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.
NOAA 2010-11 Winter Outlook - Temperature
NOAA 2010-11 Winter Outlook - Precipitation
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
episode expected to peak sometime in the late fall or early winter and last
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
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.
Updated by JRK 11/9/2010