I get what you're saying. Maybe I'm just not making my point. I think of what you are describing as a rogue wave in the ocean. Can't forecast it. Never know when one might hit and it is worse than any of the other waves. But it happens when the sea is already rough and choppy. You can't rule it out when you're sailing a rough sea but it is not likely. You'd never be sitting on a calm, flat sea and then get blind sided by a rogue wave.
That is how I think of a "surprise storm" - a rogue wave on a calm ocean. It doesn't happen. But if you're out in marginal weather, maybe no worse than forecast, maybe even better, and a localized area of nasty weather hits it's hard to see that coming. No forecast can be sufficiently micro-level to predict that with much accuracy. But you should already be on some level of alert from a marginal weather forecast and the current conditions because it is possible. A nasty storm does not simply materialize instantly out of nothing, which you pointed out in your reply. There have to be elements present - which are observable - that indicate it is possible and allow us to predict changes coming, i.e. dropping temperatures, clouds building, increase in wind, etc.
Hopefully that clarifies my point. If not, then to hell with it. I tried. We should just go ahead and carry on with our lives....