"sharp warning" is correct and usable in written English.
You could use it to emphasize the urgency of a message or situation. For example: "The teacher gave a sharp warning that students who didn't turn in their assignments by the deadline would fail the course."
In welcoming them, he issued a sharp warning.
In March of last year, just before Kabul Bank's problems surfaced, Karzai got a sharp warning.
North Korea sent a sharp warning in comments before the vote.
Presumably for that reason, Mr. Bush's chief spokeswoman, Karen P. Hughes, issued a sharp warning after Ms. Harris spoke.
But last month, the China Banking Regulatory Commission issued a sharp warning, ordering investment trust companies to stop selling such products in cooperation with banks.
The revolts in North Africa appear to have made Mr. Mugabe's inner circle nervous — and the arrests were a sharp warning to those emboldened by them, they said.
Senior American and European Union figures issued sharp warnings that the country's future was at stake.
At least two leading environmental organizers have been prosecuted in recent weeks, and several others have received sharp warnings to tone down their criticism of local officials.
And Mr. Ocalan's death sentence has drawn sharp warnings from European human rights courts and organizations that he must not be executed.
In "Sharp Warnings as Hurricane Churns In," James Barron writes on Oct. 29 about anticipating the Hurricane Sandy: The National Hurricane Center said it expected the storm to swing inland, probably on Monday evening.
This week, as the New York region recovers from Hurricane Sandy, we pair Melanie Braverman's "'I used to love the run-up to a storm,'" a poem about a child's memory of anticipating severe weather, with "Sharp Warnings as Hurricane Churns In," an Oct. 29 article about preparing for Sandy.
