Unfortunately, the NWS API is missing machine readable condition codes/icons in the daily and hourly forecast, which is why it isn't available in briefsky. The API has nearly everything else, though. If I had a solution for those icons, I probably would have added support for their weather alerts too, and also made it the default for US locations.
See here for notes on other unsupported providers: https://github.com/vsergeev/briefsky#unsupported-providers