First, go to your device settings to applications, to 1Weather and toggle the storage permission off and back on. Does that help?
If not, next step is to go to device settings to apps to 1Weather and "Clear Cache" this will clear out any bad data. Issues can arise with this, especially after upgrades.
If not, we have had several customers with Nougat and higher (Oreo, Pie) who have reported this issue and it appears that adjusting the new Display Size setting added in Nougat has helped. There's some helpful info on that setting here:
After toggling the font and display size settings, then toggling them back, the radar should work properly.