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 volunteer 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 DataStreme 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, teams 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 (SCS), 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 DataStreme WES Homepage. These charts are updated daily at approximately 1600 UTC (8:00 a.m. PST).
As of the beginning of this week (13 March 2005), many of the SNOTEL sites across the Northwest reported below to near average water content in the snow pack, with some stations in Washington State and Oregon having 25 percent or less of their long-term average, while Idaho, Nevada, Montana and western Wyoming had 25 to 75 percent of normal. However, a map of the precipitation totals measured by the SNOTEL sites from the beginning of October to mid March (not shown here) shows that precipitation across the region ranged between 25 and 75 percent of average, indicating that the precipitation that fell across the Cascades was in the form of rain, as the region has been relatively warm. Elsewhere, across the West, the central Sierras of California, the mountains of southern Nevada, along with the higher terrain in the Four Corners area (southern Utah, northern Arizona, southwest Colorado and New Mexico) have experienced above average water content in their snow pack, reflecting the numerous winter storms that moved across the region, along with a flow of moisture-laden air from the tropical Pacific.
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 DataStreme 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 (12 March 2005), snow covered the northern tier of states from the Dakotas eastward to New England, the higher terrain of the Rockies in Colorado, Wyoming and Utah, along with the Sierra in California. Scattered snow was detected over the Cascades in Washington and Oregon. Ice was detected in some of the bays along many of the northern Great Lakes, while the Lake Erie and the Gulf of St. Lawrence in eastern Canada were essentially ice covered. Hudson and James Bays in Canada were also ice 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 the 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. As of the start of this week (13 March 2005), much of the Northeast (95%) was covered by an average of nearly two feet of snow (58.5 cm), with nearly 5 inches of average water equivalent. In contrast, only 13 percent of the Pacific Northwest had a snow cover, with an average depth of less than three inches and an average water content of one inch.
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URL: DSWES/supl.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2005, The American Meteorological Society.
