NOAA 97-R804
 
 Contact:  Pat Slattery - NWS                   FOR IMMEDIATE RELEASE
           Jeanne Kouhestani - NOAA Corps       4/17/97

Rising rivers caused by record snowpack in the Dakotas, Minnesota, and elsewhere in recent weeks has brought new public awareness of the importance of flood forecasting. Measuring snowpack is a vital part of the National Weather Service's efforts to provide accurate and timely flood forecasts and outlooks.

The art of measuring the water content of snow has advanced significantly from the days when weather service employees simply used rulers to measure snow depth. Light, fluffy snow contains less water than wet, slushy snow. For this reason, the water content of the snowpack, commonly referred to as "water equivalent," rather than the snow depth, is needed for hydrologic forecasting. Until the use of airborne measurements of snow water equivalent, collection of this information was difficult and not always accurate.

Airborne snow data collection is the responsibility of a single federal installation in Chanhassen, Minn., a Minneapolis suburb. The National Operational Hydrologic Remote Sensing Center (NOHRSC) employs a small group of specialists who collect the data, then analyze it and produce accurate estimates of snow water equivalent over most of the nation's river systems. NOHRSC provides critical data input to the National Weather Service River Forecast System.

Hydrologists at the weather service's River Forecast Centers blend NOHRSC data with data from ground-based sensors, satellites, and volunteer observers to arrive at spring flood outlooks and flood forecasts

NOHRSC's two-member flight crews collect snowpack data from Maine and New Brunswick, Canada, westward to California, north to British Columbia, and south to Arizona. The airborne network encompasses more than 1,900 flight lines over 27 states and seven Canadian provinces.

Flights are conducted by officers of the National Oceanic and Atmospheric Administration Commissioned Corps (NOAA Corps), the smallest of the seven U.S. uniformed services. The NOAA Corps is responsible for operating and managing NOAA's fleet of research ships and aircraft in support of the agency's scientific and environmental programs, including the National Weather Service.

Snowpack data are collected by NOAA Corps flight crews flying an Aero Commander in the upper Midwest and East and a Turbo Commander in the western United States. Both planes are equipped with instrumentation that records natural gamma radiation emitted from the soil under the snow.

"The process involved with airborne snow survey is pretty simple," according to NOAA Corps pilot Lt. Cmdr. Rob Poston. "The gamma signal is diminished by the overlying snowpack. The deeper the snow in a given area, the weaker the signal picked up by the sensors.

"Determining the snow-water equivalent is done on board with sophisticated hardware and specialized software. The measurement is accurate to within one centimeter of water, and becomes the hard, quantitative data used by weather service hydrologists." Poston added that soil moisture data collected in summer months are accurate to within four percent, and are used to determine naturally occurring gamma radiation patterns that allow more accurate airborne snow water equivalent measurements in the winter.

Piloting NOAA aircraft on a survey flight can be extremely dangerous, requiring intense concentration and knowledge of location and conditions. Poston and Lt. Andrea Hrusovsky were surveying the snow covered headwaters of the Green River in Wyoming recently. The river drains the western side of Wyoming's Wind River range that includes some of the most spectacular and foreboding sections of the Rockies. Poston pointed out what appeared to be clouds forming behind one of the higher peaks.

"That's not a cloud," Poston explained. "That's snow blowing off the mountain tops. Today the winds are blowing hard up there and that means extreme turbulence if you try to fly over those passes. That's why we've always entered the upper Green River Valley from the south side - less turbulence and we've got a good downhill escape route. No pilot should try flying over those passes when snow is blowing like that; the winds can make any aircraft uncontrollable."

Poston wrote the software program that collects global positioning satellite data to record the aircraft's navigational position every five seconds during flight. "We can combine today's data automatically now with other geographic information. It's given hydrologists more flexibility to process and individually disseminate the snow information for each individual user," he said.

The purpose of collecting all this data is to provide National Weather Service hydrologists and meteorologists with the best information to help prevent loss of life and minimize property losses caused by flooding. But the data are employed by many outside the weather service as well.

Water managers all over the nation utilize the data to more accurately plan the use of the country's limited water resources. Government and academic researchers use the data to enhance a number of interdisciplinary research programs. "It would be easy to get lost in the scenic vistas and beauty of this country during a flight," Poston said, "and we certainly enjoy the scenery when we're up there. But we always know our main purpose is collecting data that may well save someone's life. That's what it's all about."

The National Weather Service is an agency of the National Oceanic and Atmospheric Administration, which operates under the U.S. Department of Commerce.

NOTE to Editors: Additional information about the National Operational Hydrologic Remote Sensing Center can be found on the Internet at: http://www.nohrsc.nws.gov. For information about the Office of NOAA Corps Operations, see: www.noaa.gov/nchome .