Drought - May 2010
Contents Of This Report:
National Drought Overview
Detailed Drought Discussion
May 2010 was wetter than normal with near-normal temperatures when weather conditions are averaged across the country, with the national precipitation ranking in the top one-third of the historical distribution (36th wettest). But considerable variability occurred throughout the month (weeks 1, 2, 3, 4) and on a regional basis. Beneficial rain and snow fell across parts of the West, while much of the Southwest, Lower Mississippi Valley, southern Plains, and Northeast, and parts of the central Plains to western Great Lakes, were drier than normal. The dryness in the Northeast was accompanied by unusually warm conditions which enhanced evaporation. Prolonged dryness continued over the western Great Lakes and Hawaii. By the end of May, drought or abnormally dry areas intensified or expanded in the western Great Lakes, Northeast, Lower Mississippi Valley, Hawaii, and Alaska.
The weather pattern for May was a typical active spring pattern, with many cold fronts and Pacific low pressure systems moving across the country in the westerly flow. Precipitation occurred along favored storm tracks, especially in northern and Midwestern states. During the last half of the month, a warm and dry high pressure ridge became entrenched over the eastern half of the country with a cool and wet trough persisting over the West (temperature and precipitation pattern for last week of May).
By the end of May, core drought areas in the U.S. included:
- parts of Hawaii, where moderate (D1) to exceptional (D4) drought persisted;
- the western Great Lakes, where drought intensified from moderate (D1) to extreme (D3) conditions;
- parts of the West, High Plains, and Lower Mississippi Valley, which had areas of moderate (D1) to severe (D2) drought; and
- parts of the Northeast and Alaska, where areas of moderate drought (D1) had developed or abnormal dryness (D0) had expanded.
The Palmer drought indices measure the balance between moisture demand (evapotranspiration driven by temperature) and moisture supply (precipitation). The Palmer Z Index depicts moisture conditions for the current month, while the Palmer Hydrological Drought Index (PHDI) and Palmer Drought Severity Index (PDSI) depict the current month's cumulative moisture conditions integrated over the last several months. Unusually dry conditions were evident on the May 2010 Palmer Z Index map over the Northeast, western Great Lakes, and Lower Mississippi Valley and adjoining southern Plains, with moderately dry conditions over parts of the Southeast. The May 2010 PHDI map indicates that the drought conditions over the Great Lakes and parts of the West were due to long-term dryness, whereas the Southeast dryness was a relatively recent development. The Z Index map also shows recent wetness in the Northwest where areas of long-term dryness persisted.
The Standardized Precipitation Index (SPI) measures moisture supply. The six SPI maps here show the spatial extent of anomalously wet and dry areas at time scales ranging from one month to 24 months. Parts of the Northeast have been dry for the last one to three months and dryness is evident in the Southeast at the two to three-month time scales. The Lower Mississippi Valley and adjacent southern Plains have been dry for the last one to six months. Long-term dryness has afflicted the Great Lakes for the last nine months (eastern Great Lakes) to at least the last 24 months (western Great Lakes). The last couple months have been wet across parts of the Northwest and northern Rockies, but dryness is evident across parts of the West at all time scales.
Did You Know?
Soil Moisture Water Balance Models
In an ideal world, measurements of soil moisture content — at several levels from the surface of the ground to five feet below or deeper — would be available on a daily basis for every backyard and field across the United States. This type of observation network would give us a good idea of how dry or wet the ground is and help tremendously with drought monitoring. Such a national soil moisture observation network doesn't exist. At present, a few hundred soil moisture stations are scattered around the country. So, water balance models are run on a gridded spatial scale in order to get an idea of what the national soil moisture conditions are. Examples of soil moisture water balance models include the "Leaky Bucket", North American Land Data Assimilation System (NLDAS), and VegET models. Water balance models typically use precipitation as the water supply component. They calculate a water demand component (evapotranspiration) using temperature and other variables such as humidity, wind speed, and insolation (solar energy). Then they calculate fluxes (how water and energy change over time and space) to estimate things like soil moisture, soil temperature, snow water content, and stream runoff. Some of the models use station soil moisture observations and satellite observations of surface wetness as "ground truth" for calibration. While not perfect, using a variety of models gives us a good idea of where soils are drying across the country, especially in areas where no soil moisture observations exist.
In Hawaii, the percent area of the state in moderate to exceptional (D1-D4) drought stayed about the same compared to the end of April, but the abnormally dry (D0) to exeptional drought (D4) percent area expanded from 68 percent to 88 percent by the end of May. This resulted from below-normal May precipitation at most stations across the islands, marking a continuation of sub-normal rainfall which has characterized much of 2010 and the last 6 to 12 to 24 and even 36 months. Agricultural issues continued to plague the Big Island with below-normal streamflow and rapidly dropping reservoir levels — irrigators were looking at a 30 percent reduction of water on Oahu.
Most stations in Alaska reported below-normal precipitation and above-normal temperatures for May 2010. The drier-than-normal pattern is evident at most stations for the year so far, the hydrological year to date (October to present), and last 12 months. Interior basins had below-normal streamflow and snowpack water content, and numerous large wildfires had developed by the end of the month (details can be found at the U.S. wildfire report).
Precipitation was generally above normal across Puerto Rico during May 2010, resulting in above-normal streamflow. Below-normal precipitation was evident across parts of the East and South for the water year to date (October-May), but it was not as evident at other time scales (last 60 or 180 days).
On a statewide basis, May 2010 was drier than normal for states in the Northeast, Southwest, and southern Plains. Spring 2010 (March-May) ranked in the top ten driest for one state (Louisiana). The excessive dryness for the year to date (January-May), however, resulted in a top ten driest rank for Louisiana (fifth driest), Michigan (fifth driest), and Wisconsin (sixth driest). Spring (March-May 2010) has been unusually dry across parts of the Northeast, but the dryness has been accompanied by record and near-record warmth both for spring and the year-to-date, which increased evaporation and enhanced the dryness.
Record dryness occurred across parts of the western Great Lakes and Lower Mississippi Valley. March-May 2010 was the driest spring in the 1895-2010 record for West Upper and East Upper Michigan (climate divisions 1 and 2, respectively) and North Central and Northeast Louisiana (climate divisions 2 and 3, respectively). East Upper Michigan (climate division 2) also had the driest year-to-date and last nine months (September-May).
Abnormal dryness and drought were evident in several indicators. Much of the Southwest, and parts of the southern Plains and Upper Peninsula of Michigan, had just a few days with precipitation during May 2010. The dry spell in the Southwest lasted most of the month, while dry spells lasted two weeks or longer in parts of the Northwest, southern Great Plains, Northeast, and Michigan's Upper Peninsula. Soil moisture, as observed (percent of topsoil dry and very dry, and comparison to 5-year average and 10-year average) and as monitored by several models (NOAA Climate Prediction Center anomalies and percentiles, Leaky Bucket, NLDAS [North American Land Data Assimilation System] top soil layer and total soil layer), was drier than average across much of the Great Lakes to Northeast States, Southwest to Lower Mississippi Valley, and Alaska, and parts of Hawaii and the West.
Satellite monitoring of vegetation health (Vegetation Drought Response Index, Vegetation Health Index) indicated stress on vegetation from the Upper Mississippi Valley to the Great Lakes and in parts of Alaska, and lingering stress in parts of the West. The U.S. Geological Survey (USGS) agro-hydrologic model (Soil Water Index, Water Requirement Satisfaction Index) indicated stressed vegetation across much of the Southwest and parts of the southern Plains, Southeast, and western Great Lakes. U.S. Department of Agriculture (USDA) pasture and rangeland condition reports (percent in "poor" or "very poor" condition, anomaly) revealed just a few states with deteriorating (above average percent in "poor" or "very poor") conditions. May 30th USDA reports indicated that 69 percent of the nation's rangeland and pastures were rated in good to excellent condition, while only seven percent were rated very poor to poor. A year ago, those numbers stood at 58 and 16 percent, respectively. At the end of May, row-crop conditions were better than this time last year for all major U.S. commodities, including corn (76 percent good to excellent on May 30), winter wheat (65 percent), spring wheat (85 percent), cotton (63 percent), and rice (74 percent).
More well monitoring stations in the Northeast were at low levels at the end of May (real-time network, climate response network, total active network) compared to the end of April (real-time network, climate response network, total active network), indicating drying conditions. There were also sporadic reports of low groundwater in parts of the West, Great Lakes, southern Plains, Southeast, and Hawaii. Streamflow (observed and modeled [CPC, VIC]) was below average across much of the Great Lakes, New England, northern Rockies, and Hawaii, and parts of the Southwest, Deep South, and Alaska.
Percent area of the Western U.S. in moderate to extreme drought, January 1900-May 2010, based on the Palmer Drought Index.
October through the following September is defined as the water year for the West. Water-year-to-date (October-May) precipitation has been mixed at low elevation stations, but a spatial pattern of above-average conditions in the southern portions of the West and below-average conditions in the northern portions is evident at high elevation (SNOTEL network) stations. Mountain snow associated with low pressure systems moving in a late spring cold upper-level circulation pattern increased the snowpack in many high elevation locations, resulting in a late-season snow water content pattern that was mixed (WRCC depiction, USDA depiction). The snow water content ranked in the wettest five percent for several locations, but the end of May is late in the snow season where normals rapidly decrease and records are readily set. Winter and spring mountain snowpack provides a crucial water source for the summer melt season across much of the West. An analysis by the USDA indicated that reservoir levels were generally below average across much of the interior basins. According to the USDM, 14 percent of the West was experiencing moderate to severe drought at the end of May, a decrease from April, while the Palmer Drought Index statistic had dropped to 3 percent.
As explained by the Midwest Regional Climate Center, most of the region received normal to well above normal precipitation in May with the exception of a band from western Iowa northeastward across the Upper Peninsula of Michigan. Precipitation in this band was generally only 50 to 75 percent of normal. The area of Extreme Drought depicted on the USDM in northeastern Wisconsin and the Michigan Upper Peninsula expanded by the end of the month. In contrast, precipitation over northwestern Minnesota and much of Kentucky was twice normal. Spring precipitation was normal to above normal from Missouri into the southeastern half of Iowa, northwestern Illinois, and across northern Indiana and southern Lower Michigan. In contrast, precipitation was less than 50 percent of normal across the Upper Peninsula of Michigan.
As noted by the High Plains Regional Climate Center, May 2010 was cooler than normal and precipitation was variable over the Region this month. A large swath of below normal precipitation stretched from the four corners region northeast through portions of Nebraska, Kansas, and South Dakota. Western Wyoming, where drought conditions were ongoing, also received below normal precipitation. Areas receiving at least 150 percent of normal include much of North Dakota, western and central South Dakota, central Wyoming, and pockets of Kansas. The precipitation in south central Kansas and western North Dakota helped eliminate the abnormally dry conditions which had developed in April. Heavy snows across Nebraska, Colorado, and Wyoming led to record breaking seasonal snowfall totals in some locations. One strong storm occurred May 11-12 and brought snow to Wyoming, Colorado, and Nebraska. In central Wyoming, unofficial reports indicate that up to 40 inches (101.60 cm) of snow fell on the east slopes of the Wind River and southern Absaroka mountains. The late season snowstorm also bumped Scottsbluff, Nebraska up to the snowiest season on record and Cheyenne, Wyoming up to the second snowiest season on record. Scottsbluff, Nebraska received 82.2 inches (208.79 cm) of snow which beat out the old record of 78.5 inches (199.39 cm) recorded during the 1979-1980 snow season (period of record 1893-2010). Cheyenne, Wyoming received 103.4 inches (262.64 cm) of snow this season which came in second to the record 121.5 inches (308.61 cm) of snow received in the 1979-1980 snow season (period of record 1883-2010).
This month, abnormally dry conditions (D0) were eliminated in central Kansas and the northwest corner of North Dakota. Although abnormally dry to moderate drought conditions (D0-D2) remained in western Wyoming, much of the drought conditions in central Wyoming were eliminated or reduced in intensity due to the late season snow storms.
As summarized by the Western Regional Climate Center (WRCC), winter seemed reluctant to relinquish its grip throughout the West in May with numerous cold fronts ushering in cool and blustery weather for the month. Temperatures were below normal for the western U.S. except for a small portion of New Mexico. Despite the many cold frontal passages, and often unstable atmosphere, precipitation was mostly below normal except for pockets of the Pacific Northwest, California and eastern Montana. Thanks to cool, cloudy weather and occasional snowfall throughout April and May in the Sierra Nevada of California, the percent of normal snow water equivalent increased from 90 percent on April 1 to 120 percent on May 1 to roughly 150 percent on June 1. Snow melt was delayed throughout the rest of the West as well with percent of snow water equivalents increasing from 50 percent on May 1st to over 100 percent in the Pacific Northwest and northern Rockies by the end of the month. The increase in percentage was less through addition than through minimal snow loss divided by decreasing long-term daily climatologies.
As pointed out by the USDA, the reservoir level at Lake Mead (in northwest Arizona) at the end of May 2010 was near record low levels. While much of the Colorado River basin has recovered, in meteorological terms, from the drought of the last decade, ever-increasing demands on the basin's water from a variety of user interests have served to prolong the period of extremely low lake levels.
As explained by the Southern Regional Climate Center, the month of May was quite dry throughout most of Louisiana, southwestern Oklahoma and eastern Texas, where most stations reported less than half the expected monthly precipitation. Slightly drier conditions were observed in the western panhandle of Texas, with most stations reporting less than 25 percent of normal precipitation. In total, the state of Texas averaged 2.25 inches (57.15 mm) of precipitation for the month, which is their twenty-third driest May on record (1895-2010). In contrast, conditions were very wet in western and middle Tennessee and in north central Arkansas.
Drought conditions in the Southern Region changed dramatically over the past month. At the end of April, only 1.3 percent of the region was in moderate drought and no areas were experiencing severe drought. Drought conditions worsened during May due to abnormally dry conditions in the areas of north and central Louisiana and eastern Texas. As of June 1, 2010, 12 percent of the region experienced moderate drought or worse. The drought was confined to eastern Texas, Louisiana, and southwestern Mississippi. In Texas, moderate drought afflicted the eastern part of the state. In Louisiana, a bulk of the northern parishes were in a state of severe drought, with the severe drought covering 27.5 percent of the state.
As noted by the Southeast Regional Climate Center, precipitation was highly variable across the Southeast in May. Though the monthly average for the region as a whole was close to normal, there were many locations that were exceptionally wet (more than 300 percent of normal) and others that were exceptionally dry (less than 10 percent of normal). The precipitation pattern across the region was driven primarily by scattered showers and thunderstorms, some of which produced prodigious rainfall totals. San Juan, Puerto Rico experienced the 4th wettest May on record — this is now the wettest start to a calendar year ever at that location. In contrast, monthly precipitation was between 25 and 75 percent of normal across much of Virginia, western North Carolina, and along the Atlantic coast from southeast Virginia to northeast Florida. The driest locations (less than 10 percent of normal) were found along the south Florida coast and in the Pamlico Sound region of eastern North Carolina. New Bern, North Carolina and Melbourne, Florida experienced their 3rd driest May in records extending to 1948 and 1937, respectively.
The beneficial rainfall during the month helped to eliminate the areas of abnormal dryness (D0) in northern and southern Alabama and in central North Carolina. Much of central South Carolina and eastern North Carolina, however, remained abnormally dry at the end of May. This created concern among farmers in these regions, who were reaching critical planting periods for crops such as corn and soybean. In contrast, many farmers in Georgia reported a bumper peach crop in May due to the late spring warmth and above normal rainfall.
As summarized by the Northeast Regional Climate Center, May was a dry month in the Northeast. The average precipitation for the region was 3.08 inches (78.2 mm), or 77 percent of normal. Only West Virginia had a total that was above normal (118 percent). The driest (percent of normal) states were Delaware, with 42 percent of the normal May amount and Maine, with a precipitation total that was 50 percent of normal. The dry weather was accompanied by unusually warm conditions. Regional temperatures in the Northeast averaged above normal for the fifth consecutive month in May. It was also the third month in a row with above-normal statewide temperature averages in each of the twelve states in the region.
The 3-month total precipitation for Spring 2010 was 9.90 inches (251.5 mm), which was 89 percent of normal. Of the seven states with below-normal averages, New York was the driest, with 76 percent of the normal spring total. It was the 19th driest spring since 1895 in the Empire State. Among the major cities in the state, it was the 10th driest spring in Syracuse since 1902 and the 14th driest in Albany since 1874. Vermont's precipitation total was exactly normal and the remaining states (Massachusetts, New Hampshire, New Jersey, and Rhode Island) had 3-month totals that were above normal. Rhode Island was the wettest of the group with 160 percent of normal precipitation. The regional average temperature was 50.1 degrees F (10.1 degrees C), which was 4.6 degrees F (2.6 degrees C) above normal, resulting in the 2nd warmest spring since 1895.
Below normal precipitation in April and May left a large portion of the Northeast with abnormally dry conditions, according to the June 1, 2010 USDM. Southern Maine, northern Massachusetts and New York, western Pennsylvania, and all of New Hampshire and Vermont were the areas affected. A small area east of Lake Ontario was experiencing moderate drought conditions. In spite of the dryness, most state reports to the USDA National Agricultural Statistics Service told of above normal crop progress, good working conditions and a concern for potential fruit crop damage due to the early warmth and subsequent frost/freeze that occurred in May.
At the end of May, much of Maine was under a red flag warning for forest and brush fires. At the same time, smoke from fires in Quebec resulted in air quality alerts and reduced visibility from eastern New York to southeastern New England.
State/Regional/National Moisture Status
A detailed review of drought and moisture conditions is available for all contiguous U.S. states, the nine standard regions, and the nation (contiguous U.S.):
|northeast u. s.||east north central u. s.||central u. s.|
|southeast u. s.||west north central u. s.||south u. s.|
|southwest u. s.||northwest u. s.||west u. s.|
|Contiguous United States|
- Palmer Drought Indices
- Standardized Precipitation Index
- long-term (36 to 60 month) percent of normal precipitation maps
- airport station percent of normal precipitation maps
- statewide precipitation rank maps
- Cooperative station percent of normal precipitation maps
- percent of average maps for the SNOTEL stations in the western mountains provided by the Western Regional Climate Center
- satellite-based observations of vegetative health
- National Weather Service model calculations of soil moisture, runoff, and evaporation
- National Weather Service model calculations of soil moisture using the Leaky Bucket Model
- Midwest Regional Climate Center model calculations of soil moisture
- topsoil moisture conditions observed by the USDA and mapped by the Climate Prediction Center
- pasture and range land conditions observed by the USDA and mapped by the Climate Prediction Center
- streamflow maps maintained by the USGS
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