Using the New Floating Month Drought Index to Monitor Extreme Moisture Spells and Assess Century-Scale Climate Change

The evolution of drought indices over the 20th century culminated in the U.S. Drought Monitor (USDM) as a drought monitoring tool that incorporated the various existing drought indicators, drought impacts information, and input from local field experts. A set of objective blends was created to integrate appropriately-scaled indices which assessed short-term and long-term moisture conditions. Unfortunately, the objective blends provide indeterminate information when short-term conditions are wet and long-term conditions are dry, or vice versa. The new Floating Month Drought Index (FMDI) improves upon the objective blends by including a temporal component. The FMDI computes the precipitation percentile for the current month and for the current N-month dry spell, the length and starting year/month of the current dry spell, and the Dx dry spell category based on USDM categories (and similar statistics for wet spells). In this way, the FMDI provides an objective decision-support tool for integrating the multiple time scales of drought. This presentation will discuss the development of the FMDI and how it can be used to assess changes in extreme moisture conditions on regional and national scales over the 20th to 21st centuries.