Part of my powers is that I am attuned to changes in atmospheric conditions - pressure, temperature and the like. Though usually this awareness only applies to my immediate vicinity, large enough fronts like the one over the Midwest can affect me as well... it is not so much the flu as just a general sense of 'off'ness. Though as far as I can tell, the storm is naturally occurring, it is still a rather large shift in normal conditions, and as such, everything else around it is affected.

Much like the butterfly in Tokyo flapping its wings, in a way.