HDSC Annual Exceedance Probability

HDSC analyzes annual exceedance probabilities (AEPs) for selected significant storm events for which observed precipitation amounts for at least one duration have AEP of 1/500 or less over a large area.

AEP maps have been created for the events listed below for selected durations that show the lowest exceedance probabilities for the largest area. For most recent events, associated shapefiles can be downloaded using the links provided in the table. Because the beginning of the observation period for a selected duration is not identical across the area, the AEP map does not characterize isohyets at any particular point in time, but rather within the whole event. For some events, the maximum observed precipitation amounts were also compared with corresponding frequency estimates across a range of durations at central gauged location(s).

UNDERLYING DATA

The underlying data for the AEP analyses are grids of observed precipitation data and precipitation frequency estimates at 30-arc second resolution for a range of durations and AEPs.

Observed Precipitation Data

Whenever possible, gridded precipitation data are developed for a range of durations from measurements collected from rain gauges reporting at the time when the map is created. Rain gauges are usually from the National Centers for Environmental Information's- NCEI's Climate Data Online. When rain gauges do not provide sufficient information to depict spatial patterns, the NCEI's multi-sensor Stage IV QPE Product and radar-based NEXRAD Precipitation product are also used to represent observed precipitation data.