If you're thinking of zephyr as a gentle breeze:
* Gale: A strong wind.
* Hurricane: A powerful and destructive storm with very high winds.
* Typhoon: A hurricane in the western Pacific Ocean.
* Tornado: A violently rotating column of air.
* Squall: A sudden and violent storm.
If you're thinking of zephyr as a light and delicate fabric:
* Heavy fabric: Something like velvet, brocade, or tweed.
* Rough fabric: Something like burlap or canvas.
Ultimately, the best antonym for "zephyr" depends on the context of your sentence.