NPD released its estimates of US console sales for December last night. While neither the tracking firm nor Nintendo released specific sales figures for the Wii U in that period, the information we do have indicates the Wii U's sales in its second holiday season (e.g., November and December) were at or below those for its first holiday season.

NPD didn't offer specifics, but it did note that December was the Wii U's "highest month for unit sales thus far," meaning it surpassed the 460,000 mark the system achieved last December. However, the Wii U also finished behind the Xbox One, PS4, and Xbox 360 for the month, meaning it sold fewer than the Xbox 360's reported 640,000 December sales.

Putting that range together with the previously reported November data and sales data from last year, we can figure the Wii U selling anywhere from 3,000 to 210,000 fewer units in its second holiday season in the US, compared to its first, as shown in the chart above. Historically, that puts its year-to-year performance somewhere between that of the original Xbox and Gamecube, both of which languished behind the market-dominating PlayStation 2 in their console generation.

The Wii U's second-holiday sales figures also lag the vast majority of systems that have seen a substantial increase in sales in their second holiday season. Even systems like the Sega Saturn, Nintendo 64, and PlayStation 3 managed to show sales momentum in their second holiday season, despite going on to relatively weak performance in the US for the rest of their console generation.

While the Wii U could technically still turn itself around, 2013 was Nintendo's best chance to get a head of steam that could make the system a true alternative to the newly launched systems from Microsoft and Sony. Instead, the lack of any forward sales momentum in its second US holiday season is another worrying sign for the company, which recently slashed Wii U sales estimates for the current fiscal year. It's also likely to continue to put a damper on third-party support for the system, continuing the death spiral that often grabs consoles that fail to come strong out of the gate.