I have WeatherNation on my Rokus, and it does offer a weather radar display. However, it seems to have gone downhill since I'd last used it. It now starts with an ad, and it takes a long time for the radar display to update when I zoom in. But at least it's something.
Looking in the channel store there appears to be quite a number of weather options. Weather HD appears interesting, but of course it's a subscription.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.