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NOAA NATIONAL OPERATIONAL HYDROLOGIC REMOTE SENSING CENTER – THE ULTIMATE SOURCE FOR SNOW INFORMATION

Rocky Mountain snow survey from NOAA aircraft.March 1, 2006 — The anticipation of an approaching snow storm brings with it both excitement and trepidation, depending on who you are. While school children are delighted at the prospect of a "snow day," parents and transportation workers cringe at the thought of travel delays and the hours they will spend tackling snow removal.

Snow impacts individuals and the nation in many ways. For instance, it benefits the nation by providing a basis for the winter recreation industry. Melting snow waters can help generate hydropower. On the other hand, snow can cause significant personal/property damage, paralyze regional transportation systems and even impact the national economy.

Because the seasonal storage of water in snow pack can cause severe flooding or alleviate drought during the snow melt season, hydrologists are particularly interested in the total water content of the snow. In general, ten inches of snow yields one inch of water — although light, fluffy snow contains much less water than wet, slushy snow. Therefore, the snow water equivalent, rather than the snow depth, is needed for hydrologic forecasting.

NOAA National Operational Hydrologic Remote Sensing Center
Given the great impacts of snow, there are obvious needs to measure it accurately and consistently for a multitude of uses. The NOAA National Operational Hydrologic Remote Sensing Center, located in Chanhessen, Minn., part of the NOAA National Weather Service, gathers snow data in snow-affected regions of the United States and makes that data available to numerous customers.

NOAA image of flow chart showing NOHRSC Operations. Each day, NOHRSC ingests ground-based, airborne and satellite snow observations from sources in the United States and those portions of southern Canada that drain into the United States. These data, along with estimates of snow pack characteristics generated by NOHRSC’s national snow model, are used to produce daily NOAA National Snow Analyses. (Click NOAA image to the left for a larger view of flow chart showing NOHRSC Operations. Click here for high resolution version, which is a large file. Please credit “NOAA.”)

“The general public, private sector and numerous federal, state and local government agencies use national snow analysis products in a multitude of research and operational applications,” said Thomas Carroll, director of the NOHRSC. “The NOAA National Weather Service’s hydrologists, as well as hydrologists from the private sectors, are the primary data recipients. They use this snow data when issuing spring flood outlooks, water supply outlooks and river and flood forecasts. The NOAA National Centers for Environmental Prediction also uses national snow analysis products as input into operational mesoscale atmospheric models. Additionally, U.S. citizens use national snow analysis products directly to assess snow cover conditions associated with recreation, transportation, hydropower, agriculture, environmental monitoring and commerce.”

Snow in Portland West Hills, 1979.The NOHRSC generates snow products and data sets in a resolution, format and time frame required to support the NOAA National Weather Service Hydrologic Services Program and other users. Products are distributed, in near real-time, over the NOAA National Weather Service Advanced Weather Interactive Processing System to NOAA users and over the Internet to non-NOAA users on the NOHRSC Web site: http://www.nohrsc.noaa.gov.

NOHRSC gathers, processes, analyses and manages U.S. snow data through the following four programs:

The NOHRSC uses two low-flying aircraft to make airborne snow water equivalent measurements across the U.S. including Alaska. Each winter airborne snow measurements are made from a network of over 2,200 flight lines covering portions of 31 states and 7 Canadian provinces. The data are reported in near real-time to the NOHRSC office in Minneapolis and distributed to other NWS offices and the NOHRSC web site within an hour after the survey aircraft land each evening.NOHRSC Airborne Gamma Radiation Snow Survey Program
NOHRSC operates two terrestrial gamma radiation detection systems on low-flying aircraft to infer snow water equivalents and soil moisture measurements covering portions of 31 states and seven Canadian provinces.

The NOAA Corps commissioned officers onboard NOAA aircraft, operated by NOAA’s Office of Marine and Aviation Operations Aircraft Operations Center, fly 500 feet above the ground to make more than 1,000 airborne snow water equivalent measurements each snow survey season (October to mid-April). The principal snow survey aircraft, a NOAA Aero Commander, based in Minneapolis, Minn., operates in the north-central and northeast United States and Canada. A NOAA Jetprop Commander has been calibrated to make similar measurements on the flight line network in the West. A second NOAA Aero Commander has been calibrated to serve as a backup aircraft.

Airborne sensors on the NOAA aircraft detect and measure the natural terrestrial gamma radiation emitted from the potassium, uranium and thorium radioisotopes in the upper eight inches of soil. Since the water in the snow cover blocks the terrestrial radiation signal — the deeper the snow, the weaker the signal picked up by the sensors. The difference between airborne radiation measurements made over bare ground and snow-covered ground at the same geographic location can be used to accurately calculate a mean areal snow water equivalent value at that location.

Airborne snow survey missions are conducted over regions of the country based on input from the NOAA National Weather Service field hydrologists, who work closely with the staff at NOAA National Weather Service River Forecast Centers and Weather Service Forecast Offices, and snow cover conditions across the country.

NOAA satellite image of snow cover in the Northeast USA taken at 10:43 a.m. on Jan. 24, 2005, following a storm system that brought heavy snow and winds to the region.NOHRSC Satellite Hydrology Program
NOHRSC is also responsible for generating and disseminating remotely sensed hydrology products derived from the NOAA Satellite and Information Service’s satellite data. Specifically, NOHRSC uses image data from the Advanced Very High Resolution Radiometer on NOAA polar-orbiting environmental satellites and image data from NOAA geostationary operational environmental satellites to digitally map the areal extent of snow cover for both the United States and those portions of southern Canada that drain into the United States. When the land is blocked by cloud cover, NOHRSC users data collected by the Defense Mapping Satellite Program’s Special Sensor Microwave/Imager, which is able to penetrate non-precipitating cloud cover. Satellite snow cover mapping typically begins in October of each year and continues through June or July in regions of the country where snow cover remains. (Click NOAA satellite image for a larger view of snow cover in the Northeast USA taken at 10:43 a.m. on Jan. 24, 2005, following a storm system that brought heavy snow and winds to the region. Click here for high resolution version, which is a large file. Please credit “NOAA.”)

Photograph of researcher programming a thermal infared radiation detector.Other NOAA Snow Survey Techniques
 NOHRSC also collects snow depth and snow water equivalent observations made by NOAA National Weather Service stations and a variety of other observers, including volunteer Cooperative Observers, power companies, river association volunteers and several state and federal government agencies.

Ground-based snow water equivalent measurements are typically made using a snow tube and a scale, which is calibrated to give a direct reading of snow water equivalent. snow water equivalent can also be determined by melting the snow after it has been collected in a tube.

NOHRSC Snow Data Assimilation Program
NOAA image showing flow chart showing NOHRSC Snow Data Assimilation System (SNODAS).The Snow Data Assimilation System is a modeling and data assimilation system developed by NOHRSC to provide the best possible estimates of snow cover and associated variables to support hydrologic modeling and analysis. SNODAS provides a physically consistent framework for integrating the wide variety of snow data that are available from various locations and time scales. (Click NOAA image to the left for a larger view of flow chart showing NOHRSC Snow Data Assimilation System (SNODAS). Click here for high resolution version, which is a large file. Please credit “NOAA.”)

SNODAS consists of three components:

  • Data ingest and downscaling procedures,
  • A spatially distributed energy-and-mass-balance snow model that is run once each day for the previous 24-hour period and a 12-hour forecast period at high spatial (one kilometer) and temporal (one hour) resolutions and
  • Data assimilation and updating procedures.

NOAA image of flow chart showing NOHRSC snow model preprocessing forcing data.The snow model is driven by downscaled analysis and forecast data from a mesoscale, numerical weather prediction model, provided by a Rapid Update Cycle 2 model from NOAA Ocean and Atmospheric Research’s Earth System Research Laboratory, surface weather observations, satellite-derived solar radiation data and radar-derived precipitation data. (Click NOAA image to the left for a larger view of flow chart showing NOHRSC snow model preprocessing forcing data. Click here for high resolution version, which is a large file. Please credit “NOAA.”)

After the model is initialized, periodic observations of snow water equivalent, snow depth and satellite-derived areal extent of snow cover are assimilated into the modeled snow state variables at the appropriate time step.

A clear advantage to the SNODAS modeling approach is that it uses all of the available data — ground-based, airborne, satellite, and national weather prediction model data sets — to generate the “best estimate” of a gridded snow water equivalent field and other snow pack properties at 1 kilometer resolution for the country. As a result, NOAA can easily track the evolution of snow pack between observations and forecast future snow pack estimates.

Plow crew working to clear the roads after a lake effect snow storm on Jan. 12, 1997 in Buffalo, New York.National Snow Analysis
The National Snow Analyses generated by the snow data assimilation program provides daily comprehensive snow information for the United States. The national snow analysis includes maps and text descriptions of daily snow accumulation based on snow observations and modeled snow pack characteristics. The national snow analysis provides information about snow water equivalent, snow depth, surface and profile snow pack temperatures, snow melt, surface and blowing snow sublimation, snow evaporation, snow-surface energy exchanges, precipitation, satellite-derived areal extent of snow cover, present weather and weather forcings in multiple formats.

A principle feature of the national snow analysis is its daily national and regional snow discussions, a text product that reviews the most recent snow storm systems that have impacted the United States. The snow discussions are intended not only to provide an overview of the snow system meteorology, but also to provide an enlightened discussion on the performance of the NOHRSC snow model as it simulates snow accumulation and ablation across the country. Other national snow analysis summaries include:

  • NOHRSC's Interactive Snow Information Page. Interactive snow information with map and query functionality and user selectable physical elements, map overlays, geographic regions and zoom levels;
  • Station time-series plots of modeled forcing data and snow model state simulations along with observed snow water equivalent, snow depth and other hydrometeorological variables and
  • Alphanumeric summaries, on a basin-by-basin basis, of the mean, maximum, minimum and standard deviation of snow water equivalent, snow depth and areal extent of snow cover. (Click NOAA image to the left for a larger view NOHRSC's Interactive Snow Information Page. Please credit “NOAA.”)

NOHRSC national snow analysis output products are distributed in a variety of interactive map, text discussion, alphanumeric, time-series and gridded formats. National snow analysis product formats include:

  • Daily national and regional maps for nine snow pack characteristics;
  • Seasonal, two-week and 24 hour movie-loop animations for nine snow pack characteristics;
  • Text summaries;
  • A suite of interactive maps, text and time series products and
  • Selected gridded snow products for the United States.

NOAA Geographic Information System Support Program
NOHRSC uses geographic information system technology to integrate satellite, airborne and ground-based observations of snow into gridded products of areal extent of snow cover and snow water equivalent. NOHRSC also provides selected GIS data sets for use by NOAA National Weather Service headquarters and field personnel, as well as programs such as Advanced Weather Interactive Processing System, the nationwide network of NEXRAD (Next Generation Radar) Doppler radar systems and Integrated Hydrologic Automated Basin Boundary System.

Sunrise with warm colors on a cold morning in the Harpers Ferry , West VIrginia in 2001. NOHRSC Web Site
Descriptions and examples of all NOHRSC products can be found on the NOHRSC Web site (http://www.nohrsc.noaa.gov/), including the raw reporting station snowfall, snow water equivalent and snow depth data used to create the NOHRSC snow products and photos from airborne surveys, satellite images and forecasts.

NOAA Snow Data Archives
NOHRSC data are permanently archived and available from both the National Snow and Ice Data Center in Boulder, Colo., and the NOAA National Climatic Data Center in Asheville, N.C. NSIDC and NCDC have developed an on-line distribution system whereby any end user can access and select any of the available gridded products for their required spatial and temporal domain.

The Future of NOAA’s Snow Data Activities
NOAA is looking into more advanced ways to measure snow water equivalent, such as using satellite-based Synthetic Aperture Radar to directly measure snow water equivalent on the ground, a technique that showed promise using aircraft during the 2003 NOAA/NASA Cold Land Processes Field Experiment and will be tested again in Alaska later this year.

In addition to providing vital information to NOHRSC's many customers, NOHRSC's snow data will also help fill important gaps in U.S. data that will be needed in a larger and more comprehensive Global Earth Observation System of Systems (GEOSS) being developed by more than 60 countries.

Relevant Web Sites
NOAA Weather Service

NOAA/NASA Cold Land Processes Field Experiment

NOAA National Operational Hydrologic Remote Sensing Center (Snow Analyses)

NOAA Operational Daily Snow Cover Analysis (Satellite Imagery)

NOAA Snow Water Equivalent

NOAA U.S. Snow Monitoring

NOAA National Climatic Data Center (Archived Weather Data)

National Snow and Ice Data Center

NOAA Weather Service Suite of Official Weather Products

NOAA Winter Weather Safety/Wind Chill

NOAA Climate Prediction Center

Snow Survey and Water Supply Forecasting

Media Contact:
NOAA National Weather Service Public Affairs, (301) 713-0622