I am very far from being an expert here, but my understanding is that the practical definition of "extraordinary circumstances" is "beyond the control of the carrier", and has almost nothing to do with the frequency of the event. See for example euclaim.com, which says "We refer to an extraordinary circumstance when the airline isn’t at fault for a flight delay or cancellation." The actual text of the act is "Such [extraordinary] circumstances may, in particular, occur in cases of [...] meteorological conditions incompatible with the operation of the flight concerned", without any qualifier for the rarity or commonness of the bad weather. So I believe afternoon thunderstorms at DFW are considered "extraordinary circumstances", even though they are of course very ordinary in the plain sense of the word. The act does require that the circumstances "could not have been avoided even if all reasonable measures had been taken", which I guess is held to apply to weather at DFW or really anywhere.

(And conversely, the Huzar decision determined that tech failures are _not_ extraordinary circumstances, despite potentially being very infrequent.)

The experts in the BA forum have clear answers based on the extensive case law for lots of questions of this type.