Heidi Klum went from "Project Runway" to "Project Rainway" ... 'cause her latest catwalk resembled a wind tunnel -- except it was outside on a rooftop, where she found herself unwittingly locked in a high-fashion duel with her umbrella.

Check it out ... Heidi hit the Empire State Building in NYC on Thursday ... but between the howling wind and sideways sheets of rain, her poor little umbrella never stood a chance.

The video’s pure comedy ... Heidi was battling her rebellious canopy as it flipped inside out and thrashed from side to side as she desperately tried to keep it together.

You can really see how wild the weather was -- and that brolly looked pretty sturdy too, but let’s be real … no one’s winning a showdown with Mother Nature.

It's unclear if she weathered the storm or blew

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