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Ross Stores $ROST -0.28% reported first-quarter earnings that exceeded its own forecasts by a wide margin on Wednesday, as a 17% jump in comparable store sales drove profit well above plan.

Earnings per share came in at $2.02, up 37% from $1.47 in the same period last year and above the company's guidance range of $1.60 to $1.67. Total sales rose 21% to $6.0 billion, up from $5.0 billion a year earlier. The quarterly operating margin reached 13.4%, compared with the company's plan of 11.8% to 12.1%.

Shares jumped roughly 6% in after-hours trading, Reuters reported.

CEO Jim Conroy attributed the results to customer traffic. "Customer traffic was the primary driver of the strong sales trend as compelling merchandise assortments, higher customer acquisition and engagement from our ongoing marketing initiatives, and an improved in-store experience are resonating with shoppers," he said in a statement. "We believe our results also benefited from higher consumer spending related to tax refunds."