The Recent La Niña
Winter of 2010/2011

 COLD . . . .  The United States, east of the Rockies, endured a remarkably cold winter (Dec. 2010–Feb. 2011), with most states and counties reporting colder than normal conditions (blue tones on map A, at right. Only two states—Nevada and Maine—reported above-average temperatures. Georgia (at –3.9°F below normal) and Florida reported the greatest departures from their average winter temperatures. The averages are for the 1970–2000 period. In the western states, the month of February was unusually cold, and several cities in California and Montana reported their coldest February temperatures in over 30 years. Early in the month, extreme cold descended upon the US-Mexico border; El Paso, Texas reported that 15,000 pipes froze and broke, causing $50 million in damages.

    The atmospheric circulation was active over the lower 48 states of the US, with numerous waves in the jet stream, and strong low-pressure centers causing intense snowstorms and outbreaks of cold Arctic air behind them. The jet stream moved west-to-east over the midsection of the country or somewhat farther north. Snow cover attained its greatest extent, 71 percent of the nation, by January 12, and on February 10, all states except Florida reported some snow. For North America, the extent of snow cover was the 8th largest for the month of February in the 45-year record.

&  DRY . . . . . The active storm track over the middle of the United States divided the country into wet and dry halves. The precipitation pattern evident on map B at center right is how La Niña typically manifests itself in the winter in the United States—wet in the north, quite dry along the Gulf Coast. To the south of the storm track, where yellow and orange predominate, the winter was the third driest on record for the South climatic region, and the ninth driest for the Southeast region. Dry conditions were very pronounced in Louisiana, Mississippi, Alabama, and North Carolina.

    The winter was wet and snowy for the North Central region (green on the same map), underneath and north of the active storm track. For December-to-February, South Dakota experienced its fourth wettest period on record; Montana, its ninth.

INFLUENCE of LA NIÑA & OTHER PATTERNS

     La Niña has a strong influence on the lower 48 U.S. states during the winter months. it tends to bring dry conditions to the southern tier of states from the Southwest to the Southeast, and wet conditions to the Ohio Valley and the northern tier of states. It also is associated with warmer-than-normal temperatures over most of the US, something NOT seen in the past winter. Another constant pattern this winter was a broad ridge ver the far West, and a broad trough East of the Mississippi. This pattern tends to be dry for most of the country, except on the West coast which tends to be wet. It also tends to be cold near the trough, which was true this winter. For the full story we must look at other climate oscillations; in each month a different oscillation had a stronger influence.

     In December, the Arctic Oscillation was strongly negative. That reinforced dry conditions in the southeast and much below-normal temperatures in the US east of the Rockies.

     With the broad ridge in the far Western states, a "pineapple express" of airborne moisture lashed the Pacific coast with relentless rainfall, in December.

     In January, the Pacific-North America (PNA) pattern was positive, which is associated with a colder-than-normal Southeast and a warmer-than-normal West coast. Both of these indeed happened. The Mississippi valley and the East also tend to be dry in a positive PNA pattern, and indeed they were dry.

     By mid-February, the PNA pattern flipped and became negative. The Southeast, which had been unusually chilly, rather suddenly warmed up. The Pacific Northwest cooled down. Wet and often snowy conditions were the rule in the Ohio Valley, Great Lakes, and the northern High Plains. The combined influence of La Niña and the negative PNA favored some record-breaking snowstorms in these regions, and caused the "Lower 48" to have above-average snow cover in February.

     This winter was a great example of a curious climatic factoid: a region can be drier than normal and yet have well-above-normal snow cover, as long as its temperatures are below normal. A lot of snow – 9 inches, say, melts down into not much water: 0.9 inch, often less. Where both rain and snow fall during a typical winter, the rain contributes the most to the seasonal precipitation total, the snow contributes less—but it is the snow that is remembered.