Severe weather already made its presence known a few days ago as violent thunderstorms and tornadoes ripped through the Midwest.

Today won’t be much different. The Storm Prediction Center (SPC) has already issued a “High Risk” for severe weather in the Ohio and Tenessee Valleys. These storms have the potential of producing winds in excess of 75mph, large hail, and tornadoes.

SPC Convective Outlook

Now, lets take a look at the synoptic set-up that will lead to this severe weather outbreak.

There are 3 major “ingredients” or parameters, that are necessary for severe weather to occur. The first is lift. Lift is provided by a low pressure system. Taking a look at this surface map, we will notice a strong low pressure system along with both a warm and a cold front.

The Arctic Oscillation

Atmospheric teleconnections have been something of a mystery to meteorologists for almost a century. One of the most well-known teleconnections is the Arctic Oscillation (AO).  Usually we measure the AO using geopotential height anomalies, typically at 500 or 700 mb. Figure 1 shows the two phase of the AO. During the positive phase there are negative height anomalies over the Arctic and positive height anomalies surrounding it. Since heights are in general lower at the poles than the midlatitudes, the positive phase of the AO represents an enhanced meridional height gradient, and the negative phase of the AO represents a weakened meridional height gradient.

An enhanced meridional height gradient is equivalent to an enhanced meridional temperature gradient. Using thermal wind balance arguments, one can show that this indicates an enhanced upper-level jet. Indeed, when the AO is positive, the polar jet is anomalously strong. Because of this, many people (myself included) like to think of the AO as a measure of the strength of the polar jet.

The North Atlantic Oscillation

The AO was identified in the 1950s by Ed Lorenz. About 30 years prior, Sir Gilbert Walker discovered an oscillation in sea level pressure between Lisbon, Portugal and Reykjavik, Iceland, and this has been termed the North Atlantic Oscillation (NAO). For our intents and purposes, the NAO is nothing more than a regional manifestation of the AO, and we will consider them to be the same thing. Be warned, however, that this topic is of some debate in the scientific community.

What’s the AO have to do with winter?

The AO can be statistically associated with winter weather throughout the Northern Hemisphere. The second figure shows a correlation map of the AO with winter (DJF) temperatures. During the positive phase of the AO, the eastern U.S. and Europe tend to be anomalously warm. Alaska, in particular, tends to be anomalously cold. These associations are reversed during the negative phase.

During its positive phase, the AO is associated with a strong polar jet and polar vortex. This allows the cold air to be “bottled up” in the Arctic and keeps most areas just south of the Arctic circle warmer than usual. When the polar jet breaks down, the AO turns negative and the cold air “spills” out into the world.

To the left is an image of the AO index for the past few months. Note that it was positive for most of the winter. During the last half of January to the middle of February the AO turned negative. That was also around the same time that Europe got really, really cold. Remember that the negative phase of the AO

is associated with cold in Europe. Now it’s positive again.

Why do we care?

I do not consider the AO/NAO to have its own internal dynamics, but this, again, is subject of much debate. I prefer to think of the AO as the result of weather events – lots of different synoptic patterns, such as cyclonic and anticyclonic wave breaking, can affect the phase of the AO.

The most important part of the AO, though, is its predictability. It is much easier to predict the phase of the AO than it is to predict individual weather events over a significant amount of time. This is mainly because the AO covers a large geographic region over a long range of time, similar to how climate predictions work. Short term variations (noise) just aren’t as important and don’t throw the forecasts off as much – they tend to average out.

The last is a prediction of the AO for the next 14 days from the Climate Prediction Center. Note the end where the forecast is – most of the ensemble members (identified by the yellow shading – ensemble spread) are forecasting the AO to stay positive for the next 2 weeks.

This would imply that the Eastern U.S. will stay warm for the foreseeable future. We’ll keep our fingers crossed!

Figure 1

Anchorage is Alaska’s largest city and is home to nearly 40% of the entire states population. Located in South-central Alaska, Anchorage is a low lying coastal city with the Chugach Mountains located to the East. To the north, The Alaska Range blocks the city from interior cold air, keeping Anchorage’s temperatures 20 to 30 degrees warmer than the rest of the state.age is Alas

Since December 1, Anchorage received 71.9” of snow. With a snow pack of 31” and temperatures just averaging just above zero the snow isn’t expected to melt until late spring or early summer. The average January temperature this year was just 2.5 degrees.

So why so much snow?

Well there is a combination of factors that have led to this record snowfall. First we have an Aleutian Low. The Aleutian Low is a semi-permanent low pressure system located near the Aleutian Islands in the winter. As you can see in figure 2, the dark purple shades represent the low mean sea level pressure associated with the intense Aleutian Low.

Second, we have the jetstream (Fig. 3). In the left-exit region of the jet, located just south of Alaska, there is a maximum area of upper level divergence. Upper level divergence leads to intensification of the surface low.

So, we already established that the Aleutian Low occurs each winter, and that there is a jet stream located south and west of Alaska. But what do these two factors have to do with the record snow fall?

Well, a composite anomaly map shows that this year, the Aleutian Low been more than 12mb stronger than normal. So the combination of a strong area of upper level divergence and a low pressure system, leads to a period of persistent snowfall, which is what Anchorage has experienced this season.

January 25-27 2011, Courtesy of NASA

The biggest headline this winter so far has been the lack of snow. Mild temperatures have been sweeping across the nation and with Spring only a few weeks away, Mother Nature has many people wondering if we are going to see any more snow.

Lets take a look back…

January 2011 was an unforgettable month for people living in New York City. Central Park was hit with 36.0” of snow, almost 5 times the normal amount of 7.5”.  Most of the monthly snow fell during January 26 and 27^th when a Nor’easter came up the coast dumping 19” of snow. This storm came just a month after the Christmas Blizzard brought 20 inches of snow. January 2011 set a new record for Central Park naming it the snowiest January on record.

This January, Central Park has had an average temperature of 37.3 degrees with only 4 days with high temperatures that fell below freezing. Unfortunately this was not the warmest January on record. In 1932 Central Parks average temperature for the month of January was 43.2 degrees. With mild temperatures like these it’s no wonder Central Park only saw 4.3” of snow.

With February temperatures already above normal, can we rule out the possibility of more snow?