"Rainfall in any form will provide some drought relief. A good analogy might be how medicine and illness relate to each other. A single dose of medicine can alleviate symptoms of illness, but it usually takes a sustained program of medication to cure an illness. Likewise, a single rainstorm will not break the drought, but it may provide temporary relief.
A light to moderate shower will probably only provide cosmetic relief. Its impact is short term. Thunderstorms often produce large amounts of precipitation in a very short time, and most of the rain will run off into drainage channels and streams rather than soak into the ground.
Soaking rains are the best medicine to alleviate drought. Water that enters the soil recharges ground water, which in turn sustains vegetation and feeds streams during periods when it is not raining. A single soaking rain will provide lasting relief from drought conditions, but multiple such rains over several months may be required to break a drought and return conditions to within the normal range."
- Groundwater Storage - The Water Cycle (Webpage), U.S. Geological Survey
An article on the storage of groundwater and the place of groundwater in the water cycle.
- Physical Processes that Cause Drought (Website), NASA Earth Observatory
Part of a NASA piece on drought in North America, explaining the complex interplay of processes that cause and relieve droughts.
- WaterWatch (Map tool), U.S. Geological Survey
Provides state drought watch information.