2010-11 U.S. Winter Outlook and

La Nina Winters in the Washington D.C.-Baltimore Area

2010-11 U.S. Winter Outlook

The National Oceanic and Atmospheric Administration/Climate Prediction Center (NOAA/CPC) 2010-11 winter outlook shows the mid-Atlantic region placed in a transition zone between a weak signal for above normal temperatures (centered over southern and central parts of the country) and equal chances for above, near or below normal temperatures. Meanwhile, the winter precipitation outlook shows equal chances for being above, near or below normal in the region. Although some may think a forecast of equal chances is not very useful, this still provides important information. A prediction of equal chances implies that the probability of most likely category (above, neutral or below) could not be determined because the predictive tools used to construct this forecast was not able to show a strong enough seasonal climate signal to shift the statistical probabilities one way or another.

La Nina and Washington D.C.-Baltimore Winters

La Niña, which is a climate phenomenon characterized by unusually cool ocean temperatures in the tropical Pacific Ocean, influences the winter climate pattern across the United States. With the current borderline moderate to strong La Niña episode expected to peak sometime in the late fall or early winter and last through the rest of the winter, La Niña is expected to play a key role in the 2010-11 winter’s climate. Looking back at past winters since 1950, approximately 20 winters were influenced by a La Niña 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. and Baltimore. During La Niña years (independent of strength of episode), DJF temperatures averaged near to slightly above normal while DJF precipitation and seasonal snowfall averaged near to slightly below normal.

These composites are further broken down by the strength (weak, moderate and strong) of the La Niña episode. Based on the small dataset, there appears to be some historical correlation between the strength of the La Niña episode and seasonal temperatures locally at Washington D.C. and Baltimore: the stronger the La Niña, the warmer the temperatures averaged. Winter precipitation averaged slightly drier than normal during all La Niña intensities. Seasonal snowfall during La Niña winters averaged below normal during moderate and strong episodes.

For moderate to strong La Niña episodes (like the one expected in the 2010-11 winter), above normal temperatures and below normal snowfall indicated in the composites could in part be due to a westward shift in storm tracks that is often seen during La Nina winters. More storms tracking to the west of the Appalachians would imply the mid-Atlantic region being located on the warmer side of storms, resulting in more mixed or rain events for the area and less snow events.

Not all La Niña winters are alike. Many other shorter- [e.g., North Atlantic Oscillation (NAO), Pacific-North American Pattern (PNA)] and longer-term climate patterns influence the local weather and climate. For example, while almost all of the La Niña episdoes are linked to near or below normal snowfall at Washington D.C. and Baltimore, the weak La Niña episode during the 1995-96 winter was an outlier in the dataset with well above normal snowfall for the season. In this case, the above normal snowfall was weighted heavily by the 6–8 January 1996 blizzard, when 17.1 (22.5) inches of snow fell at Washington D.C. (Baltimore). Aside from the 1996 blizzard, the only other big snowfall events with double digit snowfalls during a La Niña winter (since 1950) were the 10.2 inches recorded on 16–17 December 1973 at Washington D.C. and the 14.9 inches recorded on 25 January 2000 at Baltimore.