The site would have to provide a compatible image format to use with the Media Tile. From what I can tell, Windy.com only provides the live webpage (no images).
Edit: You can now embed other websites using the Custom Tiles feature. There’s even a nice Refreshable URL Embed custom tile that can be used.
One option would be to use the weather image from a different source in a Media Tile with the action set to ‘Hyperlink’ to open Windy in a modal. One quirk here is that you’ll have to use the URL from Windy.com’s embed feature - they block embedding the base windy.com domain directly.