Yes, they're not hosting it over unencrypted web access at that location. Whether it's available at any other unencrypted location, I don't know, but it's available at the same location with the https(encrypted) scheme.
https://www7320.nrlssc.navy.mil/hycomARC/navo/arcticictn_nowcast_anim30d.gifIt's ultimately up to you and your computer to decide whether the credentials they present are good enough. Some browsers and online tools are apparently rejecting the credentials presented for encryption, but others are fine with them.
They're misconfigured or they decided that unencrypted data access is not worth the [regulatory and compliance] headache and they selected credentials appropriate for their main target audience: .mil. Either way, it's like asking Johnny Canuck to check an American passport in Manitoba. Looks authentic, but you'd never know for sure, even if your machine beeped yes.
To you, it is at a minimum as trustworthy as hosting it unencrypted was. If you were comfortable clicking the first link, you should be comfortable clicking the second link and overriding the warning. Your browser is telling you that you're on your own in either case.