Average Snowfall for the Washington DC Metropolitan Area:
- The two snowiest winters in the past 15 years at Reagan National (DCA) occurred in 1996 (46″) and 2003 (40″).
- The snow-less winters that have occurred in recent years can not simply be blamed on increasing carbon dioxide in the atmosphere.
There are climatic patters (El Nino, La Nina, the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), the Pacific Decadal Oscillation (PDO) and other synoptic patterns that affect storm tracks and the behavior of the Jet Stream). If climate change were the only reason for some of DC’s snow-less winters, accumulations would only decrease each subsequent year since the Industrial Revolution.
- For example, 1998 was the warmest year on record for the globe yet the winter of 2002- 2003 was the coldest winter in 25 years. 2002-03 was the second snowiest winter at Reagan National Airport & Dulles International Airport & the 3rd snowiest winter at Baltimore-Washington International Airport (NOAA).
Factor #1 – El Nino and its effect on the southern branch of the jet stream:
The image below shows sea-surface temperature anomalies in September of 2009
This winter, the National Oceanic and Atmospheric Association believes that the current El Nino pattern in the Equatorial Pacific will strengthen and last through the Winter 2009 – 2010. El Nino means “The Child” and occurs on average around Christmas. During an El Nino pattern the easterly trade winds over the equatorial Pacific ocean weaken causing the surface ocean water to warm. The warm waters heat the air above activating the southern branch of the jet stream. For good snow storms in the Washington Area we need storms traversing the Deep South that then track up the eastern seaboard (placing us on the western side/cooler side of the storm track).
I looked at past moderate to strong El Nino winters here in the Washington Area and found some interesting facts: Out of nine El Nino winters, four were snow-less and five were very snowy. So based on probability alone there is a 56% chance of a snowy winter (44% chance of a snow-less winter). El Nino years and snowfall accumulations at DCA: 1957-58 (40″); 1965-66 (28″); 1972-73 (trace); 1982-83 (27″); 1986-87 (25″); 1991-92 (6″); 1994-95 (10″); 1997-98 (trace); 2002-03 (40″). The variations in snowfall accumulations during past El Nino oscillations are a result of other synoptic patterns around the globe such as the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO).
Factor #2: A suppressed southeast ridge of high pressure
Winter 2008-09 was a huge disappointment for snow lovers. Last year the United States was under the influence of La Nina or abnormally cool waters in the equatorial pacific. The southern branch of the jet stream was inactive and a strong northern branch diving out of central Canada steered most of the storms to our west. The storm track to our west brought more rain storms to the Mid-Atlantic versus the classic Nor’easter track (to our east) that brings the Washington Region big snow storms. Reagan National Airport received only 7 inches of snow the last weekend in February of 2009. Another factor in the more westerly storm track was a strong ridge of high pressure anchored off the southeast coast. The area of high pressure pulled mild air out of the Gulf of Mexico and kept the storm track over the spine of the Appalachians.
This ridge of high pressure eventually broke down during the spring of 2009 resulting in an extremely chilly summer for Chicago, Washington DC, Philadelphia, New York and Boston.
This winter, this ridge of high pressure should remain suppressed well south and east of the coast enabling southern tracking storms (with the active southern branch of the jet stream) to pass to our southeast increasing the chances for several Nor’easters.
Factor #3 – Pacific Decadal Oscillation (PDO)
The summer of 2009 featured a stubborn ridge of high pressure anchored just off of the Pacific Northwest coast.
- California has remained in a severe drought over the past several years. Cooler ocean waters cause the air above to sink, scouring out clouds and producing drought conditions.
Climatologists have noticed that the northeast Pacific basin has gone through patterns of cooling and warming. These patterns referred to as the Pacific Decadal Oscillation (PDO) have a large impact on the placement of the jet stream over the Pacific.
- The recent negative phase (cooling phase) of the PDO may help to explain the drought conditions over California as well as the cooler summer in the Mid-West and Northeast. The stubborn ridge over the west during the summer of 2009 led to more troughs (dips in the jet stream) over the Mid-West and Northeast. The ridge out west and the trough in the east as well as the suppressed southeast ridge over the Atlantic led to abnormally cool temperatures and the “Year without Summer in the Northeast”.
Negative PDO and Winter 2009 – 2010
- The negative PDO should continue into the upcoming winter. The positive phase of the Pacific North American (PNA) ridge will keep much of the interior west mild and dry. A ridge out west typically favors troughs (dips in the jet stream) over the eastern United States. More troughs in the east increase the probability of coastal tracking mid-latitude cyclones (Nor’easters).
History can help to predict future weather events:
- Looking back 15 years there have been six average or below average winters between 1996 and 2003. Since 2003 we have had six more average or below average winters. Are we over due for a snowy winter? The statistical game says “yes”.
Some folks look at folklore:
- Woolie Bears (Furry Caterpillars) – This year the light brown stripes are thin. Folklore has it that the thinner the stripe the more severe the winter.
The North Atlantic Oscillation (NAO):
- The NAO index can be positive, neutral or negative. When the NAO is negative there is a ridge of high pressure sitting over Greenland. This ‘blocking’ forces the jet stream to buckle (to drop south) over the eastern United States increasing the probability for coastal storms to develop and slowly move up the eastern seaboard. Snow lovers want a negative NAO index (During the Blizzard of 1996 the NAO index was negative 2). Last winter (2008-2009) the NAO was in a positive or neutral phase for much of the winter.
My prediction for snowfall in the National Capital Region for the Winter of 2009-2010