100% Chance of a Thunderstorm... so Ballsy

There is ZERO chance it will not rain in NYC tonight.  It WILL happen.  Accuweather says there is 100% chance.

If I were the weathermen, given my track record, I think I'd couch that and just take it down to 90% just in case.  What do you gain by telling us its 100%?  Its just all downside.