Costco Wholesale Corp (COST) is about to deliver a reality check. The warehouse giant reports its December sales numbers on Wednesday, January 7, after the market closes, and the forecast isn't exactly thrilling.

Telsey Advisory Group expects total comparable sales growth of just 2.2%, a steep decline from last year's 7.4% clip. That's the kind of deceleration that makes investors sit up and pay attention.

Analyst Joseph Feldman, who maintains an Outperform rating and $1,100 price target on the stock, breaks down what's happening beneath the surface. Core merchandise comps (stripping out gas and foreign exchange effects) could land around 2.0%, compared to a blistering 9.9% in the year-ago period.

The Gas and Currency Factor

Here's where it gets interesting. Falling gas prices are creating a headwind of approximately 20 basis points to comparable sales. Gas is a weird part of Costco's business: when prices drop, revenue takes a hit even though the company still sells the same volume.

But there's a silver lining. The weakening U.S. dollar should provide an estimated 40 basis point tailwind based on recent currency movements, Feldman notes. When you convert stronger foreign sales back into dollars, you get a nice bump.

Regional Breakdown

Feldman's estimates paint a picture of diverging performance across geographies. U.S. comps excluding gas are projected at just 1.0% versus 9.8% last year. Canada should fare better at 4.0% (ex gas and FX) compared to 10.3% previously. Other international markets are expected to deliver 5.0% growth, down from 9.8% a year ago.

Costco shares traded down 0.46% at $858.34 on Friday.