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Winter Outlook Part 5 - AO/NAO

This is my last update. I will issue my winter outlook before December 1st.

The Arctic Oscillation (AO) is an index which is used to monitor the polar vortex over the Arctic Circle. Here is a link to a table of the AO index since 1950:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/monthly.ao.index.b50.current.ascii.table

(POSITIVE AO LEFT AND NEGATIVE AO RIGHT)
The Arctic Oscillation can have a major impact on temperatures over the eastern half of the United States. All else being equal, a positive AO usually correlates with warm temperatures and a negative AO typically equates to cold.

(ARCTIC OSCILLATION AND TEMPERATURES)

Though the correlation with precip isn't as strong, a positive AO can bring wet conditions to the Tennessee River Valley and the Pacific Northwest, while negative will bring drier than average conditions to these same areas. 

(ARCTIC OSCILLATION AND PRECIPITATION)

There has been interesting research about early-season Siberian snow cover affecting the polar vortex and bringing cold and snowy conditions to the East Coast (a negative AO scenario). 

(POSSIBLE CONNECTION BETWEEN SIBERIAN SNOW AND EAST COAST WEATHER)

In fact, the historic winter of 2009-2010 showed strong snow anomalies in the Arctic during October (indicated by the blue and purple on the map below). AO quickly turned negative in December, January and February. This combined forces with El Nino, which provided moisture from the south, and the East Coast experienced record-breaking snowfall.

(SNOWFALL ANOMALIES OCTOBER 2009)

This October featured some areas within the Arctic Circle with above-normal snowfall. However, it doesn't come close to the amount of snowfall in October of 2009, especially in Canada.

(SNOWFALL ANOMALIES OCTOBER 2011)
AO has been generally in the positive phase since September, which is one of the reasons why the eastern half of the U.S. has seen a mild autumn.

(AO INDEX SINCE AUGUST)

Latest computer forecasts have been hinting at a drop in AO by early to mid December, which could lead to colder weather for the East Coast and Great Lakes region. 

(ECMWF ENSEMBLE FORECAST FOR AO)

The North Atlantic Oscillation (NAO) is somewhat related to the AO. This index is highly dependent on the  strength and placement of the Icelandic low and Azores high. This website does a good job explaining NAO:

http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html

Also, here is a link to the NAO index since 1950:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/norm.nao.monthly.b5001.current.ascii.table

If you want snow along the East Coast, then you want positive height anomalies in western Greenland and eastern Canada (west-based negative NAO). This will decrease the westerly flow of the jet stream and allow cold air to plunge into the eastern half of the United States.

(WEST-BASED NEGATIVE NAO)

(TEMPERATURES ASSOCIATED WITH WEST-BASED NEGATIVE NAO)

NAO has generally stayed positive over the past three months, which is not good news for snow lovers along the East Coast.


There are some indications of NAO becoming negative in December, though it doesn't look too promising.


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