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Air temperature is still the most important factor in melting snow. Rain will usually not add much heat to the process. At 40 degrees, one inch of rain will only produce a tenth of an inch of added water from snow melt. At the same time, frozen ground will result in more of the available water running off directly to streams.
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Pieces of floating ice carried with a stream's current can accumulate at any obstruction to the stream flow. These ice jams can develop near river bends, mouths of tributaries, points where the river slope decreases, downstream of dams and upstream of bridges or obstructions. The water held back can cause flooding upstream, and if the obstruction suddenly breaks, flash flooding can then occur downstream as well.
An ice jam can occur anytime from early winter to late spring in Michigan, depending upon changes in temperatures that cause alternate freezing and melting of water surfaces. The most likely times are early winter before the surfaces are completely frozen and early spring when the ice cover begins to break up due to melting.
When the snow melts, it adds water to the ground that drains away in the same way as water from rainfall. On average, one inch of fresh snowfall contains about a tenth of an inch of water. However, as snow accumulates and becomes compacted during the winter, the ratio of snow to water decreases. Thus, ten inches of snow remaining on the ground into early spring may contain as much as five inches of water.
Three days with the maximum temperature of about 50 degrees would create enough melting to cause ice breakup on small streams. That amount of warming could also melt two inches of snow.
The Spring Snowmelt Flood potential Outlook defines the flood potential from snowmelt based on normal precipitation and rate of melt projected through the normal snowmelt period. If the actual conditions bring more rapid melt or heavier rains than normal, or if ice jams occur, the flood threat would increase substantially. On the other hand, a gradual or intermittent melt, with minimal additional precipitation, would decrease the flood threat.
Outlooks are based on calculations of existing conditions (snow cover, soil conditions, and stream flow) together with predicted future weather conditions. Normal precipitation and snowmelt rates for the future period are presumed in making these projections. An earlier melt than expected may reduce flood potential. Alternatively, if snow persists into late March, the flood potential increases.
The river crest stage values given in the outlooks are only an indication of potential stream crests rather than specific forecasts. An increase in the potential can be expected if above normal precipitation and/or rapid melting develops. Likewise, the potential will decrease if below precipitation and/or more gradual melting occurs.
The main factors contributing to spring snowmelt flooding are:
A general term indicating minimal or no property damage but possibly some public inconvenience.
The inundation of secondary roads; transfer to higher elevation necessary to save property, some evacuation may be required.
A general term including extensive inundation and property damage (usually characterized by the evacuation of people and livestock and the closure of both primary and secondary roads).
Large-scale inundation, requiring substantial resources from outside the local communities; record or near record flooding.
The 2001 Spring snowmelt Flood Potential Outlooks are tentatively scheduled to be issued February 9 and 23 and March 9 and 23.