CPC Long Range Forecast

I find the recently updated monthly and seasonal forecasts from the CPC to be intriguing for the Deep South. The graphic below shows that the CPC expects temperatures to be above normal across most of Mississippi and the eastern half of the country from December 2009-March 2001. The accompanying discussion is very good and well worth a read, but there are a couple of factors that I did not see included in the discussion.


The first is the potential impact of the Pacific Decadal Oscillation. Clearly, the PDO has been in a cold phase for the last couple of years, coinciding with the persistent La Nina conditions. However, since the first of May, SST anomalies have become positive across much of the North and Northeast Pacific Ocean; a cold pocket remains off the Alaskan Peninsula. It's interesting to note that the upswing in the SST anomalies across the North Pacific has coincided with the developing El Nino in the equatorial Eastern Pacific. If this pattern of positive SST anomalies continues or strengthens, the PDO may likely transition back to the warm phase. How soon this occurs remains to be seen, but a warm PDO would make it very difficult to have above normal temperatures across the Deep South and the eastern third of the United States.

Although the developing El Nino was the basis for their forecast, El Nino conditions typically cause cooler and wetter than normal conditions across the South. The key to this will be the placement of the subtropical jet stream. It tends to fluctuate between the Deep South and Florida during El Nino years, and a more suppressed STJ would keep above normal precipitation to our south. Regardless of the STJ placement, a warm PDO would make it difficult to have above normal temperatures for a four month period.

The second factor that was not considered, not surprisingly, is the Global Wind Oscillation (GWO). Given that we are headed for a weak to moderate El Nino, westerly winds will increase atmospheric angular momentum and should lead to the GWO persisting between phases 5-7 for much of the winter. If this indeed occurs, GWO composites for these phases correspond to cooler than normal temperatures across the South and Eastern U.S. As a caveat, more than wind direction (frictional torque) is factored into the GWO, but El Nino conditions generally cause the global circulation to vary from phases 4-7. The intent of this post isn't to disagree with the CPC's forecast, but I think the two factors mentioned above should be followed closely as we head into the fall. It should be interesting!