While many people view an extensive snow cover as an inconvenience, the winter snow that piles up across the northern portion of the nation and the mountains of the West is important for water management. According to a fact sheet supplied by the National Snow and Ice Data Center, the mountain snowpack in the West contributes up to 75 percent of that region's annual surface water supply. In this discussion, the term snow cover refers to the areal extent of the snow on the ground, whereas the term snowpack denotes the accumulation of snow especially over mountainous terrain.
For more than a century weather observers have measured and reported snowfall, the liquid water equivalent of the snow, and the depth of the snow on a daily basis. These observations are made not only at the various weather stations operated by the National Weather Service, but also by thousands of cooperative observers. They measure the depth of newly fallen snow once daily at a particular time (usually 1200 UTC or 7:00 a.m. EST). These observers also melt this snow to determine the liquid water equivalent of the snow. They also measure the average depth of the snow cover, taken at several representative locations. A link to current snowfall and snow depth information can be obtained from the WES Homepage. The 24-hour snowfall and the snow depth (in inches) as observed at 1200 UTC are made available by NOAA's Climate Prediction Center. These data are available in several formats, including plots on individual state maps or the entire nation.
In some areas of the country, especially in the West, team of observers conduct snow surveys to determine the water content of the snowpack in the mountains. However, over the last several years, automated weather stations have been deployed in remote regions to determine the snowfall and other weather conditions that affect the snowpack. The Natural Resources Conservation Service (NRCS), formerly known as the Soil Conservation Service, maintains a network of at least 600 automated stations across the West (including Alaska) called the SNOTEL Network (SNOTEL=SNOw TELemetry). These remote unattended sites have instruments that measure the water equivalent of snow and other data, and then transmit the collected data to a centralized location, often via satellite relay. One of the quantities plotted is the snow water content, expressed as a percentage of long-term average conditions. The current water equivalent of the snowpack chart produced by the SNOTEL network is available on the WES Homepage. These charts are updated daily at approximately 1600 UTC (8:00 a.m. PST).
As of the beginning of this week (14 March 2004), many of the SNOTEL sites across the West reported below to near average water content in the snow pack, reflecting some alleviation of the extreme drought conditions experienced throughout the West for the last several years. With numerous winter storms slamming into the Pacific Northwest, the Oregon Cascades had a snowpack that contained between 90 and 130 percent of the average amount of water. Stations in the Sierras of California, across northern Nevada and southern Idaho also experienced average to above average water content. However, some locations in interior Washington, northwestern Montana, western Wyoming, Colorado, Arizona and New Mexico reported below average water content. A map of the precipitation totals measured by the SNOTEL sites from the beginning of October to mid March (not shown here) reveals that many stations in the Pacific Northwest and the Great Basin have received between 90 and 110 percent of the long-term average, while below average precipitation fell across the northern, central and southern Rockies. Stations in Arizona received approximately one half their usual precipitation.
For nearly 35 years, the fleet of polar-orbiting and geosynchronous satellites has been routinely monitoring the areal extent of snow and ice on a weekly basis. Reflected visible sunlight as well as the emitted infrared radiation are used to determine the extent of the snow cover under relatively cloud-free conditions. Microwave radiation is now used to assist the snow cover determination under cloudy skies. Since 1997, NOAA's Satellite Analysis Branch (SAB) of the Satellite Services Division (SSD) produces a daily snow and ice chart with a resolution of 25 km over much of the Northern Hemisphere based upon satellite surveillance, supplemented by surface observations. The current snow cover chart for the continental United States and adjoining portions of Canada and Mexico is found on the WES Homepage. White areas depict the snow, while yellow regions show ice cover on the major lakes and bays. As of the end of last week (13 March 2004), snow covered the northern tier of states from Montana eastward to New England, the higher terrain of the Rockies in Wyoming and Colorado, the Cascades in Washington and Oregon and the Sierra in California. Ice was detected in some of the bays along the Great Lakes, while the Gulf of St. Lawrence in eastern Canada was essentially ice covered. Hudson and James Bays in Canada were also covered.
Recently, low flying aircraft have been used to monitor the amount of water in snowpack through the measurement of upwelling gamma radiation emitted from various radioisotopes that occur naturally in the upper 8 inches of soil. These aircraft fly along prescribed flight paths at elevations of about 500 feet and at designated times measure the terrestrial gamma radiation. Since snow cover tends to attenuate the gamma radiation in proportion to the amount of water in the snowpack, a comparison of the amount of gamma radiation detected from a flight over a snow cover with that collected over bare ground permits the determination of the snow water equivalent in the snowpack. An operational Airborne Gamma Radiation Snow Survey Program is maintained by the National Operational Hydrologic Remote Sensing Center (NOHRSC) to determine snow water equivalent and soil moisture for the National Weather Service and the River Forecast Centers. This survey program typically operates from January to April. For example, aircraft surveillance across the Northeast during the exceptionally snowy winter of 2000-01 found that the mountain snowpack in places had a water equivalent of about 12 inches (up to 150% of the long-term average). Such analyses allowed water managers and the public some early guidance in preparing for the spring melt. Flight-line status maps and other information for the current snow season surveys are available.
The NOHRSC produces a variety of national and regional snow analyses that provide daily comprehensive snow information for the continental United States. These current displays include snow cover, snow water content, the average temperature, and several other derived statistics not only for large portions of the nation, but for the watersheds within the areas of responsibility of the individual River Forecast Centers. Individual maps are based upon satellite-derived statistics, supplemented by airborne surveys of the snowpack.
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URL: DSWES/supl.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2004, The American Meteorological Society.
