For Universal App Campaigns (UAC), AdWords uses its own predictive algorithm to adjust bids, and doesn’t allow any other types of bid adjustments e.g. geo. For this reason WeatherAds likely won’t work for Universal App Campaigns.
An easy workaround would be to run a search or display campaign for app installs (targeting only mobile devices), and trigger those ads based on weather using WeatherAds. You can read more about search campaigns for app installs in this article.
Alternatively, you can run an app install campaign on Facebook (which is compatible with WeatherAds) and apply weather targeting to that using WeatherAds.
Further Reading: Which AdWords campaign types are compatible with WeatherAds