El Nino Southern Oscillation (ENSO) is a main player in U.S. winter weather activity, especially when it’s into the moderate or strong level of either phase (El Nino/warm or La Nina/cool). The warm phase of ENSO often delivers D.C. some of its biggest snow, but it’s a tough line to walk, particularly in strong or super El Ninos like the one we are seeing unfold for winter 2015-16.
If you’re not a weather geek, this post may leave you confused. It’s basically highlighting the various mid-level patterns seen during the strongest El Nino winters. The images below piggyback off historical ENSO refinement work done by the talented Eric Webb of North Carolina State.
Rather than add a lot of analysis, since I’m still learning myself, I’ll mainly just share the images of the predominant 500mb pattern for the winter and by month from December through March. Just keep in mind, one winter in the list also included the extremely unusual Veterans Day snowstorm of 1987.
Meteorological winter 500mb pattern
In the above maps and also in all of the below, a quick layman way of understanding what you’re seeing is to assume warm colors are high pressure areas and cool colors are low pressure areas. Spots where the colors get the most intense fall in the zone in which the high or low is most focused during the period. Lower-level colors stretched out often indicate the general storm track when it comes to the cool colors, or a “ridge bridge” with the warm.
The maps with the deepest colors primarily include fewer comparison years because the scale is standardized. Averaging over multiple years tends to wash out single-season ideas and intensities a bit. Given that there are not a ton of strong El Ninos in the record, even including those pre-1950 thanks to Webb’s work, the “above average” snowfall looks in particular are made up of very small sample sizes. It’s thus hard to make absolute judgments, even though the maps do tend to nicely portray what we might look for in advance or as it plays out across the winter of 2015-16.
One other thing you’ll notice is that other than December’s maps, some sort of a Greenland block (or -NAO) is shown for above average snow periods. A negative phase of the North Atlantic Oscillation is a commonly snowy experience across D.C. and northeastern U.S. snow history.