HDSC creates maps of annual exceedence probabilities (AEPs) for selected significant storm events that typically have AEPs of less than 0.2% (i.e, exceed 500-year average recurrence interval (ARI) amounts).

• Underlying data. The underlying data for these analyses are rainfall observations and point rainfall
frequency estimates for a range of durations and AEPs (ARIs). Rainfall observations are collected from all available reporting rain gauges at the time when the map is created. We also look at related radar images to better represent spatial patterns. Point rainfall frequency estimates come from the latest NWS precipitation frequency document, usually from NOAA Atlas 14.

• Map development. We look at a range of durations and select the one that shows the lowest
exceedance probabilities for the largest area, i.e., the worst case. The beginning and ending of the worst case observation period is not necessarily the same between observing gauges. As a result, these maps do not represent isohyets at any particular point in time, but rather within the whole event.

• Maps have been created for the following events:

1. Tennessee, 1-2 May 2010. Average recurrence intervals for the 48-hour rainfall (created in May 2010).


2. Duluth, Minnesota, 19-20 June 2012. Annual exceedance probabilities for the worst case 24-hour rainfall (created in July 2012).


3. Tropical storm Debby, Florida and Georgia, 24-27 June 2012. Annual exceedance probabilities for the worst case 48-hour rainfall (created in July 2012).


4. Ohio Valley, 23-27 March 1913. Annual exceedance probabilities for the worst case 96-hour rainfall (created in September 2012).