December was marked, not surprisingly by anomalously high 500-mb heights (synonymous with high pressure aloft) over the northern Rockies and northeastern Pacific Ocean. Conversely, anomalously low 500-mb heights were observed over the southwest United States and northwest Mexico.
These circulation anomalies wreaked havoc on the Northern Hemisphere in December, resulting in the split flow that plagued us and an anomalously strong North Atlantic jet. Both can be seen in the 250-mb mean and anomalous wind analysis below.
Why the strong split flow and drought? That is the $64,000 question, and a good answer requires more than just "La Nina," as we noted over the summer.