My first suggestion would be to use a gradient wipe, with blob shaped gradients (maybe created with Fractal Noise and left as a still) that would approximate the random way in which snow melts. The only thing missing with this would be the soft shadows on the edges of the snow as it melts into separate blobs. If your transition isn't too long, you might paint in a soft shadow on the sides away from the light source, so as to enhance the effect. This would only work if the two shots are identical when superimposed.
Creative Director / Multimedia Specialist
B&S Exhibits and Multimedia