Is feeding birds only after a snowstorm helpful or harmful?
Feeding for a short period should be helpful, not harmful to the birds, particularly if it is during or following a cold storm.  (Ground feeding birds, such as the Junco or Towhee shown in your picture have particular difficulty after a storm covers the ground with snow.)  A concern people have with feeding is that feeding will create a dependency of the birds on the artificially supplied food.  The worst case of this would be expected to occur during fall migration, where an unusual supply of food might delay birds from migrating, which may be detrimental if they cannot survive the colder weather they may encounter because of the delay.  Other than that period, short term feeding should not be problematic since the birds will not develop a strong visiting habit over a short period.

Dependence on human feeding was studied by Brittingham and Temple in an article "Does Winter Bird Feeding Promote Dependency?".  This study evaluated chickadees in two areas, one which had supplemental food available for a year.  The next year, without food available for either area, showed no difference between the resident birds of the two areas.  Although they didn't test suddenly discontinuing food in midwinter, they do comment on that scenario:


  We did not test what happens when feeders are removed unexpectedly from a site in the middle of winter, but we suspect that this would not be as detrimental as is typically thought. In winter, a natural food patch may disappear suddenly as a result of a winter snow or ice storm or the foraging activities of other flock members. As a result, chickadees apparently track a number of food patches at all times and sample a number of areas continuously no matter how abundant food is in any one patch. This strategy is necessary for surviving in an unpredictable and fluctuating environment. People consider bird feeders to be a very predictable food source, but in terms of evolutionary time, they have only been available for a very short time. For birds, they are probably no different than any other food patch.