As a reminder, this year we are in a moderate La Nina that may briefly reach strong levels. Whereas Klein found snow averaged above normal during weak La Ninas (the very snowy 1995-96 winter coincided with a weak La Nina, for example), it’s usually been considerably below normal during moderate to strong La Ninas like this year. So that’s the first piece of bad news for snow lovers - nicely illustrated in the above graph.
But this second graph (above), which plots the above normal snow years since 1950 based on prevailing weather patterns, is what I found particularly depressing. It’s a little complicated, so let me walk you through it.
As we’ve discussed frequently on this blog, D.C. winter snowfall is strongly related to the phase of the North Atlantic Oscillation (NAO) (or Arctic Oscillation). When the NAO is negative, we tend to get more snow and when it’s positive, we tend to get less snow.
So far this year, the NAO has been highly positive. On the graph above, note that the area below the x-axis represents negative NAO territory, and that the overwhelming majority of points representing above normal snow years are plotted in that area. There have been a few years where the NAO averaged positive (above the x-axis) and we had (somewhat) above normal snow, but none among the top 10 snowiest years.
The picture becomes grimmer when you also consider the El Nino Southern Oscillation (ENSO) phase (i.e. El Nino or La Nina) on top of the NAO phase. When the NAO has been positive and we’ve had a La Nina (see the upper left quadrant of the graph), there have only been two years with above average snow in the last 60. And those two years are at the very bottom of the list of snowiest years (to the right of the plot, #21 and #22) and just slightly above average.
Most of our snowiest winters have occured during one of the two combinations: 1) a weak La Nina and negative NAO (bottom left quadrant on graph), and 2) a moderate to strong El Nino and negative NAO (bottom right quadrant on graph)
The bottom line here is that, given the moderate (to strong) La Nina pattern, if the NAO does not flip to negative in January and February, our odds of having above normal snow are remotely low - somewhere in the neighborhood of 5%. Ouch.