming days and weeks ahead, while we're looking for signs of real winter weather, you must keep this in mind when using these tools. The odds right now favor that any forecast of the NAO by NCEP is going to be too low/negative, and you'd be wise to assume it will come in more positive. At some point this will change, but as of this moment, these facts have to be considered when attempting to make any sort of forecast beyond day 5 or 6.

Again, it doesn't mean that it cannot or will not get cold (see the snow in the interior today and the upcoming weekend for an example). But sustaining it like we have the last couple winters is going to be a daunting task, at least for the foreseeable future.