Shopping centers' pain continues to be online retail's gain.
In wake of a ShopperTrak report that store traffic posted yet another steep decline last week, comScore has given retailers some hope that shoppers are shifting their spending to the Web, instead of just cutting down on purchases.
The analytics firm on Wednesday said that last week, for the first time ever, online sales eclipsed $1 billion for each of the five individual workdays. The streak contributed to a total online spending increase of 21 percent so far this season, when comparing the days since Thanksgiving to the same period last year.
(Read more: Storms walloped retail, ShopperTrak says)
What's more, comScore said it has seen strong online growth coming from weekend spending, a period that is typically lighter for the Web. In the past, online sales have been stronger during the workweek, when people are in front of their computers for longer periods of time.
"One of the most interesting stories we are seeing this holiday season is the explosion in weekend online spending," said comScore's chairman, Gian Fulgoni. "While we have seen solid growth during the days between Monday and Friday, weekend growth rates have surged to nearly 4 times what we're seeing during the workweeks."
The news offers a small sign of relief to retailers, who have so far been struggling to reel shoppers into their stores. ShopperTrak on Tuesday said that last week's foot traffic fell nearly 20 percent compared with the same period in 2012, a dramatic decline that was deepened by the winter storm that kept consumers indoors in the Midwest and Northeast.
(Read more: Shoppers, beware: Time is running out for online)
Some experts have expressed concern that not all of retailers' sales will be made up following the storm—the second one to hit the Northeast over a holiday season that is already challenged by one fewer weekend between Thanksgiving and Christmas.
When people push back their buying to avoid nasty weather conditions, "the risk for retailers is something might come up, [and] they don't necessarily make up 100 percent of the sales," said Paul Walsh, chief meteorologist at The Weather Channel's WeatherFX.
The upcoming weekend is crucial for retailers, who in addition to snow and sleet, have faced a challenging environment plagued by a lack of must-have gifts and heavy discounts. ShopperTrak expects that Friday, Saturday and Sunday of this weekend will be three of the four busiest shopping days this season, as consumers make their last-minute runs to get their presents by Christmas.
(Read more: Holiday gifts for procrastinators)
But even as delivery windows are getting tighter, online spending hasn't tapered off just yet. ComScore forecasts that Wednesday, known as Free Shipping Day, will once again be one of the year's heaviest spending days.
"Given the shorter holiday season this year ... we should see some aggressive last-minute buying—particularly on Free Shipping Day," a comScore spokesperson said.
—By CNBC's Krystina Gustafson. Follow her on Twitter @KrystinaGustafs.