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Exceedance Probability Analysis for Selected Storm Events



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).


Event AEP map duration(s) Shapefile
Post-Tropical Depression Ida, Northeast, 31 August - 2 September 2021  3-Hour, 6-Hour, 12-Hour, 24-Hour link
West Central Tennessee, 20-22 August 2021  12-Hour, 24-Hour link
Imelda, 16-20 September 2019  12-Hour, 24-Hour, 48-Hour link
Remnants of Barry, Arkansas, 15-16 July 2019 24-hour link
South-Central Nebraska, 8 July 2019 6-hour link
Arkansas River Basin, April - May 2019 30-day link
Hurricane Florence, 13-18 September 2018 72-hour link
Michigan and Wisconsin, 14-18 June 2018 6-hour, 24-hour, 72-hour link
Ellicott City, Maryland, 27 May 2018 3-hour
Hurricane Maria, 20 September 2017 12-hour link
Hurricane Harvey, 25-31 August 2017 4-day  
Missouri, 28 April - 2 May 2017 48-hour link
Hurricane Matthew, 6-10 October 2016 12-hour link
Louisiana, 11-13 August 2016  48-hour  
Ellicott City, Maryland, 30 July 2016  3-hour  
Northern Wisconsin, 11-12 July 2016 6-hour  
West Virginia, 23-24 June 2016  24-hour  
Lower Mississippi River Valley, 8-12 March 2016  48-hour  
Corsicana, Texas, 24-25 October 2015  24-hour  
Austin, Texas, 30 October 2015  3-hour, 6-hour  
South Carolina, 2 - 4 October 2015  24-hour, 72-hour  
Central Texas, 23-24 May 2015  3-hour, 6-hour  
Oklahoma, April - June 2015  20-day,30-day, 60-day  
Phoenix, Arizona, 19 August 2014  12-hour  
Islip, New York, 13 August 2014 3-hour  
Pensacola, Florida, 29-30 April 2014  6-hour  
New Mexico, 9-16 September 2013 7-day  
Colorado, 9-16 September 2013 24-hour, 48-hour, 7-day  
Southern Missouri, 29 July - 8 August 2013 10-day  
San Antonio, Texas, 25 May 2013  6-hour  
Oklahoma City region, 31 May-1 June 2013  4-hour, 6-hour  
Tropical storm Debby, 24-27 June 2012  48-hour  
Duluth, Minnesota, 19-20 June 2012  24-hour  
Tennessee, 1-2 May 2010  48-hour  
Southeastern New England, March 2010  20-day  
Southeastern United States, September 2009  24-hour, 4-day, 7-day  
Ohio Valley, 23 - 27 March 1913  96-hour  



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.


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