Data Formats Post-March 22nd, 2017 - Derived from Stage IV QPE

Tips for using these precipitation data formats on common GIS platforms are available here.

The new QPE NetCDFs generated from the NCEP Stage IV data are CF-compliant multi-variable NetCDFs. The variables they contain are:

• observation - Last 24 hours of QPE spanning 12Z to 12Z in inches
• normal - PRISM normals in inches (see "Normal Precipitation" section on the About page)
• departure_from_normal - The departure from normal in inches
• percent_of_normal - The percent of normal

The new QPE GeoTIFFs generated from the NCEP Stage IV data are multi-band GeoTIFF. The bands they contain are:

• Band 1 - Observation - Last 24 hours of QPE spanning 12Z to 12Z in inches
• Band 2 - PRISM normals - PRISM normals in inches (see "Normal Precipitation" section on the About page)
• Band 3 - Departure from normal - The departure from normal in inches
• Band 4 - Percent of normal - The percent of normal

In the output files, the special value of -10,000 indicates the cell is expected to have valid data, however, no data has been received. Since data is submitted by individual RFCs, if an RFC does not submit data for their area of responsibility, all the cells within the RFC will be filled with a value of -10,000 and display as dark gray on the mapping interface.

The output masks are used to determine which cells within the coverage domain are expected to contain precipitation data and may be downloaded here.

In all three masks, a value of 0 marks a cell that does not fall within the bounds of valid input data. A three digit value will indicate which RFC is responsible for providing data for a given grid cell. In many cases, any value greater than 1 can simply be considered a cell which should contain valid data. Lastly, the CONUS mask contains cells with a value of 1. These cells indicate they contain offshore readings from coastal RFCs. A value of 1 fills the cells in the Gulf of Mexico and of the eastern coastline of the United States.

To symbolize the downloadable accumulations to match the look of the mapping interface, use the prebuilt ArcMap layer files or the QGIS color map text files:

• ArcMap v10.5 QPE Symbology (NetCDF only)
• ArcMap v10.3 - v10.4 QPE Symbology (NetCDF only)
• QGIS QPE Symbology

*Note: The pre-built layer files, tested with ArcMap 10.5, does not support the "Classified" symbology type for the multiband GeoTiff files. When using QGIS (tested with QGIS 2.8.6-Wein), either file format can be used.

Applying symbology in ArcMap

1. Drag the NetCDF file into ArcMap
2. (optional) Select the appropriate variable in the NetCDF tab if it's not already asked for a selection
3. Open the Symbology tab of the layer's properties
4. Select the "Classified" symbology type in the left pane
5. Click the folder icon to import the symbology from a file or other layer
6. Click the folder icon in the "Import Symbology" window
7. Select the appropriate .lyr file for the dataset being viewed
8. Click "Ok"

Applying symbology in QGIS

1. Drag a NetCDF file or GeoTiff into QGIS
2. (optional) Select the appropriate band. If using GeoTiff, see band definitions above
3. Open the Style pane of the layer's properties
4. Select "Single band pseudocolor" as the Render Type
5. Click the folder icon for "Load color map from file"
6. Select the appropriate color map file for the dataset being viewed
7. Click "Ok"

Data Formats Pre-March 22nd, 2017 - Derived from Stage III QPE

The shapefile contains the following fields:

1. ID - a unique value for each grid bin
2. HRAPX - column number of the HRAP grid cell (higher numbers are further east)
3. HRAPY - row number of the HRAP grid cell (higher numbers are further north)
4. LATITUDE of the HRAP grid point
5. LONGITUDE of the HRAP grid point
6. GLOBVALUE - 24-hour precipitation value in inches. "-2" values correspond to "Missing Data", i.e. an incomplete dataset.

The second download option "Legend File" will help you reproduce the color scheme we are using. If using ArcView, you will need to copy it into your c:\esri\av_gis30\arcview\legend_avl\ directory and load it manually through the legend editor. This file does not change from day to day.

Projection files (.PRJ extension) is available at https://water.weather.gov/precip/archive/nws_precip_projection.prj

Please note, grids with no precipitation (i.e. 0.00") are not in the observed data shapefiles. A shapefile with all points is available at https://water.weather.gov/precip/archive/nws_precip_allpoint.tar.gz.

A precipitation data legend file can be downloaded at https://water.weather.gov/precip/archive/nws_precip_legend.avl.

A problem was discovered on August 9, 2007. The shapefile coordinates for each HRAP grid were indicative of the lower left corner of the grid rather than the center of the grid. The entire shapefile archive has been updated to correct the lat-lon issue. The shapefile https://water.weather.gov/precip/archive/nws_precip_allpoint_conversion.tar.gz shows both the old and new coordinates for each point.

The three netCDF files (one for CONUS, one for Puerto Rico, and one for Alaska), may be used independently of one another. Each file contains the following fields:

1. hrap_xor and hrap_yor - the lower left corner of the grid
2. hrapx and hrapy - grid dimensions
3. amountofprecip - an array with the 24-hour precipitation values in hundredths of a millimeter (a value of 2540 corresponds to 1 inch). "-1" values correspond to "No Data", e.g. data over the ocean. "-2" values correspond to "Missing Data", e.g. an incomplete dataset.
4. other metadata - date and time of creation, lat-lon of the bottom-left, bottom-right, top-right and top-left corners of the grids, et al.

NOTE -- Latitude and Longitude are not explicitly stated in the netCDF file. The second download option "HRAP-to-LatLon" contains source code for a C-program. The program contains a function that reprojects HRAP coordinates to Lat-Lon coordinates. The fully compiled program reads the netCDF files and writes ascii files with the following fields:

1. id - a unique value for each grid bin
2. hrapx - column number of the HRAP grid cell (higher numbers are further north)
3. hrapy - row number of the HRAP grid cell (higher numbers are further east)
4. latitude of the HRAP grid point
5. longitude of the HRAP grid point
6. value - 24-hour precipitation value in inches

To compile the C-program you will need to install the netCDF library on your computer. More information about using netCDF files (including the netCDF C-library) is available from the University Corporation for Atmospheric Research (UCAR).