I just finished discussing seasons, mentioning 2 weeks ago that largest snows often occur during early Spring. Well, as I type this, snow is falling here in Mount Pocono, PA - about a foot and still accumulating - the largest storm snow total during this year ! Of course, tomorrow is April; so the snow shouldn't stay around very long. Temperatures of 50's to 60's during the end of this week should make most of the foot & a half or so of snow a recent memory then. Thus, I picked a timely topic. Next week I plan discussion of the California Cooperative Snow Surveys.
Regarding snowmelt, another type of season is often referred to - the water year, 1 October - 30 September. Snowmelt obviously occurs at all elevations where snow falls, but most relevant to hydrology are the large snowpacks which occur yearly at high elevations, particularly in the Rockies and northwest United States. In fact, permanent snowpacks (i.e., glaciers) exist in some parts of the Rockies. Most notable episodes involving snowmelt are those associated with storms, during which rain contributes to major flooding problems, such as occurred during this mid-winter in the Pacific Northwest (1), (2), especially if a large snowpack exists. Snowmelt contributes to dam breakages and other problems. More commonly, snowmelt mainly occurs during April-July, months during which large snowpacks as deep as 40 feet melt during a long process for which most rivers, dams, and other structures can handle well. Long range weather forecasts can help hydrologists at various river forecast centers prepare for changing conditions. Hydrological forecasts (1), (2) are issued, and watches and warnings when necessary.
Accurate assessment of existing snowpacks is essential for their forecast. This is done both with human and airborne surveys. Gamma rays emitted from elements in shallow soil beneath snow are absorbed by snow, water, and ice directly above. Knowledge of background amount and measurements allow accurate snowpack water equivalent estimation, which is interpolated for areas between measurement locations. Human surveys are essential for verification of remote data, often accurate to within a centimeter for large-scale areas. Consider if you wish, current snowpack water equivalent for the northwest United States, indicating a potential of nearly 100 inches of snowmelt at very high locations. When snow first falls, about 8.5 inches of snow will melt to 1 inch of water (if very heavy). Compaction occurs as snowpacks develop during winter, such that the ratio becomes about 3:1 during January and 2:1 during May. Thus, a snowpack with a water equivalent of 100 inches during late March might be 250 inches thick.
Elevations where snowpacks remain presently vary from about 2000 feet north to 9000 feet south, and amounts are above normal in much of the northwest United States.
Text is copyright of Joseph Bartlo, though may be used with proper crediting.