Despite a persistent strong La Nina, much of the northern Plains, Midwest, Great Lakes and Northeast have seen a mild winter so far. No major arctic outbreaks have occurred yet. The reason for this mild first half of winter across much of the eastern half of the United States is the presence of a strongly positive North Atlantic Oscillation (NAO). When the NAO is positive during the winter months, the main core of arctic air tends to remain bottled up across Canada.
Also, more of a spilt jet stream pattern has been in place across the country, which is more reminiscent of an El Nino setup. These two factors have contributed heavily to the mild temperatures, as well as to the lack of snowfall across many locations in the northern and eastern United States so far this winter. The latter of these two factors also brought unseasonably cool conditions to the Desert Southwest during December.
So what’s in store for the second half of winter? Current climate models and indices suggest the ongoing La Nina phase will continue through at least the rest of winter. Cooler than average February temperatures are forecast across the Pacific Northwest, northern Rockies and northern Plains. A surplus of heating degree-days by between 30 and 90 is projected throughout these regions.
In contrast, milder conditions are expected across the Ohio Valley, Tennessee Valley, lower Mississippi Valley and Gulf Coast with heating degree-day deficits likely in these parts of the country. The wild card for the rest of the winter, again, is the NAO. If the NAO becomes negative it will open the door for colder temperatures across the eastern third of the country, especially the Midwest, Great Lakes and Northeast.