Gilda's murdered in the middle of a storm (complete with off-stage chorus impersonating the wind)

There's an electric storm going on during most of HELP! HELP! THE GLOBOLINKS! and Emily - who shows herself a lot braver than the grown-ups - gets caught in it...  where she encounters her former School Principal (who has now been turned into a Globolink).  (The plot is largely a spoof of DAY OF THE TRIFFIDS).

And our old friend Elviro (whom I'm now playing for the third time in a row, for extra points) from SERSE gets caught in a rainstorm which brings down the newly-built bridge... and he runs off in case anyone thinks that he was responsible.

Both BARBIEREs - Rossini & Paisiello - have thunderstorms.

At the end of OSUD, the students are rehearsing Zivny's new opera when a huge electrical storm begins outside. Zivny goes out onto the balcony, where he has a vision of his wife Mila (who had been killed falling from a balcony in a domestic incident several years previously). One of the students suggests that this could be the end of the autobiographical opera, which Zivny has failed to complete with only days left to the premiere.  Zivny responds angrily that the end of the opera lies in the hands of God.  Zivny is struck by lightning and dies - the opera remains unfinished.

Is 1 from Bizet's The Pearl Fishers (Les Pêcheurs de Perles)?

I still haven't actually had the opportunity to listen to any of the storm clips - but your clue alone leads me to make a guess of Sadko for number 4...

I don't know Meyerbeer's L'africaine, but I believe there is a ship board scene which must surely get wrecked in order to display the Opera's stage craft.

George mentioned the one in Kat'a as a possible musical example.  pw reeled off loads.  There's a storm outside in La fanciulla, I believe, which may have been mentioned.  I remember a very funny performance of it by the dear old Gran Scena Opera Company with the cliche of snow swirling in whenever the door was opened.  (A conceit used to great effect in Grimes.)

I know someone MUST have mentioned it but I can't find it by trawling back over the last few pages:

Berlioz: Royal Hunt and Storm in The Trojans

And I can't resist dragging out again (it's been ages) my old Quiz Answer Favourite David Blake: The Plumber's Gift which has a pastiche of the storm scene in Peter Grimes with a seaside landlady forever closing the front door of her B&B to a burst of Britten when a new guest fails to close it properly against the gale blowing along the promenade (well, it made me larf at the time; perhaps you had to be there).

And Gershwin: Porgy and Bess?   

If we've run out of storms, I have another possibility.  I'll wait for some more storms to pass.

Tchaikovsky's MAZEPPA has a nice mother/daughter dysfunctional relationship, and although Lyubov Kochubey is definitely of the "old dragon" type of mezzo,  asking her daughter if she's please cooperate in saving their husband's/father's (respectively) life doesn't seem too outlandish?

Mila Valkova's mother (she doesn't ever seem to have a name of her own) in OSUD is the nightmare mother-in-law from hell, finally jumping over a balcony and dragging her own daughter with her to their death, merely to spite a man she considers an inappropriate son-in-law?

Cio-Cio-San is a mother.