National Overview - Annual 2011
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Climate Extremes Index | Tornadoes | Tropical Cyclones | Snow
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2011 National Events Map
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Issued January 19, 2012: The data presented in this report are final through August and preliminary from September-December. Ranks, anomalies, and percent areas may change as more complete data are received and processed.
National Temperature and Precipitation Analysis
In 2011, the contiguous United States (CONUS) average annual temperature of 53.8 degrees F (12.1 degrees C) was 1.0 degree F (0.6 degree C) above the 20th century average, and was the 23rd warmest year on record. Since 1895, the CONUS has observed a long-term temperature increase of about 0.12 degree F (0.07 degree C) per decade. Precipitation across the CONUS in 2011 was 0.36 inch below the long-term average (LTA). Over the long-term, precipitation averaged across the CONUS, is increasing at a rate of about 0.18 inch (4.6 mm) per decade.
On a statewide and seasonal level, 2011 was a year of temperature and precipitation extremes for the United States. Most states east of the Rockies had annual temperatures which were above average. Sixteen states had annual temperatures among their ten warmest. Delaware was record warm for the period, with an annual temperature of 58.2 degrees F (14.6 degrees C), or 3.5 degrees F (1.9 degrees C) above average. Texas had its second warmest year on record, with an annual temperature anomaly of 2.2 degrees F (1.2 degrees C), just shy of the annual record of 2.5 degrees F (1.4 degrees C) set in 1921. The western states had annual temperatures which were near average, with the exception of Oregon and Washington, which were the only two states with annual temperatures below average.
Although the CONUS as a whole was drier than average for the year, several states and cities were record wet during the year. Above-average precipitation was widespread across the northern CONUS during 2011, with the wettest part of the country being the Ohio Valley and Northeast. Seven states across the two regions — Connecticut, Indiana, Kentucky, New Jersey, New York, Ohio, and Pennsylvania — had their wettest year on record. Below-average precipitation was present across the West, and along the southern tier of the country. Georgia (5th driest) New Mexico (6th), Louisiana (7th), and South Carolina (8th) had annual precipitation totals among their ten driest. Texas was record dry for the year, with 14.89 inches (378 mm) of precipitation — 13.03 inches (338 mm) below the 20th century average. The year 2011 surpassed 1917 as the driest year on record for Texas, when 14.99 inches (381 mm) of precipitation was observed across the state. When the wetter-than-average conditions across the northern CONUS are averaged with the drier-than-average conditions across the southern CONUS, they nearly cancel each other out in the nationally averaged precipitation total. When the precipitation extremes are combined cumulatively, like in the U.S. Climate Extremes Index (CEI), they tell a different story. The combined percent area of the country experiencing either extremely-wet or extremely-dry conditions during 2011 was record high at 58 percent.
Seasonal highlights in 2011 include a cooler-than-average winter (December-February) across much of the CONUS, with the coolest temperature anomalies (the temperature compared to the 20th century average) anchored across the Southeast. Spring (March-May) brought precipitation extremes to much of the CONUS, with ten states across the northern U.S. having a wettest spring on record, with flooding occurring across the Upper Midwest and the Ohio and Mississippi Valleys. The Southern Plains were extremely dry during spring, a precursor for the record drought which persisted into summer. Wildfires ravaged the Southern Plains and Southwest during spring, with record acreage burned during the 3 months. Numerous tornadoes impacted the Southeast in April, with 748 confirmed during the month — a new national record for any month. Summer (June-August) 2011 was the second warmest thee months on record for the CONUS, with much of the U.S. having much-above-average temperatures, with the exception of the Northwest. Oklahoma and Texas both exceeded the previous records for warmest summer on record for any state. Hurricane Irene, the first land-falling U.S. hurricane in three years, made three landfalls along the Atlantic coast in late August, causing damage from the Carolinas to Vermont.
This annual report places the temperature and precipitation averages into historical perspective, while summarizing the notable events that occurred in 2011. More detailed analysis on individual months can be found through the Climate Monitoring home page.
|1||April 25th–28th Tornado Super Outbreak|
|2||Southern U.S. Drought (spring–summer)|
|3||Joplin, Missouri EF-5 Tornado (May 22nd)|
|4||Mississippi River and Ohio River Flooding (spring)|
|5||Oklahoma and Texas — Hottest 3-month Statewide Temperatures on Record (summer)|
|6||Hurricane Irene (August)|
|7||Northern Plains and Upper Midwest Flooding (early summer)|
|8||U.S. Wildfire Season (spring-summer)|
|9-tie||Tropical Storm Lee (September)|
|9-tie||April 14th–16th Tornado Outbreak|
The 2010/11 winter season was marked by anomalously cold temperatures across the eastern half of the country, with the coolest temperature anomalies across the Southeast. The CONUS had an average winter temperature of 32.2 degrees F (0.1 degrees C), which was 0.7 degrees F (0.4 degrees C) below average. Florida had its tenth coldest winter on record. A strong negative phase of the Arctic Oscillation (AO) was associated with much-cooler-than-average temperatures across the eastern U.S. during the first half of winter. Florida and Georgia both had their coolest December on record, with temperature anomalies of more than 9 degrees F (5 degrees C) and 8 degrees F (4 degrees C) below average, respectively. By mid-January, the negative phase of the AO had subsided, and more seasonal to above-average temperatures returned to the Southeast by February. A different scenario played out across the West, with the winter season beginning warmer than average but ending much cooler than average. On the regional scale, only the West climate region had winter temperatures that were above normal. The Southwest and Northwest were near-normal, while all climate regions east of the Rockies were cooler than normal.
The presence of La Niña during winter influenced precipitation patterns during the season. The 3-month average CONUS precipitation of 5.71 inches (145 mm) was 0.76 inch (19 mm) below average. Drier-than-average conditions were observed across much of the southern U.S., from New Mexico to Virginia. Louisiana, Arkansas, Alabama, Mississippi, and Virginia had winter precipitation totals ranking among their ten driest. A persistent storm track brought above-average precipitation to the Northern Plains and Upper Midwest, where Montana, North Dakota, South Dakota, and Minnesota had a winter season ranking among the ten wettest. The above-average precipitation across the Northern Plains contributed to a much-above-average snowpack across the region. A massive winter storm impacted the central U.S. during February, dropping 2 feet (61 cm) of snow in the Chicago area. The storm caused at least 1.8 billion U.S. dollars worth of damage, and killed at least 36 people. The storm track across the Northern U.S. and few storms making it into the South and Southeast are consistent with La Niña conditions. The wetness across the Northern Plains primed the region for spring and summer flooding, while the dryness in the South was a precursor to the spring and summer drought.
The spring (March-May) average CONUS temperature of 52.3 degrees F (11.3 degrees C) was 0.5 degree F (0.3 degree C) above average. Regional spring temperatures were consistent with an upper level trough across the West and an upper-level ridge across the South and East. Above-normal temperatures were present from New Mexico, across the South, and along the Eastern Seaboard. Texas had its second warmest spring on record. The West, Northwest, and Northern Plains were cooler than average during spring — Washington had its fifth coolest spring on record and Oregon its seventh coolest.
The precipitation (and lack of precipitation) was the big story during spring 2011. The nationally-averaged precipitation during the season was 1.41 inches (39 mm) above average. Consistent with a spring La Niña, a persistent pattern set up in which the Pacific Northwest and Northern Rockies saw frequent storm systems and invasions of cold air. In the South, storm systems repeatedly developed in the mid-Mississippi Valley, just a few hundred miles from the drought-stricken Southern Plains, then raced northeastward. In general, drought intensified in the South, while much of the Northeast, Ohio Valley and Northwest were subjected to historic or near-historic wetness. A total of 10 states were record wet during spring, and an additional 11 states had spring precipitation totals ranking among their ten wettest. The storms brought snow pack totals across much of the West to more than 180 percent of normal. The record precipitation and snow melt caused significant flooding in late spring and early summer along several rivers, including the Mississippi, Ohio, Missouri, Souris, and James. The runoff from the precipitation put significant stress on the levee systems along these rivers, and several levees were deliberately breached to flood farm lands and protect cities. In contrast to the wetness, Texas was record dry for the three-month period. Its statewide-averaged precipitation for spring was a paltry 2.56 inches (65 mm), more than 5 inches (127 mm) below normal. The dryness across the Southern Plains was associated with record wildfire activity during the spring period, when approximately 3.2 million acres (1.3 million hectares) burned — a new spring record. The active storm pattern brought a record-breaking severe weather season to the South and Southeast. As strong storms moved into the warm and moisture-rich atmosphere across the Southeast, 1,155 tornadoes were spawned, killing hundreds of people and causing over 20 billion U.S. dollars worth of damage.
The summer period (June-August) was warm for much of the CONUS, with only Oregon and Washington having summertime temperatures ranking among the bottom third of their historical distributions. The CONUS, as a whole, had its second warmest summer period on record with an average temperature of 74.5 degrees F (23.6 degrees C), 0.1 degree F (0.1 degree C) below the warmest summer on record in 1936. A persistent ridge across the eastern U.S. brought the epicenter of the hot temperatures to the Southern Plains, where New Mexico, Texas, Oklahoma, and Louisiana had their warmest summer on record. The statewide average summer temperature of 86.9 degrees F (30.5 degrees C) in Oklahoma marks the warmest three-month period for any state on record. During the summer of 2011, all states across the contiguous U.S., with the exception of Vermont, experienced at least one day with a location having a daily maximum temperature exceeding 100 degrees F (37.8 degrees C).
The drought across Texas was amplified during the summer of 2011, due to the very warm temperatures and the lack of precipitation. The state had its driest summer on record with a statewide average of 2.44 inches (62.0 mm) of rain accumulating. This is 5.29 inches (134.4 mm) below the long-term average, and 1.04 inches (26.4 mm) less than the previous driest summer in 1956. At the end of August, 81 percent of Texas was in the worst category of drought (D4, 'Exceptional' Drought), and analysis of tree-ring records dating back to 1550 indicated that the summer of 2011 drought is matched by only one summer (1789) in the 429-year tree-ring record. A persistent dome of high pressure was present for much of the summer across the Southern Plains, limiting storms from entering the region and causing temperature to soar. The Northeast was wetter than average during the summer period, partially because of Hurricane Irene bringing heavy rainfall to the region the last week of August. Connecticut and New Jersey were record wet for the summer. As a whole, the CONUS was drier-than-average with a nationally average precipitation total 0.9 inch (23 mm) below the LTA.
Although autumn (September-November) 2011 brought several extremes to the U.S., it was a significantly quieter season compared to the spring and summer. The average autumn temperature for the CONUS was 1.3 degrees above average. Most states had autumn temperatures near average, while Massachusetts, Rhode Island, and Vermont had their record warmest fall. Eight other states had an average temperature that was one of the ten warmest on record. Conversely, four states in the Southeast and along the Gulf Coast had below-average autumn temperatures.
Two significant storms impacted the CONUS during autumn. Tropical Storm Lee made landfall along the Louisiana coast on September 4th, and moved along a frontal boundary into the Ohio Valley and eventually into the Northeast. The storm brought much needed precipitation to the drought stricken Gulf Coast, but added to the annual precipitation totals of the Ohio Valley and Northeast. An early season storm brought heavy snow accumulations to the northeastern United States on October 29th-31st. The heavy, wet snow falling on the autumn foliage, combined with strong winds, caused havoc across the region. During autumn, the western half of the CONUS had precipitation which was near to slightly below the LTA. Beneficial precipitation fell across the core drought areas of Texas and Oklahoma. Minnesota had its third driest autumn on record. In contrast, many states across the Ohio Valley and Northeast had a very wet autumn. Pennsylvania and Ohio were both record wet for the period, and five other states had autumn precipitation ranking among their ten wettest.
Alaska Annual Summary
Alaska temperatures in 2011 were above the 1971-2000 average, continuing the upward trend of the last 20 years. However, there was variation between the seasons. Winter temperatures in 2010-2011 were 0.4 degrees F (0.2 degrees C) below average. Spring temperatures were 0.9 degrees F (0.5 degrees C) below average, summer temperatures were 0.7 degrees F (0.4 degrees C) below average, and fall was 0.4 degrees F (0.2 degrees C) warmer than the average. For the annual period, Alaskan temperatures were 0.2 degrees C above average, driven almost entirely by very warm temperatures in December (third warmest December).
Precipitation in Alaska in 2011 was slightly above average. The winter season brought near-normal precipitation to Alaska, while the spring was much drier than average. Summer was wetter than average across Alaska and autumn precipitation was slightly below average.
In November, a large and powerful extratropical cyclone slammed into western Alaska, with extremely high tides, strong winds, heavy rain, and blizzard conditions. Winds gusted to over 80 mph (130 km/hr) and the storm surge topped 8 feet (2.4 m), marking the strongest storm to impact the region in decades.
Very Warm/Cold and Wet/Dry Percentages
One way to assess the magnitude of warm/cold and wet/dry episodes is to compute the percent area of the contiguous United States that was "very warm/very cold" and that which was "very wet/very dry". The figures above depicts these values for each month in the past 30 years. These percentages are computed based on the climate division data set. Those climate divisions having a monthly average temperature/precipitation in the top ten percent (> 90th percentile) of their historical distribution are considered "very warm/very wet" and those in the bottom ten percent (< 10th percentile) are "very cold/very dry".
During 2011, the U.S. experienced a cooler-than-average winter, which transitioned into a warm spring, summer, and autumn. In terms of the area of the contiguous U.S., 18 percent experienced temperatures that were in the bottom 10th percentile or categorized as "very cold" in December 2010. This definition correlates well with "much below normal" in terms of NCDC ranking methods. The extremely cool temperatures dissipated during January and February. The percent area of the country experiencing “very cold” conditions reached its highest value of 2011 during May, when it reached 22.7 percent. Most of those cool temperatures were present across the West, where Washington, Oregon, and Wyoming had a top ten cool month. Influenced by a strong Bermuda High and dome of high pressure, “very warm” conditions prevailed for April (22 percent of the country), June (29.7 percent), July (48.8 percent), August (43.5 percent), and September (25.44). The warm season, which as defined at April-September, was the seventh warmest for the CONUS, and Texas and Oklahoma were record warm.
Large areas of the CONUS experienced “very wet” and “very dry” conditions during 2011. The wetter-than-average conditions across the northern regions of the CONUS were counterbalanced by drier-than-average conditions across the southern regions. The percent area of the country experiencing very wet conditions peaked in April, when 24.3 percent of the CONUS was “very wet”. The smallest percent area of the country classified as “very wet” during 2011 was in June when the percent area dropped to 6.7 percent. The drought across the Southern Plains and Southeast contributed to large areas of the CONUS being “very dry” in 2011. The percent area of the U.S. as “very dry” peaked in August at 21.7 percent and again in December at 25.0 percent. A large area of the western U.S. was near-record dry during December 2011.
When the area of the country that experienced “very warm” conditions in the summer period (June, July and August) are averaged, the total area extent of 40.7 percent surpasses the previous record of 38.1 percent set in 1934, the fourth warmest summer on record. The effects of the record warmth in the Southern Plains during the summer period were compounded with the parched soils. More than 16 percent of the country was considered “very dry” most of it concentrated in the southern plains. The area of “very warm” conditions combined with “very dry” conditions was only second to 1936.
Climate Extremes Index
The U.S. Climate Extremes Index (CEI) measures the occurrence of several types of climate extremes, such as record or near-record warmth, dry spells or rainy periods. In 2011, extremes in both temperature and precipitation were observed around the country and had a large impact on several regions. Persistent drought plagued much of the South and southeastern U.S., while parts of the Ohio Valley and the Northeast experienced record rainfall. In addition, much of the Mid-Atlantic, Northeast and South experienced much-above-average temperatures throughout the year, while the Pacific Northwest remained relatively cool. For the contiguous U.S., the spatial extent of extremes, as measured by the CEI, during the annual season was approximately 12 percent greater than the historical average. This above-average extent of extremes was primarily due to extensive extremes in warm maximum and minimum temperatures, extreme wetness and dryness, as denoted by the Palmer Drought Severity Index (PDSI), and an abundance of days in which precipitation fell. Regions which experienced some of the most wide-spread extremes during 2011 include the South, Southeast and Northeast. Nearly half of the South region was impacted by a blend of extremes in warm maximum and minimum temperatures as well as PDSI dryness. Over one third of the Southeast was impacted by a combination of warm maximum and minimum temperatures as well as PDSI dryness. Over 70 percent of the Northeast experienced extremes in warm maximum and minimum temperatures in addition to PDSI wetness, extremes in 1-day precipitation, and an abundance of days in which precipitation fell. During 2011, the most prominent and widespread extremes occurred during two seasons: spring and summer.
At the National level, the spring season was near normal for both temperature and precipitation, despite large regional differences. Warm extremes were confined to the Deep South and parts of the Mid-Atlantic states, while cool extremes occurred across the Northwest and northern-tier states. Record wet conditions persisted across much of the Ohio Valley and into the Northeast as well as across parts of the Northwest. Record dryness existed across Texas with much below average precipitation realized in other parts of the South.
For the contiguous U.S., the spatial extent of the CEI was nearly twice the historical average during spring. Factors contributing to the elevated 2011 spring value were large footprints of warm and cold maximum temperatures, areas of extreme PDSI wetness, and an abundance of days in which precipitation fell. Regions of the country which were most significantly impacted by extremes during the spring season were the Northwest, Northern Rockies and Plains, South and Northeast regions. In fact, the Northeast and the South regions had their 2nd highest extent of extremes on record with 57 percent and 43 percent of each region affected by extremes during this season, respectively.
The summer season was second warmest on record for the U.S. with much above average temperatures to record heat extending from the South though the Mid-Atlantic states. Precipitation received from Hurricane Irene impacted parts of the Northeast while drought conditions persisted across one third of the U.S. The CEI for the contiguous U.S. was approximately twice the historical average during summer. A record 63 percent of the South region was impacted by extremes during the summer months. Approximately 96 percent of the region succumbed to extremes in warm maximum temperature, 95 percent to extremes in warm minimum temperature and 78 percent to extremes in PDSI dryness. For the Southeast, extremes impacted a record 53 percent of the region with the largest contribution coming from extremes in warm maximum temperature, warm minimum temperature and PDSI dryness.
National Snow & Ice
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.
U.S. Winter Snow Cover Extent Anomalies
Data Source: Rutgers Global Snow Lab
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.
Western U.S. Snowpack
May 1, 2011
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.
Hurricanes & Tropical Storms
The 2011 North Atlantic hurricane season had 19 named storms, seven hurricanes, and three major hurricanes. The 19 named storms tied with 2010, 1995, and 1887 as the third busiest year for tropical cyclones in the basin. The second most active tropical cyclone year for the Atlantic basin was 1933. An average season has 11 named storms, six hurricanes, and two major hurricanes. The seven storms which reached hurricane strength and the three that reached Category 3 status (major hurricane) are both near the average, despite the high number of tropical storms. One hurricane (Irene) and one tropical storm (Lee) made landfall in the U.S. during the 2011 season. Hurricane Irene was the first Hurricane since Ike in 2008 that made landfall in the nation.
The U.S. spring and summer of 2011 will likely be remembered as one of the most destructive and deadly tornado seasons to ever impact the nation. During the time period, there were seven individual tornado and severe weather outbreaks with damages exceeding one billion U.S. dollars, and total damage from the outbreaks exceeding 28 billion U.S. dollars. This represents the most property damage from severe weather in a single year since record keeping began in 1980. As of mid-January 2012, the 2011 confirmed tornado count stood at 1,625, with 93 tornado reports still pending for November and December. This places 2011 as the second or third most active year on record for number of tornadoes since the modern record began in 1950, depending on the confirmation rate of the end-of-year tornado reports. There were six EF-5 tornadoes confirmed during 2011, the strongest category on the Enhanced Fujita scale. This ties with 1974 as the most (E)F-5 tornadoes during a single year (Please note that NOAA switched form using the Fujita (F) scale to the Enhanced-Fujita (EF) scale in 2007). The annual total number of fatalities from tornadoes was 551, the most in the 62-year period of record.
Looking beyond the modern tornado record (1950-present), 2011 ties with 1936 as the second deadliest year for tornadoes in U.S. history. In 1925, 794 tornado-related fatalities were reported, the deadliest tornado year on record. The extensive damage and loss of life due to tornadoes during 2011 was a product of an above-average tornado year and strong tornadoes hitting densely-populated areas. During the year, the two largest tornado outbreaks on record for the nation impacted the southern U.S., and the single deadliest tornado since the modern record began in 1950 hit Joplin, Missouri.
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