National Overview - May 2011
Drought and Wildfire
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.
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 5th 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.
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 24th marked the 110th 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:
|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|
|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.
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)].
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 1st and April 30th, 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.
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.