| | Re: THE before an adjective
No, "the" is necessary because the name of the country is "The United States". When a proper noun includes an adjectival component (here, United) the definite article is used. Other examples include The Western Isles, as above. The adjective specifies which states, which isles etc and because of the specific reference, the definite article is used.
Other examples, though "hidden", are names like The Netherlands and The Midlands, where the adjective has "merged" with the noun to make one word.
I'm afraid I don't really understand your last question. However, even if this was a full sentence, there would be no articles : There have been warnings of flash floods in southern England.