I don't know if there is theory to support the claim of enhanced propagation. However, I do remember my impressions from when I lived in a place where it actually snowed. During the snowstorm, static would be higher than normal (listening on wire beams or multi-wire diversity arrays). For a period of several hours after a storm ended, I would experience better reception due to a much lower noise floor (making signals seem to "pop"). Atmospheric noise was obviously lower during this time, but I believe there may have also been a reduction in man-made electrical noise due to the power lines being covered with snow. Maybe it made them less dry, and thus, less prone to arc?
"Life is too short for QRP!"