@underlircs I know you can get a “weather” device by installing Nest Manager. But I don’t see any way to select it for conditions/triggers in CoRE. You can get Lux, Humidity & Temp since those tie to sensors. Maybe @ady624 knows differently.
However, you could do this with iFTTT and their MakerChannel. Just setup the Weather Underground service, chose your trigger (Rain, Snow, Cloudy, Clear) as the “IF” then have the “Then” be a Maker Channel. Then just setup a piston with IFTTT as the trigger. If you need some info check out this post on iFTTT and CoRE
Hope that helps.