Airborne Snow Water Equivalent Program

The National Operational Hydrologic Remote Sensing Center (NOHRSC) has developed, and currently maintains, an operational Airborne Gamma Radiation Snow Survey Program to make airborne Snow Water Equivalent (SWE) and soil moisture measurements. Airborne SWE measurements are used by NWS Weather Forecast Offices (WFO) and NWS River Forecast Centers (RFC) when issuing river and flood forecasts, water supply forecasts, and spring flood outlooks. Each flight line is typically 16 km long and 300 m wide, covering an area of approximately 5 sq. km. Consequently, each airborne snow water equivalent measurement is a mean areal measure integrated over the 5 sq. km. area of the flight line. More background on this program is provided on the NOHRSC web site at:

An airborne gamma snow water equivalent (SWE) data collection program was initiated in Alaska in August of 2002 with the collection of background gamma radiation data on 107 flight lines. Associated soil moisture data were collected and analyzed. Additional flight line background data have been collected since then to complete the Alaska set of potential flight lines. The flight lines are shown on a map at the follow web page:

Flight lines are typically flown in mid-April to evaluate the water equivalent of the snowpack on the flight line. The pilots disseminate products each evening following the flight. Standard dissemination of the data is via a SHEF encoded product that lists the flight line number, date, percent of snow cover on the flight line, average SWE for the flight line, and other supportive information that may include comments by the pilots about snow or ice conditions in the vicinity of the flight line. A guide for interpreting the SHEF message can be found at: http://www.nohrsc.nws.gov/html/opps/asw_key.htm. The pilots also take digital photos during the flights if conditions warrant. All the products on the NOHRSC web site on the airborne snow survey page and clicking on Alaska Snow Survey in the table in the middle of the page:

The most recent airborne snow survey data are also summarized on the following web page: