You're also assuming that such an advanced species would necessarily care about the resulting shockwave.
- -There is no snowstorm and the officials shut the city down. At worst, people lose a day's worth of work, some businesses are affected. Whiners abound.
- -There is no snowstorm and the officials leave the city running. Nothing happens, nobody notices.
- -There is a snowstorm and the officials shut the city down. Everyone congratulates them for their foresight.
- -There is a snowstorm and the officials leave the city running. Possible severe damage to infrastructure, possible death toll, cleanup is significantly more complicated and takes far longer. Officials are berated for their carelessness.
The best course of action by far is to shut the city down. The downside of doing so when there is no snowstorm is far lesser than the opposite. Those who complain have no idea what the fuck they're talking about (and who really expects a cabaret singer to have any knowledge of risk assessment and weather prediction?).
This isn't to say that "visual cues" and "checking stuff" aren't ridiculous, but you're also not a character in a Bond movie.
Group velocity is the speed at which the signal carried by a photon propagates. Essentially, if you look at a moving sine wave, group velocity is the speed at which it's moving. We already know that this velocity can be altered and can even be faster than c. This is different from signal velocity, which is the speed at which the individual photons carrying the signal propagate. Each photon is also a wave thanks to particle-wave duality, so the wave you're analyzing when you look at photons is the wave embodied by every photon you catch. You can't have faster than light communication even if group velocity is higher than c because the signal is still only going at c. The little packets carrying the wave travel slower than the wave's oscillation, essentially.
Altering group velocity is neat and cool, especially doing so in a vacuum, but it's not what a lot of people here believe.