As if losing $500M in sales this holiday season wasn't bad enough already, apparel retailers could be dealt an even bigger blow this spring, "thanks" to El Niño.

According to Planalytics, a consulting firm that analyzes the effect of weather on consumer demand, specialty stores lost $421M between Nov. 1 and Dec. 19—which ballooned to more than $500M through Dec. 26. 

Population-heavy centers in the Northeast are on track to post record—or near-record—high temperatures for November and December, leaving retailers no choice but to sell winter goods for "pennies on the dollar," Planalytics president Scott Bernhardt tells CNBC.

"They'll practically be giving it away, but they'll clear it," Bernhardt says. A string of retailers, most notably Macy's, posted brutal quarterly earnings to end the year, attributing the decline to hotter-than-usual fall months.

Meanwhile, Ron Friedman, national leader of Marcum's retail and consumer products group, says he's already receiving emails offering 50% to 70% off. 

"The level of markdown that you're going to see," Bernhardt says, "we haven't seen that in a very long time." [CNBC]