[Vision2020] NOAA: La Niña 2011-12 Winter: Pacific Northwest: Colder and Wetter

[Ted Moffett]

U.S. dealt another La Niña winter but ‘wild card’ could trump it
Devastating drought in Southern Plains likely to continue
October 20, 2011

http://www.noaanews.noaa.gov/stories2011/20111020_winteroutlook.html

The Southern Plains should prepare for continued drier and warmer than
average weather, while the Pacific Northwest is likely to be colder
and wetter than average from December through February, according to
the annual Winter Outlook released today by NOAA.

For the second winter in a row, La Niña will influence weather
patterns across the country, but as usual, it’s not the only climate
factor at play. The ‘wild card’ is the lesser-known and less
predictable Arctic Oscillation that could produce dramatic short-term
swings in temperatures this winter.

NOAA expects La Niña, which returned in August, to gradually
strengthen and continue through the upcoming winter. It is associated
with cooler than normal water temperatures in the tropical Pacific
Ocean and influences weather throughout the world.

 “The evolving La Niña will shape this winter,” said Mike Halpert,
deputy director of NOAA’s Climate Prediction Center. “There is a wild
card, though. The erratic Arctic Oscillation can generate strong
shifts in the climate patterns that could overwhelm or amplify La
Niña’s typical impacts.”

The Arctic Oscillation is always present and fluctuates between
positive and negative phases. The negative phase of the Arctic
Oscillation pushes cold air into the U.S. from Canada. The Arctic
Oscillation went strongly negative at times the last two winters,
causing outbreaks of cold and snowy conditions in the U.S. such as the
“Snowmaggedon” storm of 2009.  Strong Arctic Oscillation episodes
typically last a few weeks and are difficult to predict more than one
to two weeks in advance.

With La Niña in place Texas, Oklahoma, New Mexico and parts of
surrounding states are unlikely to get enough rain to alleviate the
ongoing drought. Texas, the epicenter of the drought, experienced its
driest 12-month period on record from October 2010 through September
2011.

Stormy periods can occur anytime during the winter season. To improve
the ability to predict and track winter storms, NOAA implemented a
more accurate weather forecast model on Oct.18.  Data gathered from
the model will support local weather forecast office efforts to
prepare for and protect the public from weather events. This service
is helping the country to become a Weather-Ready Nation at a time when
extreme weather is on the rise.

According to the U.S. Winter Outlook (December through February) odds
tilt in favor of:

Pacific Northwest:  colder and wetter than average. La Niña often
results in below-average temperatures and increased mountain snow in
the Pacific Northwest and western Montana during the winter months.
This may set the stage for spring flooding in the Missouri River
Basin;

California: colder than average and wetter than average conditions in
northern California and drier than average conditions in southern
California.  All of the southern part of the nation are at risk of
having above normal wildfire conditions starting this winter and
lasting into the spring;
Northern Plains: colder and wetter than average.  Spring flooding
could be a concern in parts of this region;
Southern Plains and Gulf Coast States: warmer and drier than average.
This will likely exacerbate drought conditions in these regions;
Florida and south Atlantic Coast: drier than average, with an equal
chance for above-, near-, or below-normal temperatures. Above normal
wildfire conditions;
Ohio and Tennessee Valleys: wetter than average with equal chances for
above-, near-, or below-average temperatures. Potential for increased
storminess and flooding;
Northeast and Mid-Atlantic: equal chances for above-, near-, or
below-normal temperatures and precipitation. Winter weather for these
regions is often driven not by La Niña but by the Arctic Oscillation.
If enough cold air and moisture are in place, areas north of the Ohio
Valley and into the Northeast could see above-average snow;
Great Lakes: colder and wetter than average;
Hawaii: above-average temperatures in the western islands with above
normal precipitation during the winter.  Some drought recovery is
expected across the state with Kauai and Oahu having the best
potential for full recovery.
Alaska: colder than average over the southern half of the state and
the panhandle with below average precipitation in the interior eastern
part of the state.
This seasonal outlook does not project where and when snowstorms may
hit or provide total seasonal snowfall accumulations. Snow forecasts
are dependent upon winter storms, which are generally not predictable
more than a week in advance.

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Vision2020 Post: Ted Moffett