Citing an abrupt dropoff in sales this month, Radio Shack ( RSH) said its second-quarter earnings will fall short of estimates. The announcement knocked about a quarter of the value off its shares in the premarket session. Fort Worth, Texas-based Radio Shack expects to earn 23 cents to 26 cents a share in the second quarter, short of the 33 cents a share it previously predicted and the consensus 34-cent analyst estimate. The company said overall sales rose 2% in July, but August sales trends "deteriorated dramatically." "Sales softened abruptly in August and despite some promotional activities to stimulate business, we have been unable to overcome the falloff in consumer demand," the company said. The weakness occurred over "many" categories. The shares were recently down 23% to $22.20 on Instinet.