A cool and snowy winter for the U.S.; severe weather hits Louisiana

The winter of 2010 - 2011 is in the history books, and ranks as the 39th coldest winter for the U.S. in the 116-year historical record, according to statistics released this week by the National Climatic Data Center. The cooler-than-average weather was primarily due to Arctic air spilling southwards over the eastern 2/3 of the nation due to an unusually weak Arctic Oscillation. This natural pattern in the atmosphere (whose North Atlantic version is called the North Atlantic Oscillation) allows cold air to spill southwards over the Eastern U.S., Western Europe, and East Asia when low pressure over the Arctic weakens, and high pressure over the North Atlantic also weakens. The state most affected by this unusual winter pattern was Florida, which recorded its 10th coldest winter. No other states had a top-ten coldest or warmest winter.


Figure 1. The winter of 2010 - 2011 featured colder than average temperatures over many states in the eastern 2/3 of the country, with Florida suffering its 10th coldest winter in the 116-year record. Image credit: National Climatic Data Center.

Wet in the Upper Midwest, dry in the South
The winter of 2010 - 2011 featured very heavy precipitation over the Upper Midwest, with South Dakota and Montana recording top-ten wettest winters. The South and mid-Atlantic were very dry, with eight states experiencing top-ten driest winters. This pattern is a typical one for a La Niña winter, since the cooler than average waters off the Pacific coast of South America act to deflect the jet stream so that the preferred track for winter storms takes them to the north over the Upper Midwest. However, the Ohio Valley typically gets above average precipitation during a La Niña winter, and that did not happen this year. This is fortunate, since very heavy rains the past two weeks have inundated Ohio, Indiana, Kentucky, and Illinois, leading to moderate flooding on many rivers.


Figure 2. The winter of 2010 - 2011 featured very heavy precipitation over the Upper Midwest, with South Dakota and Montana recording top-ten wettest winters. Image credit: National Climatic Data Center.

A snowy winter for the U.S.
It was a very snowy winter for the contiguous U.S., with December, January, and February having the 7th, 5th, and 9th greatest snow extents in the 45-year record, respectively, according to the Rutgers Snow Lab. This is the 2nd consecutive very snowy winter in the U.S.; during the winter of 2009 - 2010, December, January, and February had the 1st, 7th, and 3rd greatest snow extents on record. However, an unusually early onset of spring over North America in 2010 led to April and May 2010 having the 4th lowest and 1st lowest snow extents on record for the U.S., and the snow extent numbers for North America were near average for the calendar year 2010 (Figure 3.)


Figure 3. Twelve-month running anomalies of monthly snow cover extent over Northern Hemisphere lands (including Greenland) as a whole and Eurasia and North America separately between November 1966 and December 2010. Anomalies are calculated from NOAA snow maps. Mean hemispheric snow extent is 25.0 million sq. km. for the full period of record. Monthly means for the period of record are used for 9 missing months between 1968 and 1971 in order to create a continuous series of running means. Missing months fall between June and October, no winter months are missing. Image credit: Rutgers Snow Lab.

Flooding concerns continue in the Upper Midwest
The heavy winter precipitation that hit the Upper Midwest primarily fell as snow, and recent snow water equivalent charts show that a wide swath of North Dakota, South Dakota, Minnesota, and Wisconsin have the equivalent of 4 - 6 inches of rain locked into their snowpack. If a sudden spring thaw with heavy rain occurs later this month or in early April, record or near-record flooding is likely. The latest long-range forecasts from the G FS model do not show such an event is likely to occur over the next two weeks, though. A continuation of winter-like weather over the region with below-average temperatures and light snow is expected during the coming week, with a several-day period of thawing the week of March 20. Significant melting the massive snowpack will not begin to occur until the week of March 20, at the earliest.

Tornadoes, severe thunderstorms hit the South
A strong cold front pushing across the nation's southern states has brought severe thunderstorms, and tornado warnings have been issued for Louisiana, Mississippi, and Alabama this morning. A tornado touched down in Bush, LA at 5:20am CDT today, injuring one person and destroying one trailer. High winds from a thunderstorm hit Slidell, LA, causing roof damage, downed trees, and power outages in the city. NOAA's Storm Prediction Center has placed the region in its "Slight Risk" region for potential severe weather. You can track the action today on our severe weather page.

Jeff Masters