Three important climate indices can be deduced from the 500mb height anomalies (top left): 1) The AO is clearly negative, which is apparent from the well above average heights that exist over the polar region. 2) The PNA exhibits a positive signal, which can be seen in the trough-ridge-trough pattern that extends from the Pacific across the U.S. into the Atlantic. 3) The NAO is in a negative phase, evident by the above normal heights over Iceland, and the below normal heights in the Azores region. The interaction of these three indices allowed cold, Arctic air to push into North Carolina, as seen in the bottom left figure, setting the stage for a winter storm.
By examining the sea surface temperature anomalies (top right), the ENSO and PDO phases can be determined. Forecasters realized that the 2009-2010 winter season would likely feature a moderate El NiĆ±o, suggesting increased precipitation would be likely across the southeastern United States. Meanwhile, a weakly cold PDO (evident by below average sea surface temperatures near the western U.S. coastal region and slightly above average temperatures in the central Pacific) meant that it would have little effect on the El NiĆ±o episode. The result was numerous strong coastal low pressure systems coming out of the Gulf of Mexico, one of which was the powerful storm system on December 18-19th, 2009.