Hope you don't mind, but i've just had a play with the small one you posted there.
Nothing too complicated, in GIMP i hit the 'desaturate' based on luminosity.
Then just value curves, bring the black point up and the white point down, darken all the lows to black-out the coastline and define the top of the tornado, darken the upper-highs a bit for that 'foreboding sky' look, and bring the midrange up for some detail.
So here's what it looks like (has that crappy quantized look because I'd just edited the jpg you posted). But it should give you somewhere to start:
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