August 2009 Drought Report

In total, drought coverage across the United States held fairly constant during August, as small areas of improvement were more or less counter-balanced by regions in which drought emerged or intensified. The end-of-month drought footprint of 14.2 percent (of the contiguous U.S.) was up only slightly from the July value of 13.8 percent. This number remains fairly low relative to the history of the U.S. Drought Monitor (since 1999). In the more intense categories of drought, the overall size of the footprint and the placement of the features remained near constant, with only minor variations.

Three of the nation's most significant ongoing drought episodes persisted or worsened through August. Extreme and Exceptional Drought remained entrenched in Central and South Central Texas. Another drought episode featuring widespread D2 (Severe) Drought persisted in California and Nevada. This Western drought feature expanded to the north during August, where drought emerged or intensified along the Pacific coasts of Washington and Oregon. The eastern islands of Hawaii saw little improvement to drought focused there. A fourth area of Severe to Extreme Drought in Wisconsin and Minnesota improved somewhat, especially in the most intense areas.

The most notable drought eruption came in the desert Southwest, where Moderate (D1) drought emerged across half of Arizona when monsoon rains delivered less-than-typical precipitation. Pockets of Moderate Drought also appeared in the Carolinas.

Early August's general weather pattern was somewhat typical of summer, with the jet stream and associated storm track displaced to the northern U.S., and high pressure dominating the southern tier of states. However, on several occasions during the month, this pattern broke down, as cooler air advanced southward into the Plains and Midwest. The associated upper-level trough persisted for much of the month. The Ohio Valley received the most substantial precipitation from the combination of frontal boundaries and abundant available moisture. This region's wetness continued a trend that had developed early in the summer. In contrast to the early summer months, August's significant precipitation expanded northward into the Upper Midwest, helping to abate intense drought in the western Great Lakes region. In the south, precipitation was spottier, as is typical of summer. Late in the month, the cool-air incursions reinforced each other, bringing rains further south and east, along the Atlantic coast. However, the late-month precipitation was not vast enough to stop the development of drought and dryness in the southern Atlantic states. In the Southwest, the monsoon feature was weaker than average, and this was largely responsible for the introduction of drought in Arizona.

By the end of August, core drought areas in the U.S. included:

• South Central Texas, where widespread extreme to exceptional drought remained entrenched;
• The western and southwestern U.S., notably:
  • most of California and western Nevada, where moderate to severe drought persisted;
  • western Oregon and western Washington, where moderate to severe drought emerged;
  • much of Arizona, where moderate drought emerged;
• Hawaii, where moderate to severe drought continued across the central and eastern islands;
• the western Great Lakes, where moderate to severe drought improved somewhat in Wisconsin and Minnesota.

Improvements to smaller drought areas occurred in western Oklahoma, southeastern Alaska, and Montana.

Reported topsoil conditions indicated typical summer dryness in parts of the United States, but few agricultural areas reported topsoil moisture deficits less than the long-term summer averages. According to observations from the US Dept. of Agriculture (USDA), four states (Texas, New Mexico, Oregon and Washington) reported that more than half of agricultural topsoil was in "Very Dry" condition. However, of these, only the New Mexico and Washington values were above the long-term average. It should be noted that California, Arizona and Nevada, where drought is pervasive, are not accounted for on these maps. In fact, California and Arizona reported a a large percentage of pasture and range land in poor or very poor condition. This rangeland dryness, relative to normal, stretched along the nation's southern border.

Streamflow conditions reported by the US Geological Survey (USGS) were largely consistent with other drought indicators east of the Rockies. Texas gauges were near historic lows for the season, while low flows spread up the southern Atlantic states and dominated Arizona and the coastal weatersheds of the Pacific Northwest. Deficits in northern Georgia were striking. Hawaiian streamflow levels were also quite low in most locations.

Five states (Arizona, Colorado, New Mexico, Texas, and Utah) observed August precipitation among the ten driest of the instrumental record (1895-2009), while no states placed among their ten wettest August values of the same period.

• 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
• hydrologic year precipitation
• snow water equivalent of snowpack
• 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

• The U.S. Drought Portal