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:
- High soil moisture in the fall
- Significant frost in the ground
- High water content of existing snow cover
- Rapid, continuous melting
- Moderate to heavy rain during melting
- Ice jams
Flood Potential Categories (Assumes Normal Precipitation and Melt Rates)
Low Snowmelt Flood Potential
A general term indicating minimal or no property damage but possibly some public inconvenience.
Moderate Snowmelt Flood Potential
The inundation of secondary roads; transfer to higher elevation necessary to save property, some evacuation may be required.
Major Snowmelt Flood Potential
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).
Severe Snowmelt Flood Potential
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.