Northwest Airlines Corp. said labor actions and continued weakness in the Asian economies could cause second-quarter earnings to be significantly below year-ago levels. The St. Paul, Minn.-based company also noted it had some flights delayed or canceled during the quarter as mechanics seeking a contract engaged in a slowdown. Severe weather damaged and grounded more than two dozen aircraft for several days during the quarter. In the 1997 second quarter, the airline reported earnings of $136.2 million, or $1.15 a diluted share. One analyst said earnings might not break $1 a share in this year's second quarter. After the profit warning, Northwest's stock tumbled to $40 in after-hours trading from the closing price of $42.63 on Nasdaq. The stock's regular-session settlement represented a drop of 37.5 cents.