Pennsylvania Water Science Center
The Palmer Drought Severity Index is a value computed as a function of both meteorological and hydrologic data to measure soil moisture conditions. It is compiled weekly by the Climate Prediction Center of the U.S. National Weather Service for each of their climatological regions. The computed value ranges from +6.0 to -6.0. Pennsylvania uses values between -2.0 and --2.99 to indicate watch, between -3.0 and -3.99 to indicate warning and -4.0 or below to indicate emergency.