What your asking to do is not possible, and here's why. The IP address your referencing probably hosts multiple websites. Often web servers look at the address header to see which site your requesting from the IP address, and uses it to determine which content to return. It has no site configured on the IP alone, but when you connect using the hostname it knows exactly what you want.
Bigger question is why isn't your in house DNS resolving it for you?
- Fix your DNS server, reverse forwarding should allow it to resolve all external names.
- Add a manual entry in your hosts file "140.172.17.21 www.swpc.noaa.gov" (while this may work I can see complications, what if a link on the site points to www2.swpc.noaa.gov?)