Arctic Oscillation (AO) Winter 2011-2012 Forecast

In the last couple days, the NOAA released their winter forecast which recognized the La Nina but also called out an index named the AO that may be the 'wild card' for temperature averages this winter. I've heard quite a few people ask what the AO is.

The AO is the 'Arctic Oscillation' index. It is a varying phenomenon that exists year-round but varies the most in the cold season. Here's an image to offer a better explanation:

Left side: Positive index.

Right side: Negative index.

Image courtesy of J. Wallace

If the Arctic Oscillation is positive, the US basically sees more moderate temperatures. However, when in the negative phase, the AO often brings down colder temperatures to the US. For more advanced weather folk, below is a 500 mb heights image showing a typical AO+, AON (AO Neutral) and AO- conditions during the DJF (December, January, February) time frame.

Colors are pressure anomalies. Blue= lower pressure= colder

Orange/red=higher pressure=warmer, sunnier

Notice the swath of blue over the eastern US when the AO index is negative (bottom image), and the opposite when positive (top image).

That said, it is anticipated that we will be seeing an AO that should end up negative over the course of this winter, according to our analogues being used for this winter.

However, the AO greatly varies over a season, and this might be completely wrong. It varies just too much to be completely sure.

North Atlantic Oscillation (NAO) Winter 2011-2012 Forecast

Another factor is the North Atlantic Oscillation (NAO). The NAO is based on pressure differences between the Icelandic Low (a permanent low pressure system stationed over Iceland) and the Azores High (a permanent high pressure stationed over the Azores). When the pressure differences are large, the NAO is considered positive. When the pressure differences are lower, the NAO is considered negative. Below is an image describing it.

The NAO has major variability and is something special to watch in the winter. Below is an image displaying the effects the NAO has on the US in the winter months.

When the NAO is positive, the Eastern US is warmer. However, when the NAO is negative, the East US experiences colder and snowier conditions as the jet stream dips southward, releasing colder air into the region.  Below is a general overview of the different NAO conditions and subsequent effects thanks to Ian Bell's website:

Positive NAO Index	Negative NAO Index
More and stronger winter storms crossing the Atlantic Ocean on a more northerly track	Fewer and weaker winter storms crossing on a more west-east pathway
Warm and wet winters in Europe	Moist air into the Mediterranean and cold air to northern Europe
Cold and dry winters in northern Canada and Greenland	Milder winter temperatures in Greenland
US east coast experiences mild and wet winter conditions	US east coast experiences more cold air outbreaks and hence snowy weather conditions

Did it seem cold last year? If you thought it did, it was. Last year had a negative NAO for much of the winter months. This year, we are getting some indication that the NAO may also be negative this winter, but the specifics remain to be seen.

Final 2011-2012 Winter Forecast: South Central Plains Forecast

BREAKDOWN:

SOUTH-CENTRAL ANALYSIS

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The South Central Plains will be making headlines this winter- and it’s not a good thing. The drought that has dried up lakes, killed crops, damaged the economy and brought Texas to its knees will only continue in a merciless act by Mother Nature. As we enter another La Nina, conditions will be favorable for a dry winter of 2011-2012. It can be expected that, with this continuing drought, wildfires will be in the headlines as well, right beside the drought. The heat will continue, and there’s no telling if the deadly high pressure ridge will return again this summer over Texas. We all hope it will not, but it’s just too far off for us to tell if it will. There is a bit of good news, though. It looks like the cooler weather and precipitation will extend a bit farther south than last winter, which theoretically increases the chances of both for the rain-starved region. But that’s theoretically. We cannot label the specifics and give a conclusion on that just yet.

Regional times are as follows:

Nationwide Winter Forecast Release: 12:00 PM CDT
Southwest Winter Forecast Release: 12:10 PM CDT
Southeast Winter Forecast Release: 12:20 PM CDT
Northeast Winter Forecast Release: 12:30 PM CDT
Midwest Winter Forecast Release: 12:40 PM CDT
Northwest Winter Forecast Release: 12:50 PM CDT
South Central Plains Winter Forecast Release: 1:00 PM CDT

Final 2011-2012 Winter Forecast: Northwest Forecast

The Northwest will have a slightly more exciting winter than last winter. Some details that this image does not show include that the extreme northwest (by the coast in Washington/Oregon) will likely experience cooler than normal temperatures in at least a few time frames this winter. That said, snow is likely for these regions as systems will come ashore to eventually affect the rest of the US. Montana eastward will be the focus for the main US cold region this winter. The West Coast area will be wet as well as systems continuously come onshore to eventually affect the rest of the US.

Regional times are as follows:

Nationwide Winter Forecast Release: 12:00 PM CDT
Southwest Winter Forecast Release: 12:10 PM CDT
Southeast Winter Forecast Release: 12:20 PM CDT
Northeast Winter Forecast Release: 12:30 PM CDT
Midwest Winter Forecast Release: 12:40 PM CDT
Northwest Winter Forecast Release: 12:50 PM CDT
South Central Plains Winter Forecast Release: 1:00 PM CDT

Final 2011-2012 Winter Forecast: Midwest Forecast

BREAKDOWN:

MIDWEST ANALYSIS

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The Midwest is in for it this year. Temperatures will be below average for pretty much the entire region. More frigid air will be stationed in the Upper Midwest region extending westward into the North Plains. This frigid air will be colder than air in, say, the lower Great Lakes or Ohio Valley. Colder air, in a nutshell, makes snow ratios rise. Snow ratios are defined as one inch of water equalling ‘x’ inches of snow. Let’s say a snowstorm hit and 10 inches of snow fell in an area. Theoretically, if you were to take a column of that 10 inches and melt it into water, you would have 1 inch of water. That would be written as 1:10, or 1 inch of water: 10 inches of snow. In cold environments, snow ratios can rise to 1:20, whereas warmer climates usually have snow ratios around 1:7, defined as wet snow. With this frigid air in place over the North Plains this winter, clippers that usually dump 1-3 inches may put down over 4 inches as the colder air increases snow ratios. These clippers will also come downwards into the Midwest more often this winter. With the Canadian frigid air mass in place, some clippers may be re-directed into the Midwest. Adding to the snowfall will be ‘Panhandle Hooker’ storms- the type of storm that produced the epic February Blizzard last year in Chicago. These storms dip down south near Oklahoma and can fire up north and spread snow across the Midwest. These are usually the big snow makers for the region. The pink area we outlined as a combination of the effect cold air may have on the clippers, as well as the Panhandle Hooker storms.

Regional times are as follows:

Nationwide Winter Forecast Release: 12:00 PM CDT
Southwest Winter Forecast Release: 12:10 PM CDT
Southeast Winter Forecast Release: 12:20 PM CDT
Northeast Winter Forecast Release: 12:30 PM CDT
Midwest Winter Forecast Release: 12:40 PM CDT
Northwest Winter Forecast Release: 12:50 PM CDT
South Central Plains Winter Forecast Release: 1:00 PM CDT