Casual dining chain BJ's Restaurants Inc. estimated quarterly results below analysts' expectations mainly due to weaker-than-expected sales, dragging its shares down as much as 18 percent after the bell.

The company, which operates 146 restaurants under the BJ's Restaurant & Brewery, BJ's Restaurant & Brewhouse, BJ's Pizza & Grill and BJ's Grill chains, estimated revenue of about $199.8 million for the fourth quarter ended Dec. 31.

Analysts on average were expecting revenue of $204.5 million, according to Thomson Reuters I/B/E/S.

BJ's also said comparable restaurant sales fell about 2.7 percent in the quarter.

"... We experienced a significant softening in comparable restaurant sales beginning in the middle of November and continuing through the end of the year," Chief Executive Greg Trojan said in a statement.

"In addition to the shortened holiday shopping season, our comparable restaurant sales in Texas and the Ohio Valley were impacted by severe weather during parts of December," he said.

BJ's said it expected earnings of 5-7 cents per share, below the average analyst estimate of 16 cents per share.

The company's shares were down 13 percent in extended trading. The stock closed at $30.52 on the Nasdaq on Thursday.