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.

Precipitation Frequency Estimates

Except for the five US states that have no NOAA Atlas 14 coverage (ID, MT, OR, WA, WY) precipitation frequency estimates for the AEP analyses come from the NOAA Atlas 14 CONUS product. NOAA Atlas 14 CONUS product. This product combines NOAA Atlas 14 precipitation frequency estimates for durations between 60 minutes and 7 days and AEPs down to 1/1000 (or average recurrence intervals up to 1000 years) from NOAA Atlas 14 Volumes that cover contiguous US states. The estimates along the volumes' boundaries were altered to reduce discrepancies, which are unavoidable as each volume was completed independently and at a different time (for more information see Section 5 of the NOAA Atlas 14 documents).

The estimates for a user-specified area, defined by a latitude-longitude bounding box, can be accessed from the TDS using remote data access protocols, such as the Open-source Project for a Network Data Access Protocol (OPeNDAP). NOAA's Weather and Climate Toolkit (WCT) can be used to retrieve and display the data. Instructions on how to use this tool with the NOAA Atlas 14 CONUS product are available here.

There are many programming languages that can read and manipulate this data using the OPeNDAP standard; some of them are listed here. Two examples below show how to use MATLAB and R scripts for accessing, saving and plotting the data:
Example 1. Retrieving and plotting contours for 24-hour, 10-year NOAA Atlas 14 estimates for the coastal area around Florida (25 - 33 °N, 79.5 - 94 °W): MATLAB script (script in html); R script.
Example 2. Retrieving all NOAA Atlas 14 estimates and plotting the depth-duration-frequency curve for a selected location (this could also be done via the PFDS): MATLAB script (script in html); R script.