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Wal-Mart (NYSE:WMT) was falling by nearly 2.5% on Thursday after reporting disappointing results for the quarter ended on April 30. While management blamed the harsh winter and weak consumer spending for the company's uninspiring performance, competition from both online retailers such as Amazon.com (NASDAQ:AMZN) and traditional brick-and-mortar players like Costco (NASDAQ:COST) could be a more challenging threat in the long term.

Is the recent weakness in Wal-Mart a buying opportunity, or should investors stay away from the company?

Cold performance
Total sales during the quarter increased by 0.8% to $114.96 billion; this was lower than the $115.96 billion forecast on average by Wall Street analysts. Sales excluding foreign exchange fluctuations grew 2.1% versus the same quarter in the prior year, while membership and other income increased 4.8% to $793 million.

Comparable-store sales excluding fuel declined by 0.2% on the total company level in the U.S. While comparable sales at Wal-Mart stores in the U.S. were almost flat with a decline of 0.1%, performance was worse at Sam's Club, where comparable-store sales excluding fuel fell 0.5% during the 13 weeks ended on May 2.

International sales declined by 1.4% on a reported basis; however, currency fluctuations were responsible for most of this fall. International sales adjusted for currency movements increased by 3.4% versus the prior year.

There were some positive spots in the report, such as an increase of 27% in e-commerce sales and a jump of 5% in comparable sales in the company's Neighborhood Markets segment. Smaller stores seem to be helping in terms of sales performance, so Wal-Mart is planning to focus on Neighborhood Markets and Wal-Mart Express when it comes to growth initiatives.

Still, this is hardly enough to compensate for what was a broadly weak performance from the company in terms of sales.

Earnings per share also came in below expectations at $1.10 per share, a decline of 3.4% versus the same quarter in the prior year and lower than estimates of $1.15 for the quarter.

Wal-Mart vs. Costco and Amazon
Management blamed the unusually harsh winter for the company's lackluster performance during the quarter. "Like other retailers in the United States, the unseasonably cold and disruptive weather negatively affected U.S. sales and drove operating expenses higher than expected."

However, guidance for the second quarter of fiscal 2015 was also quite weak. Management expects comparable sales to be relatively flat during the quarter ended on Aug. 1, while earnings per share are forecast to be in the range of $1.15 to $1.25, versus $1.24 per share in the same period last year.

Wal-Mart's weak guidance could be interpreted as an implicit admission that the company's problems are much more permanent than unfriendly weather, and competition from the likes of Amazon and Costco seems to be a more serious problem than transitory external conditions

Amazon is unquestioningly the most disruptive force in the retail industry over the last several decades. An efficient business model, relentless innovation, and the willingness to operate with razor-thin profit margins in order to gain market share versus the competition make Amazon a dreaded competitor in the business.

Amazon has been gaining participation in different retail categories over the last several years, and the company is clearly outgrowing Wal-Mart, as well as the rest of the industry. Amazon produced revenues of $19.74 billion during the first quarter in 2014, an impressive increase of 23% versus the same period in the prior year.

Costco has also been outgrowing Wal-Mart and its Sam's Club division in recent years, and there is no reversion in the trend judging by recent sales reports. Costco announced a big increase of 7% in total sales during the month of April; comparable-store sales excluding fluctuations in gasoline prices and foreign currencies increased by 5% in the U.S. and by a stronger 7% in international markets during the month.

When compared against the stagnant, or even declining, sales reported by Wal-Mart in general and Sam's Club in particular, it becomes quite clear that Costco is proving it has a superior ability to sail through the harsh winter and unfriendly consumer environment.

Foolish takeaway
Wal-Mart is blaming its disappointing performance on weather conditions and weak consumer spending. However, competitors such as Amazon and Costco are doing materially better than Wal-Mart, while gaining market share versus the company in the savagely competitive discount retail industry. Unless Wal-Mart finds a viable strategy to overcome its weaknesses versus the competition, things could continue getting worse for the company before they turn for the better.

Andrés Cardenal owns shares of Amazon.com. The Motley Fool recommends and owns shares of Amazon.com and Costco Wholesale. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.