El Niño and DC/Baltimore Winters

Jared Klein

The official National Oceanic and Atmospheric Administration (NOAA) Winter Outlook from the Climate Prediction Center (CPC) was released back in mid-October. The outlook favors below average temperatures and equal chances for above or below average precipitation in the greater Baltimore and Washington DC area for the upcoming 2009-2010 winter season (see figures below). El Niño, which is a climate phenomenon characterized by unusually warm ocean temperatures in the tropical Pacific Ocean, is expected to play a key role in influencing this upcoming winter's weather across the United States.

Looking back at past winters since 1950, approximately 17 were influenced by an El Niño episode. The figures below are composites of average December, January, and February (DJF) temperatures and precipitation, as well as seasonal snowfall at Washington, D.C. broken down by the strength of the El Niño episode. Both DJF temperatures and precipitation averaged near normal while seasonal snowfall averaged a few inches above normal. The strength of the warming in the equatorial Pacific Ocean (i.e. El Niño episode) shows a possible correlation to seasonal temperature, precipitation, and snowfall locally at Baltimore and Washington D.C. Weak El Niño winters averaged below normal temperatures and precipitation, while strong El Niño episodes have resulted in above normal temperatures and precipitation. On average, the stronger the El Niño episode, the warmer and wetter the winters have been. These findings can largely be linked to a stronger than normal sub-tropical jet that typically occurs during moderate to strong El Niño winters, which would favor more active storm systems from the south that draw warm, moist air northward as opposed to the drier Alberta clippers from the northwest. Seasonal snowfall averaged above normal for weak and moderate El Niño winters while below normal for strong El Niño episodes. During strong El Niño episodes, the bulk of the cold air remains north of the mid-Atlantic region, often resulting in precipitation falling as rain instead of changing to snow.

Not all El Niño winters are alike as many other shorter and longer term climate patterns influence the local weather. For example, although it might seem that all strong El Niño winters in Washington D.C. and Baltimore have been associated with above normal temperatures and precipitation and below normal snowfall; these atmospheric quantities have been variable each winter. Of the 17 El Niño winters, eight had above normal snow while nine were below normal. The above average El Niño winters have been associated with some of our snowiest winters, especially during moderate El Niño episodes. With the ongoing El Niño episode expected to continue, even strengthen to moderate levels this winter, El Niño will likely play an important role with the winter climate here in the greater Baltimore and Washington D.C. area.