Monitoring Drought

U.S. Rain Days and Dry Days

Maximum Consecutive Dry Days in the past 7 days	Maximum Consecutive Dry Days in the past 30 days
Number of Rain Days in the past 7 days	Number of Rain Days in the past 30 days
Number of Days since Last Rain

The EROS Data Center of the U.S. Geological Survey computes the U.S. Rain Days/Dry Days Product Suite every week, based on data from the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS).

The maps are based on a new, experimental NWS product, Precipitation Analysis, which combines observed data (i.e., data collected with rain gauges and other equipment on the ground) with radar and satellite data. This technique allows researchers to produce credible estimates of precipitation even in areas where there is no data from the ground-level. Thus, precipitation for the entire area of the U.S. can be estimated. Previously, descriptions of precipitation were limited to records from collection points.

Technical Description

The U.S. Rain Days/Dry Days product suite consists of five graphic outputs and five image outputs in ARC/INFO grid format shown in the table below. 

The calculations for these products are performed daily at 0710 hours (Central Time). The input rainfall data is provided by the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS). A general description of the data can be found at http://www.srh.noaa.gov/rfcshare/precip_download.php
The source precipitation analysis data are equally spaced (~5 km) points in ARC/INFO shape file format. For this process, the data are first converted to a point coverage and then to grid format at 0.05 degrees resolution. At this step, each source point is now expressed as a grid cell. The spatial extent of the data is the conterminous U.S. (i.e. the lower 48 states).  The graphics are made from the output grids and all of the grids (both source and output) are retained in an archive at USGS Center for Earth Resources Observation and Science.

For each product, an automated process counts the number of days within the product interval when the precipitation in a grid cell meets the threshold criteria.  For the maximum consecutive dry days calculation, the number of days that meet the threshold criteria are counted and the largest consecutive count of days, regardless of when in the interval that count occurred, is shown on the “Maximum Consecutive Dry Days” product.  For the number of rain days, the total number of days that meet the criteria is summed for the product interval.  Finally, for the number of days since last rain, the most recent consecutive string of days that meet the threshold criteria is summed.