US Rain/Dry Days Readme | Early Warning and Environmental Monitoring Program

US Rain Days / Consecutive Dry Days products

updated: April 2007

The US Rain Days/Consecutive Dry Days products are computed daily, based on data from the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS). The input daily rainfall data are a new product from the NWS, 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.

This product suite consists of 7 graphic outputs. These are shown in the table below.

Product Name Interval "Rain" Day or Dry Day Threshold Criteria
Number of Rain Days 30 days >= 0.04 in. (1 mm)
Maximum Consecutive Dry Days 30 days < 0.04 in. (1 mm)
Number of Days Since Last Rain 30 days >= 0.04 in. (1 mm)
Cumulative Rainfall 30 days N/A
Number of Rain Days 7 days >= 0.04 in. (1 mm)
Maximum Consecutive Dry Days 7 days < 0.04 in. (1 mm)
Cumulative Rainfall 7 days N/A

The process that creates these products runs each day at 1700 hrs. US-CT. The input rainfall data is from the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS). A general description of the data can be found at https://www.weather.gov/media/srh/HourlyPrecipInformation.pdf The raw 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. Each source point is now a grid cell. The spatial extent of the data is the conterminous US (i.e. the lower 48).

The process counts the number of days, in the past 30 days (and 7 days), when the rainfall total for each day was equal to or greater than 0.04 inches. This is the "Number of Rain Days" product. At the same time the consecutive dry days are counted and the largest count of dry days, regardless of when in the past 30 day (and 7 day) period that count occurred, is shown on the "Maximum Consecutive Dry Days" product. The "Number of Days Since Last Rain" is also calculated as the count of the most recent days since a day had more than 0.04 inches of rain. Finally the "Cumulative Rainfall" is calculated as the total rainfall for each period.