There is a strong relationship between high temperatures and soil moisture. Evaporation and evapotranspiration increase with temperature, resulting in drier soils. Drier soils allow temperatures to reach even higher levels. This feedback effect can last through an entire season.
The USDA recently provided this image showing the number of 100 degree days over the summer months of June, July, and August. As you might suspect, the second image shows well-below-normal soil moisture anomalies for August.