Beginning with the setting (the rooftop, the wind) is rubbish. So begin with the man:
Sherlock was a story without a narrator; he was a narrator without a story. It wasn’t that difficult of a forecast.
Are you coming? Sherlock said.
And they scattered like crows in Marylebone Road.
It’s not enough to go on. They'll want the incoming low, winds backing around the compass, the microbursts, the downpour on their heroes’ heads. John finds the weather a bit heavy-handed, but that’s just because he’s not any good at it. (Of course he’s not; if he understood signs at all, he would have known; it wasn’t that difficult of a forecast, what would happen when Sherlock snatched a puzzle out of the air. What could it do but break over their heads?)
Well then, my former lightning seed.
If they want a storm then, it’s not a problem; he can give them drops on the sleeve of a coat, a deluge outside a coffee shop, snow ticking the Christmas windows. He can give them horror-film fog in a hollow. He can give them the right hand of the storm, the hook echo, the head of the comma, the glorious calm of the central eye: that’s what they were.
Are you coming, says Sherlock.
The question’s a gale; the statement a balm.