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 six US states that have no NOAA Atlas 14 coverage (ID, MT, OR, TX, WA, WY), precipitation frequency estimates for the AEP analyses come from the 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 NOAA Atlas 14 CONUS product is available in a NetCDF format and can be downloaded from the Unidata Program Center's THREDDS Data Server (TDS) using the following link: NOAA_Atlas_14_CONUS.nc. 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.