In a note to clients — as reported by “MacRumors” (http://www.macrumors.com) — Piper Jaffray analyst Gene Munster says U.S. Mac sales were up only 1% year-over-year in January, tracking behind predictions that have called for roughly 20% year-over-year growth for the full quarter.

The analyst’s notes are based on data from the NPD research group (http://www.npd.com). However, Munster says continued strength in international markets and the potential for new Mac updates such as a refreshed MacBook Air near end of the quarter will likely help Apple close strongly to meet expectations.

With historical evidence showing overall growth outpacing NPD’s U.S. data by 14 percentage points, Munster feels worldwide Mac growth for January could be in the 15% range.