From Wikipedia: https://en.m.wikipedia.org/wiki/Weather_Underground_(weather_service)
On July 2, 2012, The Weather Channel announced that it would acquire Weather Underground, which become operated as part of The Weather Channel Companies, LLC, which was later renamed "The Weather Company". The Weather Underground website continues to operate as a separate entity from The Weather Channel's primary site, weather.com, with its existing staff retained. Third-party web analytics providers Alexa and SimilarWeb rate the site as the 117th and 98th most visited website in the United States respectively, as of July 2015. SimilarWeb rates the site as the second most visited weather website globally, attracting more than 47 million visitors per month. The Weather Company also uses the site's San Francisco headquarters as a regional office.
On October 28, 2015, Jeff Masters noted that IBM had officially announced an agreement to acquire The Weather Company’s business-to-business, mobile and cloud-based web properties, including Weather Underground, WSI, weather.com, and also the Weather Company brand. Meanwhile, the television service (The Weather Channel) remains a separate entity. The deal was finalized on January 29, 2016.