Soil Moisture and Atmospheric Evolution Across Oklahoma During the 2011 Drought

To develop a better understanding of drought evolution across Oklahoma during 2011, an analysis of observational in situ data from the Oklahoma Mesonet in combination with the Drought Monitor was completed for the period spanning March to September. In particular, the primary focus of the study was precipitation distribution and soil moisture evolution during the period. In the early stages of the drought, the Drought Monitor showed severe drought conditions over southwest Oklahoma, with most of the state having abnormally dry to moderate drought conditions. Further, drying occurred in the soil moisture profile over western Oklahoma and transitioned south to north. Significant precipitation occurred in eastern Oklahoma during April and May, relieving drought conditions and creating a west to east dry to wet gradient in the soil moisture. Land-atmosphere coupling during this portion of the study period resulted in the anomalous eastward propagation of the dryline to the location of the greatest soil moisture gradient which further impacted convective initiation and downstream precipitation across eastern Oklahoma. The intense daytime temperatures and abnormally high near-surface winds during June resulted in rapid drying of the wet soil over most of the state. Soil moisture analyses demonstrated that drying moved from west to east and initiated at the gradient of soil moisture in western Oklahoma. With little precipitation in this same period drought rapidly developed along the evolution of the soil moisture distribution. As a result, while long-term drought was present prior to and throughout the period across western Oklahoma, the evolution of (flash) drought conditions in eastern Oklahoma, from no drought designation to severe/exceptional drought conditions occurred within weeks spanning the end of June into early July. As a result, while limited precipitation through July and August fell across eastern Oklahoma, by the end of the study period most of the state was in exceptional drought status and soil moisture conditions statewide were anomalously dry through the vertical profile.