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Note: Data in this report are compiled from preliminary statistics

Briefing Package presented by Tom Karl, Ed O'Lenic and Harold Brooks on June 15th (PDF)

Introduction

The spring (March-May) of 2011, particularly April, brought extreme weather and climate events to many parts of the United States. Tornadoes, flooding, drought, and wildfires ravaged many parts of the country during the period, and each of these extremes broke long-standing records and have been compared to the 'worst such cases' in history. While similar extremes have occurred throughout modern American history, never before have they occurred in a single month. According to the Storm Prediction Center (SPC), there were 751 tornadoes during April alone, and the confirmed number of tornadoes surpassed the all-time monthly record of 542 tornadoes set in May 2003. Record rainfall along the Ohio River Valley, punctuated with snowmelt across the upper Midwest, caused record flooding along the mid and lower Mississippi River, with water levels surpassing the historic floods of 1927 and 1937. Above-normal precipitation and vegetative growth during 2010, followed by dry and windy conditions the first five months of 2011, created ideal wildfire conditions across the Southern Plains where millions of acres of land burned. According to the U.S. Drought Monitor (USDM), the same region experienced Extreme-to-Exceptional [D3-D4] drought following consecutive months that were record to near-record dry.

April brought an active weather pattern across the contiguous U.S., with strong storms moving through the center of the country, tapping into moisture from the Gulf of Mexico as they matured across the mid-Mississippi Valley. These storms caused widespread severe weather across the Southeast and widespread heavy precipitation across the Ohio Valley. Both the number and magnitude of the severe weather events, as well as the amount of precipitation across the Ohio Valley, broke all-time records, according to preliminary data. Meanwhile, the storm track essentially blocked Gulf of Mexico moisture from entering the Southern Plains.

Across the Upper Midwest rapid melt of an above-average snowpack during late March through mid-April swelled rivers and caused near record river crests along the Red River in North Dakota and Minnesota. Farther east, across Minnesota and Wisconsin, a significant portion of the snowmelt water found its way into the Mississippi River and moved southward towards the Gulf of Mexico. Meanwhile, the above-average rainfall across the Ohio Valley, combined with snowmelt, caused the Ohio River to swell to near-record levels. At the confluence of the Ohio and Mississippi Rivers, the above-average water flow of each combined to cause the Mississippi River to crest at record to near-record levels from Illinois to Louisiana, flooding hundreds of thousands of acres. After judicial directives, the Army Corps of Engineers opened spillways and destroyed levees, flooding rural areas to save major population centers and infrastructure.

On a statewide level, during April, above-normal precipitation was widespread across the northern half of the country, while the Southern Plains and Southeast had near- to below-average precipitation. Below-normal precipitation was observed for the previous six months across the southern Plains, exacerbating drought conditions there. At the beginning of May, 73 percent of Texas was experiencing Extreme-to-Exceptional [D3-D4] drought conditions. Texas had its second driest November through April period, third driest January-April, and driest February-April and March on record. The prolonged dryness fueled several large wildfires, which burned 1.79 million acres (0.72 million hectares) nationally during the month, shattering the previous April record.

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Tornadoes

The period through the end of May, 2011 was marked by numerous large severe weather outbreaks, causing a record-breaking tornado year for the year-to-date period. An estimated 1,217 tornadoes struck the U.S. during the January-May period, according to the Storm Prediction Center and 751 of those tornado reports were during the month of April alone.

Tornado activity during April was record breaking. According to data from the Storm Prediction Center (SPC), there were 751 tornadoes during the month, and the final tornado count surpassed the all-time monthly record of 542 tornadoes (May 2003). The previous April record was 267 tornadoes, set in April 1974. The 30-year (1981-2010) average for number of April tornadoes is 135. During spring (March-May) 2011 there were several significant tornado outbreaks, mostly across the Southeast, and more information on these individual outbreaks can be found in the State of the Climate report.

Environmental Conditions

During the month of April, environmental conditions came together to create the perfect scenario for severe weather across the eastern half of the country. A persistent storm track across the central U.S. allowed frontal systems to interact with Gulf of Mexico moisture and initiate storms across the southeastern quadrant of the nation. During April, seven upper level troughs moved across the country. These upper level troughs provided dynamic forcing that allowed the development several severe weather complexes. Strong jet streams formed along and ahead of these upper level troughs providing diffluence aloft. Diffluence can facilitate rising air within thunderstorms, causing them to become stronger. Along the leading edge of the upper level troughs were surface cold fronts and surface low pressure systems. Southerly winds ahead of the surface cold fronts caused warm and moist air to surge northward. Behind the fronts, cool Canadian air swept across the central part of the country.

During the two largest outbreaks (April 14^th-16^th and 25^th-28^th), there was significant southerly flow ahead of the storm systems. Higher in the atmosphere, the wind tended to be more westerly in direction. This veering of the winds with height created a large spatial extent of vertical wind shear. This shear, under the right circumstances, can fuel long-lived supercell thunderstorms. Also at play was the difference in temperature between the warm southerly flow near the surface and the drier, cooler air aloft associated with the upper level troughs. This scenario is also favorable for severe thunderstorm development when warm moist air near the surface is lifted upwards by the surface cold front into the layer of cooler, drier air, releasing latent heat energy into the storm.

During the month of April other large scale phenomena played a role in the development of the record-breaking tornado outbreaks. Ahead of the cold fronts, warm and very moist southerly air invaded the eastern half of the U.S. from the Gulf of Mexico. A monthly analysis of winds using reanalysis shows that the southerly component of near surface winds across the Gulf Coast and southeastern U.S. more than 6.7 mph (3 m/s) above the 1971-2000 April average. This indicates that the southerly flow was persistent and strong across during April.

Another large scale feature which played a role in the development of these severe weather outbreaks was the Gulf of Mexico. Averaged for the month, the sea surface temperatures across the Gulf of Mexico were about 1 degree Celsius (1.8 degree F) above the long-term average. The warm water and atmosphere over the Gulf provided fuel for the severe weather outbreaks. The southerly winds ahead of the storms that moved across the country advected the warm temperatures and Gulf moisture over the continental United States. Many locations across the southern U.S. broke daily temperatures records when these southerly flow regimes set up across the region. The warm sea surface temperatures across the Gulf are a good indicator of the amount of moisture across the region. Warmer temperatures allow the atmosphere to hold more water vapor. The more water vapor in the atmosphere, the more readily storms form and the stronger storms can become.

Historical Perspective

On the January-May timescale, the number of tornadoes and the number of tornado-related fatalities were record breaking. It is likely that the entire country and many states will approach and/or break records for the number of confirmed tornadoes. For the year-to-date period, there were 1,217 preliminary tornadoes. Nationwide, the previous January-May tornado count record was 1,011 in 2008. In terms of the number of tornado-related fatalities, there were 561 during the January-May period. When analyzing the number of tornado-related fatalities, there were two separate comparisons — the comparison to the modern (1950-present) tornado record and the non-official (1875-present) record. The 561 fatalities is the most in the 1950-present period, and the second most in the 1875-present time period.

The severe weather outbreaks during spring 2011 caused significant property damage across the eastern United States on top of the unfortunate high number of fatalities. According to preliminary numbers from various disaster cost modeling and insurance risk reports, the estimated damage from the spring severe weather outbreaks will approach 24 billion U.S. dollars, with several of the severe weather events likely exceeding one billion U.S. dollars in damage each.

Significant Severe Weather Events

April 14^th–April 16^th Tornado Outbreak

A major severe weather outbreak impacted the southeastern U.S. between April 14^th and 16^th. A strong upper level low pressure system moved across the central plains, and ahead of the system, very warm and very moist air was advected northward from the Gulf of Mexico. As the associated surface cold front moved into the southeast, a series of severe thunderstorm complexes were initiated across the country. NOAA estimates the final tornado count for the outbreak was 176 tornadoes, marking one of the largest outbreaks on record, especially for April, which averages 135 tornadoes for the entire month. Although there were a large number of tornadoes, there were 14 tornadoes rated EF-3, and none were rated EF-4 or EF-5. A total of 38 people were killed from the tornadoes, 22 of which were in North Carolina alone. Nationwide, the tornado outbreak break was the deadliest since the 2008 Super Tuesday tornado outbreak that occurred on February 5^th and 6^th, 2008, when 57 people were killed. The 30 confirmed tornadoes across North Carolina broke the single-storm and single-day tornado outbreak record for the state. The previous record was from the infamous March 1984 Carolina’s tornado outbreak when 22 tornadoes killed 42 people in the state.

April 25^th–April 28^th Tornado Outbreak

Another major and record breaking severe weather outbreak impacted the southeastern U.S. between April 25^th and 28^th. During the outbreak, there were 345 tornadoes in 21 states. The storm scenario was similar to the mid-April outbreak, with a potent upper level low pressure system moving across the Central and Northern Plains, but this low pressure system was a little stronger and had a path more northward. Ahead of the surface cold front, very warm and very moist air infiltrated the southern U.S. from the Gulf of Mexico. Temperatures across the southeast reached into the lower 90s (°F). A strong mid-level jet streak moved into the Tennessee Valley, providing strong shear and dynamic support for severe thunderstorm development. As the storm system moved towards the east, it initiated severe weather each day between the 25^th and 28^th. The most impressive period of the outbreak was on April 27^th, and into the early hours of the 28^th, when two severe weather complexes moved across Mississippi, Alabama, Tennessee, and Georgia. NOAA’s current estimate is there were 190 tornadoes during that time period. The previous single largest tornado outbreak to impact the U.S. was the April 3-4, 1974 Super Outbreak, when there were 148 tornadoes across the Southeast and Ohio Valley. According to the most recent estimates, there were 3 EF-5 rated tornadoes, 12 EF-4s and 21 EF-3s. There were an estimated 320 tornado related fatalities from the outbreak — approximately 321 of those occurred on the 27^th. Alabama alone observed 235 of those fatalities. An EF-5 moved through northern Alabama and killed 78 people, the deadliest of the outbreak. Several major metropolitan areas were directly impacted by strong tornadoes including Tuscaloosa, Birmingham, and Huntsville in Alabama and Chattanooga, Tennessee, causing the estimated damage costs to sore. According to preliminary information, property damages might exceed six billion U.S. dollars. For more information see the monthly U.S. Tornado State of the Climate report.

April 27^th Select Station Dewpoints

City	April 27th Hourly Average Dewpoint	30-year average April Dewpoint
Birmingham, Alabama	66.2 °F (19.0 °C)	49.2 °F (9.5 °C)
Tuscaloosa, Alabama	66.0 °F (18.9 °C)	50.9 °F (10.5 °C)
Huntsville, Alabama	61.9 °F (16.6 °C)	48.3 °F (9.1 °C)

Joplin, Missouri Tornado May 22^nd

On May 22^nd a severe weather outbreak occurred across the central Plains and the Midwestern United States, and several of the storms generated tornadoes across the region. The most destructive of the storms tracked from southeastern Kansas into southwestern Missouri during the late afternoon. This supercell thunderstorm spawned a strong tornado over Joplin, Missouri, resulting in devastating damage. The National Weather Service rated the tornado an EF-5, with winds in excess of 200 mph (320 km/hr). The tornado was on the ground for approximately 6 miles (9.7km) and had a maximum width of 3/4 mile (1.2 km). The tornado struck the heavily populated southern part of Joplin, resulting in at least 157 deaths and 1,000 injuries. The tornado surpassed the June 1953 tornado which killed 116 people in Flint, Michigan as the deadliest single tornado to strike the U.S. since modern tornado record keeping began in 1950. The deadliest tornado on record for the U.S. was the 'Tri-State Tornado' which killed 695 people across Missouri, Illinois, and Indiana in March 1925.

The image on the left shows the base level reflectivity from the Springfield, Missouri NEXRAD radar site of the Joplin, Missouri tornado Sunday May 22, 2011 22:43 GMT (5:43 pm local time). The high reflectivity values occurring over southern Joplin indicate the mostly likely location of the EF-5 tornado.

The image on the right shows de-aliased storm relative velocity from the Springfield, Missouri NEXRAD radar site of the Joplin, Missouri tornado Sunday May 22, 2011 22:43 GMT (5:43 pm local time). Green colors (negative values) indicate wind moving towards the radar site (located 60 miles [97km] to the east), and red colors (positive values) indicate winds moving away from the radar site. The tight couplet of reds and greens over southern Joplin show the strong rotation occurring within the EF-5 tornado. The large scale green and red couplet indicates the larger scale rotation occurring within the supercell thunderstorm.

3-D NEXRAD Animation of Joplin Tornado

Images created with NCDC's Weather and Climate Toolkit and GRLevel2.

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Preliminary* Statewide Torando Records

Annual Tornado Records

State	2011 Preliminary Tornado Reports*	Old Record / Year
Alabama	160	94 / 2008
Kentucky	52	39 / 1997
Mississippi	136	109 / 2008
North Carolina	89	67 / 2004
Tennessee	75	46 / 2009

Jan–May Tornado Records

State	2011 Preliminary Tornado Reports*	Old Record / Year
Alabama	160	72 / 2009
Arkansas	66	62 / 2008
Georgia	56	49 / 2008
Indiana	48	46 / 1990
Kentucky	52	34 / 2008, 1974, 2003
Louisiana	70	44 / 1990
Maryland	14	13 / 2002
Mississippi	136	56 / 2008
Misssouri	85-tie	85 / 2003
North Carolina	89	53 / 1998
Ohio	31	28 / 1973
Tennessee	94	45 / 2003
Virginia	38	33 / 2008
Wisconsin	35	33 / 2005
Entire U.S.	1371	1,224 / 2008

April Tornado Records

State	2011 Preliminary Tornado Reports*	Old Record / Year
Alabama	140	35 / 2009
Arkansas	46	29 / 1979
Georgia	46	25 / 2009
Kentucky	41	29 / 1974
Louisiana	36	24 / 2000
Maryland	11	5 / 2002
Mississippi	121	26 / 2005
Misssouri	30	26 / 1994
Nevada	3	1 / 2005, 1964, 2001
New York	8	4 / 1991
North Carolina	85	24 / 1996
Tennessee	50	42 / 1974
Texas	69 - tie	69 / 1957
Virginia	35	18 / 2008
Wisconsin	16	11 / 1984
Entire U.S.	875	273 / 1974

*2011 numbers reported are preliminary tornado reports. These numbers are subject to change as more up-to-date storm surveys are completed.

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Flooding

During the spring of 2011, persistent rainfall combined with melting snowpack caused historical flooding in some of the United States' major rivers, including the Ohio, the Mississippi, and the Missouri. A relentless storm track pattern provided nearly 300 percent of normal precipitation amounts in the Ohio Valley. As the Ohio, Missouri, and Mississippi River experienced historical crests, dams and levees were breached. Those that were not breached were significantly tested throughout the enduring event. Smaller towns and farmland were intentionally flooded to save larger, more populated towns. While the slow-moving disaster provided extra lead time for the residents to prepare their homes and businesses, hundreds of thousands of acres of farmland from an area south of the Great Lakes to the Gulf Coast were swimming in 20 feet (6 m) of water for weeks.

The Lower Mississippi Basin begins at the confluence of the Ohio and Upper Mississippi Rivers in Cairo, Illinois. According to the U.S. Army Corps of Engineers, 4 million people reside within the 35,000 square mile Lower Mississippi watershed. After a massive flood overwhelmed the southernmost stretch the river in 1927, the Army Corps of Engineers built a 2,200-mile system of earthen levees, floodwalls and other controls along the Mississippi, Arkansas, and Red Rivers. The flood control system has been tested over the years, especially in 1973 and 1993 when extremely wet conditions swelled the rivers to comparable historical levels. In 2011, extreme amounts of precipitation led to unprecedented flooding in several locations along the major rivers.

The extreme amounts of precipitation was set up by a large-scale weather pattern during April which consisted of high pressure, which typically results in warm dry air, over the South and Southwest U.S. This high pressure was associated with a ridge which inhibited systems from entering the region deflecting storms into the Ohio River Valley region. The persistence of this pattern during the season exacerbated the magnitude of the precipitation amounts and subsequent flooding.

Precipitation Records

During the month of April, over 1,300 daily precipitation records were broken across the Midwest and South - 197 in Kentucky alone. For the month, 72 locations reported their wettest day in any April on record and 5 of these stations set a new all-time record for the wettest 24-hour period for any month. Rainfall totals for April exceeded 13 inches (330 mm) in cities along the Ohio River. At Louisville International Airport, the 13.97 inches (355 mm) surpassed the previous record of 11.10 inches (282 mm) set in 1970. A monthly record of 13.52 inches (343 mm) of precipitation was also a record in Cincinnati. Further inland, the 15.91 inches (404 mm) of precipitation in Paducah, Kentucky was also an April record. The 12.7 inches (329 mm) that was measured in Lexington surpassed the 1970 record of 9.3 inches (236 mm). Columbus, Ohio received 7.14 inches (181 mm) of precipitation which was also a record. The 6.89 inches (175 mm) that fell in Cleveland broke the 1961 record of 6.61 inches (168 mm). From March-May, departures were at least 150 percent of normal in an area that stretched from the Ohio Valley to the Middle Mississippi Valley.

Spring Record Precipitation Amounts

Location	Record	Amount	Previous Record (year)
Northwest Climate Region	Most March–May Precipitation	10.10"	9.39" (1993)
Washington	Most March–May Precipitation	13.67"	12.85" (1997)
Wyoming	Most March–May Precipitation	6.69"	6.41" (1906)
Indiana	Most March–May Precipitation	19.38"	18.05" (1933)
Ohio	Most March–May Precipitation	17.47"	16.22" (1964)
Kentucky	Most March–May Precipitation	23.70"	22.08" (1935)
West Virginia	Most March–May Precipitation	18.19"	17.11" (1967)
Pennsylvania	Most March–May Precipitation	18.62"	15.48" (1983)
New York	Most March–May Precipitation	16.14"	14.35" (1983)
Vermont	Most March–May Precipitation	17.18"	16.94" (1983)

Flooding

Mississippi River

In order to protect heavily populated cities from flooding, the Army Corps of Engineers opened several spillways along the Lower Mississippi River. On May 2 officials intentionally breached part of the Birds Point-New Madrid Levee near the confluence of the Ohio and Mississippi Rivers to protect the small Illinois town of Cairo, population of 2,800. The two-mile opening allowed water to pass through at a rate eight times that of Niagara Falls, flooding the Birds Point New-Madrid Floodway - which is 130,000 acres of Missouri farmland in addition to about 100 residences. The move was challenged in courts, but overturned, preventing devastating flooding in Cairo and elsewhere downstream. The Corps estimated it will take up to two months for the water to recede from the floodway and another month for the land to dry out.

One week later on May 9, the Bonnet Carre' Spillway was opened, allowing flood waters to flow into Lake Pontchartrain. When all 350 bays are opened in that spillway, it diverts 250,000 cubic feet (7,079 cubic meters) of water per second into Lake Pontchartrain and on into the Gulf. The last time all Bonnet Carre' bays were opened was in 1983. Farther upstream, a portion Morganza Spillway was opened on May 14. This move alleviated stress on the Old River Control Structure upstream and the levees which protect New Orleans downstream. The only prior time the Morganza Spillway had been opened was in 1973 and this marked the first time in history that all three spillways have been opened simultaneously.

Even with all the precautions taken, the populated cities and rich farmland along the riverside, which are normally protected by the system of levees, were flooded. The massive wall of water drifted slowly southward, overtopping its banks along the way. In Memphis, Tennessee on May 10, the Mississippi River crested at 47.9 feet (14.6 m), the highest level reached at Memphis since 1937 (48.7 feet or 14.8 m). In Greenville, Mississippi, the river crested on May 16, about one foot below the historical crest of 65.4 feet (19.9 m) set in 1927. The 2011 flood set a record in Vicksburg, Mississippi, cresting on May 18 at 57.1 feet (17.4 m) , besting the previous record of 56.2 feet (17.1 m) set in 1927. The flooded Mississippi also caused the Yazoo River to backfill, flooding out Yazoo City, Mississippi where some of the worst flooding occurred. Flood stage is 29 feet (8.9 m), but the crest reached 38.7 feet (11.8 m), just a few feet shy of the record set in 1927 of 43.4 feet (13.2 m).

Another stage height record was set farther downstream in Natchez, Mississippi on May 19. The crest was nearly 4 feet (1.2 m) higher than the previous record of 58.04 feet (17.7 m) set in 1937. To the south of Baton Rouge is the Atchafalaya Basin which, according to the U.S. Geological Survey, is the largest swamp in the United States. When the Morganza Spillway was opened as much as 1.2 million gallons of water per second flooded into the basin encroaching upon Morgan City which is perched on the banks of the Atchafalaya River. The river gauge at the riverside city, home to nearly 13,000, experienced its maximum peak at 10.35 feet (3.2 m) on May 29, just shy of the record of 10.53 feet (3.2 m) set in 1973.

Due to drastic steps of the Army Corps of Engineers that sacrificed farmland and less populated cities, major flooding was averted in the more populated cities along the southernmost sections of the Mississippi River. While the slow surge of water had dispersed some by the time it made it to Baton Rouge (flood stage is 35 feet or 10.7 m) and New Orleans (flood stage is 17 feet or 5.2 m), low-lying areas were still affected. The crest at Baton Rouge was more than 3 feet (0.9 m) below its record level and it was 5 feet (1.5 m) below the record level in New Orleans.

Missouri River

Additional flooding occurred later in the spring and into the summer along the Missouri River. The Missouri River is the longest river in North America and one of the largest tributaries of the Mississippi. Its headwaters begin in Montana and flow through several major cities including: Great Falls, Montana; Williston and Bismarck, North Dakota; Omaha, Nebraska and Kansas City, Missouri before joining with the Mississippi in St Louis, Missouri.

Record amounts of precipitation and melting snowpack contributed to historical flooding along the Missouri and its tributaries. The average rainfall across the state of Wyoming, which hosts several tributaries of the Missouri, was the most for any spring, based on records that data back to 1895. In eastern Montana, precipitation was 300 percent of normal for the month of May. The Missouri River basin experienced its fourth wettest spring and its third wettest May on record.

In Wyoming and Montana, for the month of May, a total 14 locations set precipitation records and seven locations set a new all-time record for the wettest 24-hour period for any month on record. In Glasgow, Montana, the monthly precipitation amount of 6.97 inches (177 mm) was a record in addition to the seasonal snowfall record of 108.6 inches (276 cm). This shattered the previous snowfall record of 70.7 inches (180 cm) set in 2006/2007. During the July 1 - June 30 snow season, Williston, North Dakota received a record 107.2 inches (272 cm) of snow. The previous record was 94.7 inches (241 cm) set during the 1895-1896 season. Record to near-record snowpack in the Northern Rockies and High Plains during the winter and spring contributed to high levels of runoff. The runoff quickly filled all six of the rivers reservoirs forcing the Corps of Engineers to release them. The swollen river breached levees, forcing mandatory evacuations downstream. Additional damaging flooding is expected to continue through early summer.

Lake Champlain

Flooding was not confined to just rivers and streams. Water levels at Lake Champlain, which straddles New York, Vermont, and Canada, also experienced historical crests. At Rouses Point, Vermont, water levels rose to 102.8 feet (31.3 m) on May 10 and remained at or near historical levels for several weeks. The previous record lake level was 102.1 feet (31.1 m) set in 1869 and the normal for this time of year is about 97 feet (29.6 m). Record amounts of rainfall and melting of winter snowpack in the state exacerbated the flooding. It was the wettest Spring on record in Vermont. In Burlington, it was the wettest May on record with 8.67 inches (220 mm) of precipitation - besting the previous record of 7.10 inches (180 mm) set in 2006. Warmer-than-normal spring temperatures also increased the melting and runoff of snowpack. In Burlington, 128.4 inches (236 cm) of snow fell this season, 45.3 inches (115 cm) more than normal.

Historical Perspective

The prolonged flooding during the spring of 2011 that affected the Ohio, Mississippi and Missouri River Valleys draws comparisons to the great floods during the early 20th century. During the fall of 1926, record precipitation amounts resulted in major flooding along the lower Mississippi in the spring of 1927. During the three-month period (March-April), Arkansas, Illinois, and Missouri each had their wettest such period in 117 years of record keeping. It was reported that the flood of 1927 submerged more than 165 million acres, drowning 246 people. Economic losses were estimated at 2.8 billion in 2010 dollars.

During the winter 1937, excessive precipitation during an 11-day period (January 13-24) contributed to flooding along the Ohio River. The state of Kentucky experienced a record 16.13 inches (410 mm) of precipitation in January. Individual locations had as much as 23 inches (584 mm) of precipitation during the month. From January 1-24 percent of normal precipitation in the area was approximately 600 percent of normal. Due to the copious amounts of precipitation, the Ohio River crested in Louisville at 85.4 feet (26 m). The Louisville flood stage is 51 feet (15.5 m). Further downstream in Paducah, the flood stage is 39 feet (11.9 m), but the river crested at 60.6 feet (18.5 m). It was reported that 3.3 billion in 2010 U.S. dollars worth of damage was done from the event.

As a result of a series of heavy snows in the Upper Midwest during the winter of 1972/1973 punctuated by heavy springtime rains in the South, the Mississippi swelled, overtopping its banks during the spring of 1973. Both Tennessee and Wisconsin had their wettest March-April period. Additionally, nearly every state east of the Rockies experienced above normal precipitation. Damages came to an estimated 256 million in 2010 U.S. dollars.

In the summer of 1993 frequent and excessive rainfall in the northern Plains southeastward into the central U.S. saturated soils, filling streams and rivers to capacity. For the summer period, record precipitation fell in Iowa, Illinois, Minnesota, Montana, Nebraska, and the Dakotas. Observed river crests in Iowa and Missouri easily surpassed previous record amounts by several feet. In St. Louis, at the confluence of the Missouri, Illinois and Mississippi rivers, the old record of 43.3 feet (13.2 m) set in 1973 was shattered on August 1, 1993 (49.7 feet or 15.1 m). The Missouri River also inundated towns. In Kansas City, the river set a new stage height of 48.9 feet (14.9 m), topping the old record of 46.2 feet (14.1 m) set in 1951. A total of 20 river gauges set all-time records. The devastating floods of 1993 are currently the costliest flooding disaster in the U.S. as damages neared 23 billion in 2010 U.S. dollars. In addition to the cost, more than 50 people were killed and at least 15 million acres of farmland were flooded. Other effects of the 1993 floods were: halted shipping on the Mississippi and Missouri for nearly two months, ten airports were flooded, all rail traffic in the Midwest was ceased, and both the 1993 and 1994 harvests were lost.

Impacts

The impacts of the 2011 flooding are far reaching, affecting economic sectors such as: agriculture, fishing, shipping, insurance, refineries, and tourism. Economic losses will take years to recoup. Hundreds of thousands of acres of farmland were flooded, creating a nightmare at a time of year when the growing season is just beginning for many crops. In Tennessee, corn planting has been delayed and the winter wheat crop, which is typically harvested in June, was damaged. It was reported that nearly 900,000 acres of farmland in Mississippi was flooded - roughly 10 percent of all farmland in the state. In Arkansas, it is estimated that the flood waters wiped out 1 million acres of farmland - a staggering number when you consider that the agriculture industry generates 16 billion dollars annually in Arkansas. In all, the floods washed out more than 3.5 million acres of farmland within the Lower Mississippi River Valley.

The dangerous floodwaters shut down river commerce, which would have also caused additional stress on the levees. In addition to river commerce, in eastern Arkansas a 23-mile section of Interstate-40, a major east-west thoroughfare, was closed. Businesses and homes were closed or swept away, leaving many without a job or personal belongings. As the event continues to unfold, estimated economic losses are mounting. Overall, total insured losses currently amount to approximately 2-4 billion dollars.

Selected Estimated Economic Losses

Amount	Type
$800 million	Agriculture in Mississippi
$500 million	Agriculture in Arkansas
$320 million	Damage in Memphis, Tennessee
$317 million	Agriculture and property in Missouri's Birds Point-New Madrid Spillway
$80 million	First 30 days of flood fighting efforts in Louisiana

Farming was not the only economic sector in the South that was heavily damaged. The amount of fresh water flowing into the Gulf of Mexico caused an imbalance in the ecosystem wiping out the oyster beds which need the salt water to keep their metabolisms in check. The large amounts of fresh water saturated with fertilizers, pesticides and other farming chemicals is expected to flow into the gulf causing a "dead-zone" according to Lt. Col. Mark Jernigan, deputy commander of the New Orleans District of the U.S. Army Corps of Engineers. A dead zone is as area with low oxygen levels caused by the increase of fertilizers which fuels the growth of algae.

It is too early to be able to give an exact figure of the damages done and economic loss of the 2011 flooding. However, we do know that it will take years to recover from the 6.8 million acres that were flooded. The flooding that occurred across the United States in Spring of 2011 will be one of the worst flooding disasters in modern American history.

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Drought and Wildfire

During March and April, drought and wildfires were the main headline across the Southern Plains of Oklahoma, Texas, and New Mexico. The track of storms from the Rockies into the Central and Northern Plains essentially cut off the Southern Plains from the Gulf of Mexico moisture source. The strong cold fronts and dry lines associated with the upper level troughs, which moved through the center of the country, brought strong winds and dry air into the region. The combination of warm, dry, and windy conditions worsened drought across the region and flamed out-of-control wildfires. The amount of precipitation during April across the Southern Plains stood in stark contrast to the record precipitation across the Ohio Valley, the record floods along the Lower Mississippi River, and the severe weather outbreaks across the Southeast.

Environmental Conditions

Starting in March, and lasting into April, a strong ridge of high pressure consistently setup across the Southern Plains, essentially blocking storms systems from entering the region. This led to Oklahoma and New Mexico having their top ten driest March, and Texas to have its driest March, on record. April brought continued dryness, with below-average precipitation for New Mexico, and Texas had its 5^th driest April on record. The abnormally dry conditions stretched back to October 2010 for the region, with the March-April 2011, February-April 2011, and October 2010-April 2011 periods being the driest such periods on record for Texas. Dryness across the southern tier of the U.S. is consistent with conditions expected during a winter La Niña. Under typical La Niña conditions, the jet stream and storm track are further north, leaving the southern U.S. under a ridge of high pressure. These impacts tend to be more pronounced during the winter months of a La Niña, and wane as spring progresses. During 2011, the worst of the dryness occurred later than what is typically expected with La Niña.

Drought Overview

During the March-April period little to no rain fell across portions of Texas and New Mexico. At the end of April, many locations had not received any rainfall in over 26 days, and this dry spell continued into May. In El Paso, Texas, May 24^th marked the 110^th consecutive day without precipitation in the city. This broke the previous record of 109 days set in 2002. The March Texas statewide average precipitation was 0.27 inch (6.86 mm), which is the driest March on record (1895-2011) for the state. The previous record for the state was set in 1971, with a precipitation total of 0.28 inch (7.11 mm). Precipitation totals throughout the state ranged mostly between 0 and 25 percent of normal. In the Trans Pecos climate division, 39 stations reported no precipitation. This was also the case for approximately half of the stations in the Lower Valley and Southern climate divisions. A similar scenario occurred in April, when the Texas statewide average precipitation was only 0.75 inch (19.05 mm), the fifth driest April on record (1895-2011). For the state, the majority of stations in the High Plains, Trans Pecos, Rolling Plains, South Central, Southern and Lower Valley climate divisions reported a precipitation total of 0 inches (0 mm). According to the U.S. Drought Monitor (USDM), drought conditions intensified considerably during the time period. A comparison of the spatial extent of drought at the beginning of March to the beginning of May across Texas and New Mexico can be found in the tables below:

Texas Percent Area of Drought

Week	No Drought	D0-D4	D1-D4	D2-D4	D3-D4	D4
March 1, 2011	4.99	95.01	72.91	46.34	12.72	0.00
May 3, 2011	0	100.00	98.86	93.99	73.73	25.96

New Mexico Percent Area of Drought

Week	No Drought	D0-D4	D1-D4	D2-D4	D3-D4	D4
March 1, 2011	7.79	92.21	63.97	33.32	0.00	0.00
May 3, 2011	0.00	100.00	96.54	87.36	61.02	13.63

Drought indicators across the Southern Plains show that the drought conditions during April were both a short-term and long-term phenomenon. The March Palmer Z index map shows a bull’s eye of severe and extreme short term drought across the Southern Plains and Southwest. Southern Arizona, most of New Mexico, Texas, and southern Oklahoma had the worst short-term drought conditions across the country. Little changed in April — low precipitation and hot temperatures prevailed across the same region. The April Palmer Z Index map shows that the low precipitation and hot temperatures resulted in short-term drought across much of the Southwest and Central to Southern Plains. The areas experiencing the most extreme short-term drought were central and southern New Mexico, stretching across most of Texas and along the western Gulf Coast.

The April 2011 Palmer Hydrological Drought Index (PHDI) map shows that the drought was a long-term phenomenon across most of the southern tier of the country. New Mexico, most of Texas, and northern Louisiana were experiencing severe to extreme drought, according to the PHDI. The Standardized Precipitation Index (SPI) which measures moisture supply also indicates that the dryness across the Southern Plains was both a short-term and long-term phenomenon, back to the 12-month (May 2010-April 2011) timescale, with the most severe dryness occurring in the last six months (November 2010-April 2011) across Texas and New Mexico.

Soil moisture, according to drought monitoring products from the USDA, indicated that the top soil across Texas and New Mexico was dry to very dry, and pasture lands were in poor to very-poor condition during April. Satellite monitoring of vegetation health indicated stress on vegetation across most of the Southern and Central Plains, especially over Texas during the month. According to May 1 USDA reports, roughly three-fourths of the pasture and rangeland in New Mexico and Texas was rated in poor or very poor condition, and about half so rated in Oklahoma. This is well above what is considered average and ranks as a record (based on 1995-2010 data) for this time of year. About three-fourths of the wheat crop was rated in poor to very poor condition in Oklahoma (77 percent), Texas (74 percent), and New Mexico (70 percent), and just under half so rated in Colorado (46 percent) and Kansas (45 percent). The wheat crop completely failed in parts of west Texas and significant agricultural losses were incurred in western Oklahoma. In Texas, 75 percent of oats and 41 percent of corn were rated in poor to very poor condition.

Historical Context

The spring drought of 2011 resembled the historical droughts of the 1930s and 1950s across the southern United States. It was the driest April, February-April, January-April, and November-April in the 1895-2011 record for several climate divisions in New Mexico and Texas. The spatial pattern of the current drought in the South is similar, as measured by the PHDI, to the spatial pattern of the early 1950s drought in some respects, and the intensity (short-term dryness) of the current drought is locally as severe, or more severe, than the 1950s drought. However, in terms of duration, the 1950s drought lasted much longer in Texas [climate division 4 (East Texas) and climate division 5 (Trans Pecos)].

For more information on drought conditions see the March and April State of the Climate Drought Report.

Wildfires

In a direct relationship to the drought conditions across the Southern Plains, wildfires ravaged many parts of Texas and New Mexico during the January-April period. April 2011 was the most active April in terms of wildfires this century (since 2000), when 1.79 million acres (724,000 hectares) burned across the country, with most of the activity occurring across Texas. Between January 1^st and April 30^th, 2.2 million acres (890,000 hectares) burned across Texas, a year-to-date record according to the Texas State Forest Service. In addition to the extremely dry conditions, very warm temperatures dominated the Southern Plains during April, creating very low relative humidity. Several Texas cities had a top five driest and warmest April on record, including Austin and San Antonio. The warm temperatures and lack of moisture dried the available wildfire fuels.

The dry spell across Texas and New Mexico began during October 2010, with several months prior to October bringing above-normal precipitation to the region. Summer and spring of 2010 were particularly wet, creating ideal growing conditions across the two states. This led to an abundance of shrub and grass growth. After several months of below-normal precipitation during the winter and spring, these small plants dried out, making the ideal fuel for rapid wildfire development and growth. In April 2011, many factors came together to create the perfect weather conditions for wildfire development: the abundance of dried undergrowth, very warm temperatures, two months of little to no precipitation, and strong winds. According to reanalysis, low-level winds across the Southern Plains were 2.2 mph - 4.4 mph (1-2 m/s) stronger than 1971-2000 mean. Analysis of the U-component (east-west) of the wind shows that westerly winds across western Texas and eastern New Mexico were 4.4 mph - 6.7 mph (2-3 m/s) stronger than average near the surface. This indicates an intrusion of dry continental air over the region. The persistent strong winds were associated with the same upper level low pressure systems that brought the precipitation and storms to the Ohio Valley and Southeast. On the west side of these troughs, strong westerly winds set up behind the cold/dry fronts associated with the low pressure systems. The winds also actted to fan the flames, making them much more difficult to control by firefighters.

Several of the wildfires across Texas and New Mexico burned out of control for weeks and destroyed hundreds of thousands of acres. Most of the fires were ignited by lightning strikes, although some were human caused, both intentionally and unintentionally. Several major population centers were also directly impacted across Texas including Ft. Worth, Austin, and San Angelo. Through the end of April, for the year-to-date period, 1,134 structures, including 244 homes, were destroyed by fires across Texas. In San Angelo, the Wildcat fire forced the evacuation of hundreds of people due to fears of the fires overtaking entire neighborhoods. Several of the individual fires exceeded 100,000 acres (40,500 hectares) in size. The Rock House fire, which burned near Fort Davis, Texas burned nearly 315,000 acres (127,475 hectares) of land and destroyed 41 homes and two businesses. The Rock House Fire was the largest observed in Texas for the year. Governor Rick Perry declared a state of emergency for several counties, and asked for federal funds to help the firefighting efforts, which were estimated at two million dollars a day at the worst point of the wildfire outbreak.

Impacts

According to the Texas AgriLife Extension Service, it is estimated that between November 2010 through May 2011, Texas ranchers lost 1.2 billion U.S. dollars because pastures have not greened across the state. The Texas wheat crop this year has been valued at 274 million U.S. dollars, which is less than half of the five-year average. In addition to the pasture land loss, livestock losses will also top one billion U.S. dollars due to lack of water and feed for cattle. Impacts from the 2006 drought exceeded 4.1 billion U.S. dollars across Texas and over 6 billion U.S. dollars nationwide. While the current drought impacts are not expected to exceed the impacts of the 2006 drought, if the poor crop conditions continue through summer 2011, the total economic loss will grow. The impacts of the wildfires across the Southern Plains were also large.

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Synopsis

Large-Scale Drivers

Large-scale climate and synoptic drivers played a role in the extreme weather events during April 2011, including the presence of La Niña, a persistent upper atmosphere storm track, and near historical sea surface temperatures across the Gulf of Mexico. Under a La Niña regime, an upper level ridge typically sets up across the southwestern U.S. and storms travel along the northern edge of the ridge and southward along the eastern side, bringing above-normal precipitation to the Canadian Border States and the Ohio Valley, but often blocks storms from entering the Southern Plains. This was the case during the 2010-2011 winter and into the spring, causing above-normal snowfall and snowpack across the Upper Midwest and parts of the Ohio River Valley. Once spring emerged across the region, the warm temperatures rapidly melted the snowpack, and combined with the continued above-average precipitation to cause rivers to swell. The limited storm activity across the Southern Plains and the Gulf Coast caused below-average precipitation during winter and spring across the region, and is consistent with expectations during La Niña. The nearly stationary ridge across the southwestern U.S. brought extremely dry and windy conditions to the Southern Plains during April. The windy conditions were reinforced by the active storm track to the north. When these storms intensified, wind speeds increased across New Mexico and Texas, causing rapid wildfire growth.

During spring, the seasonal re-emergence of warmer temperatures started across the southern regions of the U.S. and moved northward. During April, the boundary between the cooler air to the north and the warmer air to the south provided energy and dynamic forcing for storms to strengthen as they moved through the country. Ahead of these storm systems, warm southerly flow brought much-above-average temperatures and moisture from the Gulf of Mexico, where sea surface temperatures were about 1.0 °C (1.8 °F) above average. The above-average sea surface temperatures in the Gulf of Mexico contributed to above-average water vapor content in the atmosphere, increasing the amount of energy available for severe weather outbreaks. North of the temperature boundary, along the Ohio Valley, record precipitation fell, and to the southwest much-below-normal precipitation was observed. For the month of April, record statewide precipitation amounts fell in Indiana, Illinois, Kentucky, Ohio, Pennsylvania, and West Virginia, while much-below-normal precipitation was observed across Texas.

Conclusion

The extreme weather and climate events of April 2011 were far reaching. There was significant loss of life and economic impacts across the country, most of which will not be fully realized for months. Preliminary estimates place the total damage of property and economic impacts at over 20 billion U.S. dollars for all weather-related disasters during spring. The individual extreme events - tornado outbreaks, flooding, and drought/wildfires - have all been compared to the worst such events in U.S. history. In each of the previous ‘worst cases in history’, they occurred during different years, while several of these historical records were broken during the month of April 2011.

Spring 2011 Billion Dollar Weather Disasters*

Date(s)	Event	Estimated Damages
April 14th-16th	Severe Weather Outbreak	~1.7–3 billion U.S. dollars
April 25th-30th	Southeast Tornado Outbreak	~5–6 billion U.S. dollars
Spring	Mississippi River Flooding	~2–4 billion U.S. dollars
Spring	Texas Drought	~1.5–3 billion U.S. dollars
May 22nd	Joplin, Missouri Tornado	~1–3 billion U.S. dollars

*Based upon preliminary data

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