What: Shares of Wayfair (NYSE:W) surged late on Wednesday to close up 9.7% following an upgrade from Citigroup analyst Mark May.

So what: Specifically, May upgraded shares of the online-based furniture and home furnishings specialist from "neutral" to "buy," and assigned a per-share price target of $51 -- a 21% premium to Wayfair's closing price Wednesday even after the pop.

To justify his bullishness, May outlined his belief Wayfair is benefiting from "underappreciated seasonal strength," and is likely to extend its five-quarter streak of beating expectations and raising guidance when it officially releases fiscal fourth-quarter 2015 results in early March. May also forecasts Wayfair will report positive EBITDA and free cash flow in the fourth quarter (well ahead of analysts' consensus timeline), and expects Wayfair to continue to make progress toward sustained profitability in the coming fiscal year.

Now what: Wayfair stock more than doubled in 2015 as it kept delivering narrower-than-expected losses with each quarter. And most recently in early December, shares popped nearly 13% in a single day after Wayfair confirmed overall gross sales skyrocketed an impressive 109% year over year during its five-day peak shopping period between Thanksgiving and Cyber Monday, while direct retail gross sales climbed 130%. Wayfair CEO Niraj Shah also said the company was enjoying significant activity from both new and repeat customers.

And while I admitted at the time those sales almost certainly weren't of the high-margin variety, I also wrote, "If the company can sustain this momentum as shoppers' appetites for early season discounts begins to fade, Wayfair's upcoming quarterly report should be very strong."

For perspective, Wayfair's latest guidance calls for fourth-quarter revenue of $625 million to $665 million -- the midpoint of which represents 57.8% growth from the same year-ago period -- and for EBITDA margins of negative 0.75% to negative 1.25%. It offered no specific per-share earnings guidance.

As it stands, however, it's apparent analysts have grown to anticipate Wayfair's habit of underpromising and overdelivering; consensus estimates predict that net loss will narrow to $0.14 per share from $0.18 per share in last year's fourth quarter, while revenue is expected to jump 65.8% to $677.6 million.

It remains to be seen, then, whether Wayfair ultimately becomes a victim of its own success. But given its encouraging past and indications of a strong holiday already, I can't blame Mr. May for voicing his optimistic take.