Balloons carrying chemicals and/or virus could potentially wipe out an entire populated continent if no counter could be developed.
That's the sort of supervillain plan that would see Elon Musk in charge of China, but, yes, it's possible. Part of the madness, though, is the unpredictability of it. If the Chinese or Russians, for instance, went through Canada to get at the U.S. with one of those weapons, it would be on Canada's behalf that most of the world would shut up while the Americans sallied forth to settle the question.
And the southwest, I don't even know how the winds work, but to guess by the hurricanes, you could aim for Los Angeles, California, and take out Guerrero Negro and the Vizcaino, in the Baja.
The Chinese balloon is said to have launched from Hainan, in the South China Sea. A bioweapon launch from that location would be too dangerous for the Chinese.
But if we really want to fret about it, I suppose we could speculate about pretty much any place in the northern Pacific. Between Air Force and Navy, there should be at least someone knows the winds, and which courses to watch.