https://www.weather.gov/ does show both Fahrenheit and Celsius for the current temperature, but not everywhere.  Which is a good proof that space is not an issue.

It would be nice if they made Celsius the first class citizen, but it's a start ;)

Let them know you would like to see it everywhere.

Please call the NWS Customer Service Line at 301-427-9855, or email NWS.communications.office@noaa.gov