Tuesday, November 1, 2011

First Cold Outbreak May Come Sooner than Later as NAO may go Negative

Detailed NAO 12z GFS forecast from today (Nov. 1)
Latest NAO Ensembles

The first official cold outbreak of the winter may be coming sooner rather than later as the ensembles that forecast for the NAO appear to be aiming for a negative trend later this month.
First off, many of you are probably wondering what the NAO is.
The NAO, in a nutshell, is the pressure differences between a semi-permanent low pressure system over Iceland and a semi-permanent high pressure system over the Azores. The higher the difference: the NAO goes positive. The weaker the difference: the NAO goes negative and can persuade cool areas of air to be inserted into the US. The latest 12z GFS shows a massive low pressure system giving way to brutal air from Canada about 312 hours out. Since that is in the long range GFS time frame, it will be changing.
First off, here's what this massive low and following cold look like from the Atlantic frame.
You can see, even from here, how strong the low is and how cold the air around it is. However, notice how there's practically no tight isobars in the center of this picture or to the northwest. Respectively, that's where the Azores high and Icelandic low would usually be present. However, in a negative NAO standpoint, that's how it would look- just like it is above.
Here's a look at the same system from an 850 millibar perspective. In this perspective, we see the rush of cold air behind the system. This 850 millibar chart is a temperature chart. Before I go on, these values will NOT VALIDATE AT THE SURFACE. They do provide forecasters with a general idea on if a region will be cold at the surface.
Here's the anomalies at the same 850 millibar atmospheric level. This basically takes the temperatures above and shows if they are above normal or below normal for a region. Obviously, we can see it will be very below normal for the Northern Plains at the 850 mb level.