A wind turbine used for training and research stands outside the General Electric Co. (GE) energy plant in Greenville, South Carolina.

General Electric CEO Larry Culp may be more optimistic about the company's future than ever before but a top Wall Street analyst is warning that GE's turnaround hasn't happened yet.

"GE's guidance had an optimistic tone ... but we see little tangible here to change our view," J.P. Morgan analyst Stephen Tusa said in a note to investors Thursday. Many on Wall Street follow Tusa for his GE analysis and his notes can move the company's stock.

Culp warned in an outlook released Thursday that near-term results will remain under pressure. But Culp expects his company's performance will "be significantly better" in 2020 and 2020, he said. GE shares closed up 2.8 percent on Thursday.

But Tusa was critical of Culp's "myriad of promises" for the years ahead. Tusa said GE earnings and free cash flow are at "officially the widest gap we have now ever seen between consensus" and the company's estimate.

A turnaround for GE's struggling power business is "based on growth in transactional services," Tusa said, which has "never been done before."

"Even worse is that if recession hits, stubbornly high leverage would raise the stakes versus back then when it looked like an over-valued stock," Tusa added. "We struggle to understand the story."

J.P. Morgan has a neutral rating and a $6 price target on GE stock.

— With reporting by CNBC's Michael Bloom.