A National Weather Service infographic explaining the snowmelt processes:
Flood Science
Snowmelt ProcessesSnow Distribution
The path that weather systems take is the most important factor in determining snowpack, but terrain and vegetation also influence how snow accumulates on the ground.Snowpack Characteristics
The temperature and the amount of water (snow water equivalent) in the snowpack are important to the melting process. Before rapid melting can occur, the snowpack as a whole needs to be warmed to 32 degrees Fahrenheit.Snow Energy Exchanges
Incoming solar radiation emitted longwave radiation, turbulent transfer of heat, ground construction, and heat transferred during rainfall are all important factors in heating or cooling the snowpack.Weather Factors
Strong winds and high dew point temperatures aid in melting by limiting the effects of evaporative cooling and allow the layer directly above the snowpack to remain warm due to turbulent mixing. Rain falling on a snowpack can accelerate the melting process.Where the Water Goes
Once the rapid melting begins, the water will either infiltrate into the soil, runoff into streams and other bodies of water, pool in place, and potentially refreeze as ice, or as a combination. Ice jam flooding can occur if the river channel has excessive ice cover.
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