Adverbial adjective of the month

Or maybe "adverbial noun"? Or "adverbial verb"? Anyhow, "Long-closed Calif. mountain route surprise reopens after years", SFGate 9/2/2025:

A long-shuttered stretch of highway that cuts straight through Angeles National Forest above Los Angeles has finally reopened.

A roughly 10-mile stretch of Angeles Crest Highway, which runs roughly east-west through the national forest for over 60 miles from the wealthy suburb of La Cañada Flintridge to the small mountain town of Wrightwood, reopened with little notice on Friday after being closed for several years. Before the surprise return on Friday, the portion of the two-lane highway had been closed since the winter of 2022-2023, when “relentless storms” collapsed roadways, caused rockslides and damaged retaining walls, according to Caltrans.

The Wiktionary entry for surprise offers Noun, Verb, and Interjection sub-entries, but no Adjective or Adverb examples. It's normal for nouns to be used as nominal modifiers, but verbal modification by a noun is much rarer.

Comment from C.B., who emailed the quote and link:

I was surprised to see the following on the SFGate website, a news website run by the San Francisco Chronicle (although separate staff from the newspaper), specifically the phrase "surprise reopens." I realize it's headlinese, but still…

I don't have time this morning to find the relevant sections the Cambridge Grammar or other sources, assuming that such sections exist. And no analogous examples come to mind, though readers will no doubt oblige.

The obligatory screen shot:

Update — CuConnacht in the comments suggests that "It reads to me as a noun used as an adjective used as an adverb. That is, I think that "surprise reopening" is somewhere in the background." That makes sense — but the pattern is not a general one, so that a "surprise discovery of X by Y" would probably not be headlined as "Y surprise discovers X". And someone's surprise appearance would probably not be described by saying that they "surprise appeared".