* Second-quarter same-store sales so far up 6.4 percent
* Shares fall more than 6 percent after hours
Feb 13 (Reuters) - Whole Foods Market Inc, the largest U.S. natural and organic grocery chain, said on Wednesday that sales at established stores so far this quarter softened from the prior period and its shares fell 6.1 percent after the close.
Whole Foods is the best-performing publicly held U.S. grocer and its shares often dive when its results and forecasts do not exceed expectations.
Same-store sales, a key gauge of performance for retailers, rose 7.2 percent for the fiscal first quarter that ended Jan. 20. So far this quarter, those sales are up 6.4 percent, dampened by winter storm Nemo and a shift in the day of the week of Valentine's Day.
Analysts and Whole Food executives noted the company is only about three weeks into the current quarter.
Still, Whole Foods narrowed its 2013 forecast for growth in sales at established stores to a range of 6.6 percent to 8 percent from 6.5 percent to 8.5 percent previously.
It maintained guidance for full-year earnings per share of $2.83 to $2.87, even as it forecast lower gross margin because of efforts to lower prices and attract a broader clientele.
Austin, Texas-based Whole Foods also said first-quarter net income rose 24 percent to $146 million, or 78 cents per share.
Total sales increased 14 percent to $3.9 billion.
Whole Foods selectively passed higher food costs on to shoppers, who continued to "trade up" to pricier items such as exclusive brands and organic products, executives said on a conference call.
Whole Foods shares, which closed at $96.90 on Nasdaq, fell to $91 in extended trading.