"A 700-mile long trail of violent thunder- and windstorms ... developed in the Midwest and spread to the mid-Atlantic region on Friday cutting off electricity for millions of people," the International Council of Shopping Centers said in a weekly report.
Retail receipts rose 0.2 percent week-to-week and 1.4 percent compared to the same week a year earlier, the group said.
Customer traffic slowed "with the highest percentage of consumers reporting they did not shop at all during the past week since late May," the report said.
ICSC said there was slightly more cash available overall, as the price of gasoline fell 8.1 percent in the week to a national average of $3.356 per gallon on Monday.
The national average price has fallen by 58.5 cents in the past 13 weeks, the Energy Information Administration said.