A strong ridge of high pressure maintained its grip across the central part of the contiguous United States during this U.S. Drought Monitor week of Aug. 21 to 27. It was responsible for warmer-than-normal temperatures that stretched across the Plains and into the Upper Midwest.
Upper-level troughs of low pressure dominated the West and East Coasts, keeping weekly temperatures cooler than normal on both ends of the country. Pacific weather systems spread above-normal precipitation over northern California to the Pacific Northwest as they moved through the western trough, then triggered bands of thunderstorms over the Rockies and central to northern Plains as they bumped up against the ridge. In between the West Coast and Rockies rain areas, the West was dry from southern California to northern Montana.
Rain developed along a stationary front that was draped across Florida. But for most of the CONUS east of the Rockies, the week was drier than normal with little to no rain falling from western Texas to the Mid-Atlantic Coast. The ridge migrated eastward as the week ended, so warmer-than-normal temperatures spread into the Midwest and Southeast.
Abnormal dryness and drought expanded and intensified across the southern Plains and Tennessee and Lower Mississippi Valleys in a rapidly developing flash drought situation, as well as parts of the Southeast, Mid-Atlantic, Midwest, northern Plains, and Far West. Exceptional drought developed in parts of Ohio and West Virginia for the first time in the 25-year USDM history. Hurricane Hone’s rains brought improvement to most of the main Hawaiian Islands.
The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the U.S. Department of Agriculture and the National Oceanic Atmospheric Administration. (Map courtesy of NDMC.)
South
Hot and dry conditions continued for the South region this week. Western parts of Texas and Oklahoma were the epicenter of the heat, with weekly temperatures averaging 6 to 10 degrees Fahrenheit above average, with local areas even warmer. In the east, Arkansas, averaged 2 to 4 degrees cooler than normal. Parts of coastal Texas to the state’s interior received half of an inch to locally 2 inches of rain, but this mostly fell on drought-free areas.
Most of the South region had no rain this week. Abnormal dryness and moderate to severe drought expanded in all of the states, while extreme drought expanded in Texas and developed in Oklahoma. The heat and dryness of this August have resulted in flash drought conditions. Summer last year was a period of record and near-record heat and dryness. These extreme conditions of these two periods have combined to overwhelm the wet conditions that happened during the intervening months.
More than half of the topsoil and subsoil moisture was short or very short across all states, with Texas leading at 72% of the subsoil and 81% of the topsoil so rated. Arkansas was rated at 63%. In Texas, 41% of the cotton crop and 58% of the pasture and rangeland was in poor to very poor condition. Drought impact reports in Oklahoma included desiccated fields, dry ponds, and a high risk of wildfires, as well as low reservoir levels in the southwestern part of the state.
Midwest
Average weekly temperatures ranged from 6 to 8 degrees warmer than normal in northwestern parts of the region to 4 to 6 degrees cooler than normal in southeastern parts. Half of an inch to an inch of rain fell over western and northern parts of the Midwest, with locally 2 inches or more in Minnesota. The heaviest rain fell over drought-free areas. Abnormal dryness expanded in Iowa and Minnesota; and abnormal dryness and moderate drought expanded in Missouri.
According to U.S. Geological Survey data, the Hocking River at Enterprise, Ohio, measured the fifth lowest river level on record, and the lowest measurement ever (since 1997) at Hocking River at Athens.
High Plains
Weekly temperatures were warmer than normal across most of the High Plains region, ranging from near to 2 degrees below normal in western Colorado to 6 to 10 degrees above normal in parts of Nebraska and the Dakotas. Thunderstorm complexes moved across parts of the region, dropping locally 2 to 3 inches of rain.
In many cases, the storms were localized with one part of a county receiving rain and another part getting nothing—this was the case in other parts of the country as well. Drought or abnormal dryness contracted in a few areas of Colorado, Kansas, Nebraska, and South Dakota, but expanded or intensified in parts of all of the High Plains states. U.S. Department of Agriculture statistics indicate half or more of the topsoil is short or very short in Wyoming (73%), Colorado (52%), and Kansas (52%), and half or more of the subsoil is short or very short in Wyoming (81%) and Kansas (57%).
West
Most of the West region was cooler than normal, with temperatures more than 10 degrees below normal across interior portions. In eastern areas were warmer than normal, up to 6 or more degrees above normal in eastern Montana and eastern New Mexico.
About 0.5 to 2 inches of rain fell over large parts of the Four Corners states. The rain that fell was not enough to make up for deficits that have accumulated over several months to more than a year, so no improvement to the depiction was made. Moderate to extreme drought expanded in Montana.
According to USDA statistics, half or more of the topsoil moisture was short or very short in Montana (82%) and New Mexico (62%), and Nevada (55%) and half or more of the subsoil moisture was short or very short in Montana (82%) and New Mexico (63%).
Looking ahead
In the two days since the Tuesday valid time of this USDM, scattered showers and thunderstorms brought areas of rain to a few parts of the Southwest, northern Rockies, northern and southern Plains, and Midwest, but the rest of the CONUS was mostly dry. For Aug. 29 to Sept. 3, an upper-level ridge will build over the West, bringing warmer- and drier-than-normal weather, while a weather system moves across the eastern CONUS and a weather disturbance lingers over the western Gulf of Mexico Coast.
An inch or more of rain, with locally over 2 inches, is forecast for the southern Plains. Four or more inches could fall over parts of the southern Plains, New Mexico, and western Gulf Coast. The rain will help to improve drought conditions in the Deep South and central Appalachians, but won’t be enough to end the drought. The Rockies to West Coast, and western High Plains, are forecast to receive no precipitation during this period.
For much of the next 2 weeks, a ridge will remain anchored over the West with a trough along the East Coast, while a couple weather systems try to move through this upper-level blockade. The Climate Prediction Center’s 6-10 Day Outlook (valid Sept. 3 to 7) and 8-14 Day Outlook (valid Sept. 5 to 11) favor warmer-than-normal temperatures across the West, central and northern Plains, and along the Gulf of Mexico Coast. Near to cooler-than-normal temperatures are expected over parts of the southern Plains and from the Ohio Valley to East Coast. Odds favor below-normal precipitation across most of the West, the northern tier states, the Midwest, the northern and central Plains, and Hawaii. Odds favor above-normal precipitation across the Gulf of Mexico Coast.
Richard Heim is with the National Oceanic Atmospheric Administration and National Centers for Environmental Information.
