Here are some stormy things that you should include:
1) Diminished seventh chords
In this example from his Sixth Symphony, Beethoven starts with an F minor chord before moving to four bars of E diminished. Remember to resolve your diminished seventh in the usual way. Diminished sevenths might also be used in a chromatically ascending chain either resolving to a minor chord each time or resolving to a new diminished seventh.
- tremolo strings
- sustained FF wind chords
- melodies that arpeggiate the chord (F minor first bar of line 1, E diminished first line of page 2.)
In this further example from Tchaikovsky, an E# diminished seventh is the basis for the music but this time the gaps are filled in rather than being an arpeggio:
2) High woodwind lightning
In this excerpt (again from Tchaikovsky’s storm) notice how high clarinet, flute and piccolo do flashes of lightning:
3) Low rumblings
You can do this either with lower strings, particularly basses (see Beethoven excerpt above) or with percussion (see here for a guide to using percussion such as bass drum and timps).