NOAA's National Centers for Environmental Information
The 2010-2011 winter began with a bang for much of the Eastern U.S. with several snowstorms and cold temperatures impacting the Eastern Seaboard during December and January. However, the end of the winter season was much quitter when above normal temperatures returned to the region in February. The change in the weather regime coincided with the transition of the Arctic Oscillation (AO) from a near-record negative phase to a positive phase. Significant snowfalls occurred throughout the winter season across the Intermountain West, across the Northern Plains and into the Ohio Valley. The active pattern across these regions is consistent with the La Niña conditions which were present for the entire season across the equatorial Pacific. The heavy snowpack, combined with above-normal spring precipitation caused significant flooding across the Northern Plains and the Mississippi and Ohio Valleys when warm temperatures returned in late spring.
When conditions are averaged for the three-month winter period (December 2010 - February 2011), below-normal temperatures were anchored across the eastern half of the country, with above-normal temperatures for the Southwest. Drier-than-normal conditions were widespread across the South, Southeast, and into the Mid-Atlantic. Wet conditions were present for the Northern Plains, stretching into the Northeast. Snow cover extent, which is measured from NOAA satellites and provided by Rutgers Global Snow Lab, was above average for each winter month, stretching into the spring. Across the U.S., each month from December through April had snow cover extents were among the ten largest on record. The winter (December-February) average snow cover extent for the contiguous U.S. was 305,000 square miles (790,000 square km) above the 1981-2010 average and ranked as the fifth largest winter snow cover extent in the 1966-present period of record.
Winter and spring mountain snowpack provide a crucial water source across much of the western United States. The total annual water budget for agriculture and human use in the mountainous West is based upon the amount of snow melt that will occur in spring and is proportional to the amount of snow on the ground. The annual snowpack typically reaches its maximum value at the end of March. This year, late season snowfall across the Central and Northern Rockies contributed to a much above average snow cover extent well into April. According to data from the USDA, as of May 1st, 2011, much above normal snowpack was observed from the Cascade Mountains, southward into the Sierra Nevadas, and across the Central and Northern Rockies. Snowpack values more than 180 percent of normal were widespread. Below-normal snowpack was observed across much of the Southern Rockies of Arizona, New Mexico, and southern Colorado. Some areas of New Mexico had snowpack totals below their 25th percentile. Alaska generally had near-normal snowpack at the end of the snow season. Slightly below-normal snowpack was present across the southern coasts of Alaska while above-normal snowpack was observed across the western regions of the state.
Select Significant Events
A strong blizzard hit Minnesota, Iowa, and Wisconsin on December 10th-12th 2010, bringing over a foot of snow to the region and sustained winds of 35 mph (56 km/hr). The 17 inches (43 cm) of snow that fell in Minneapolis was the 5th largest snowstorm accumulation ever for the city and the largest December snowfall on record. Minneapolis set a new December snowfall record with a monthly total of 33.6 inches (86 cm), the previous December record was set in 1969 when 33.2 inches (85 cm) of snow was reported. Data for the city goes back to 1884. The 22 inches (56 cm) which fell in Eau Claire, Wisconsin on the 11th is the highest single calendar day snowfall total in history for the city. An unusual aspect of the storm was the high snow-to-water equivalent, which represents how heavy the snow was. The heavy nature of the snow, as well as the amount and strong winds, contributed to the collapse of the Metrodome roof, home to several professional sports teams. Fortunately the building was empty at the time of the collapse, and no one was injured.
A low pressure system moved across the southern U.S. and interacted with another system moving from the Northern Plains the last week of December 2010. The two systems brought widespread snowfall from Minnesota, southward to Alabama and Georgia and along the Eastern Seaboard to Maine on December 24th-28th. Many locations in the Southeast experienced their first white Christmas on record, and many cities broke daily snowfall records because of the storm. As the system moved up the Atlantic Coast, it strengthened, bringing heavy snowfall to the major cities in the Northeast. Central Park in New York City received 20 inches (51 cm) of snow, marking the sixth largest snowfall there, while Boston received 18.2 inches (46 cm), the city’s eighth biggest snow storm on record. The 20.1 inches (52 cm) which fell at the Atlantic City International Airport was a single snowstorm record. The Northeast Snowfall Impact Scale (NESIS) score, which measures the areal extent of snow and impact of snow extent/depth on popuated areas, was a Category 3 (major).
The first major winter storm complex during 2011 impacted the eastern half of the United States between January 9th-13th, dropping over a foot (30.5 cm) of snow for many locations across the Central Plains, Midwest, Southeast, Mid-Atlantic, and Northeast. A significant freezing rain and icing event occurred across many portions of the South and Southeast. The event began across northeast Texas on the 9th where over 6 inches (15.2 cm) of snow accumulated. The upper level low pressure system progressed along the Gulf Coast, drawing moisture from the Gulf of Mexico, causing a large precipitation shield over the Southeast. Parts of Mississippi, Arkansas, Tennessee, Alabama, and Georgia received over 10 inches (25.4 cm) of snow by the 10th. Huntsville, Alabama received 8.9 inches (22.6 cm) of snow, marking the third largest snow storm on record for the city. Parts of the Atlanta metropolitan area received eight inches (20 cm) of snow, shutting down the city for days. In the North Carolina mountains, up to 20 inches (51 cm) of snow were reported. The storm system then moved northward along the Atlantic Coast, bringing heavy snow to the population centers of the Northeast. Blizzard conditions were reported in Boston for several hours on the 12th. The 24 inches (61 cm) which fell at Bradley International Airport in Windsor Locks, Connecticut broke the previous all-time greatest storm record of 21.9 inches (56 cm), set on February 12th, 2006. Six states declared states of emergency. After the event, 49 of 50 U.S. states (all except Florida) had snow on the ground. The preliminary NESIS score, was a Category 3 (major).
On February 1st-3rd a large and powerful winter storm, dubbed the ‘Groundhog Day Blizzard’, hit the central and northern regions of the United States from New Mexico northward to Wisconsin, and eastward to New England. The storm stretched for thousands of miles, leaving behind at least five inches (12.7 cm) of snow in 22 states. The multi-faceted storm also brought an inch of ice to portions of the Ohio River Valley. Winds gusting upwards of 70 mph (113 km/hr) created widespread blizzard conditions, and snow drifts were reported as high as 10 feet (3 meters). Numerous highways were forced to close and thousands of flights were cancelled nationwide. The storm began across the Southern Plains on the 1st, where it dropped 1 to 2 feet (30.5 cm 61 cm) of snow across Texas, Oklahoma, and Missouri. The largest snow amounts occurred across northern Illinois and southern Wisconsin on the 2nd. At Chicago's O’Hare Airport, the 20.2 inches (51.3 cm) of snow was the third largest storm accumulation ever recorded for the city. Racine, Wisconsin observed 26 inches (66.0 cm) of snow during the event, breaking the city’s 48-hour and 72-hour snowfall records. Snowfall records date back to 1886 for Chicago and to 1896 for Racine. The storm then moved off into the Northeast on the 3rd, where it left behind a foot of snow. Damages from the storm exceeded one billion U.S. dollars.
A series of large Pacific storms made landfall along the U.S. West Coast between March 18th-26th. The storms brought incredible amounts of rain and snow from Washington State to Southern California. These storms contributed to many high-elevation observation stations having impressive snowfall totals for the month and the winter season. Squaw Valley at Lake Tahoe, California reported 241 inches (612 cm) of snowfall during March 2011. Alpine Meadows, California reported 315 inches (800 cm) of snow on the ground after these storms moved through, and several snow observation stations in the Sierra Nevadas received as much as 145 inches (368 cm) during the 9-day period. The snow added to the amount of water contained in the snow pack measured by snow water equivalent (SWE). At the end of March, some high elevation stations in California had SWE values approaching 80 inches (203 cm). For California as a whole, the average snowpack was 48 inches (122 cm) on April 1st, 168 percent of average. At some locations in the Sierra Nevadas, the snow depth exceeded the height of the automated weather stations in the SNOTEL network, causing underestimates in the measurement of snow on the ground.