FORTUNE — This year’s unusually severe winter — the coldest in decades for much of the U.S. — has had dramatic and far-reaching effects. Wal-Mart (WMT), the world’s largest retailer, said its same-store sales dropped in the fourth quarter because shoppers were staying home and bundling up rather than hitting the stores. Home sales fell to their lowest level in 18 months in January, due at least in part to low temperatures. And the price of natural gas spiked to a five-year high in February as consumers cranked up the heat.

But one corner of the economy has grown particularly hot in recent weeks: the long-underwear market.

With shivering consumers desperately trying to stay warm, winter sport apparel makers such as Hot Chillys, Patagonia, and Terramar Sports have all experienced a sharp increase in demand for their base-layer lines. That has put a heavy strain on supply chains — leaving the companies with tough decisions about how to fill larger-than-expected orders from retailers.

Now, with another blast of Arctic weather blowing across the country, suppliers could be tested again.

The stage was set for the layering crisis months ago when, after two mild winters in a row, most retailers decided to place relatively small orders of long underwear for this season. But by the time the Polar Vortex began to kick in and shivering shoppers were emptying shelves, long underwear suppliers were inundated with calls — and a little overwhelmed.

“It’s the middle of February, and my phone is still ringing off the hook with orders,” says Steve Lee, vice president of sales for Hot Chillys. “We fell behind in orders around the first part of December and have been catching up ever since.”

Hot Chillys had no choice but to ramp up production to keep up with an order volume that Lee says should increase the company’s revenues by 30% compared to last year. Since late November, the Hot Chillys factory in San Luis Obispo, Calif., has been operating seven days a week and 13 hours a day. Its best-selling items — like the $55 mid-weight body-fit “micro elite chamois” — are still disappearing from shelves quickly.

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Terramar Sports — the maker of the silk-based long underwear line Thermasilk — sources its products from around the world. The company is the largest global wholesaler of silk thermal underwear from China, and orders take about a month to get to the U.S., according to company president Ben Lieberman. As a result, Terramar must rely on forecasts from the National Weather Service to get product in on time for thermal-buying season.

Lieberman says forecasts indicated that this winter would be colder than in recent years, but the experts did not predict such a prolonged stretch of severe weather. By New Year’s, though, it was clear that there had been a major meteorological miscalculation. “[Retailers] were frantic by the end of December asking me to ship them anything that was on the floor,” says Lieberman. As with Hot Chillys, Terramar is projecting sales to be roughly 30% higher this year than in 2013.

Patagonia also sources fabrics from abroad and is similarly dependent on weather forecasts when making purchasing decisions. The company’s mid-weight base layer, one of its most popular styles, takes about six months to reach U.S. stores. Bruce Old, director of wholesale sales at Patagonia, told Fortune that he has plenty of long underwear on hand to fill orders, but not necessarily the exact sizes, colors, or weights that retailers want.

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The base-layer bonanza has forced retailers to scramble as well. Sports apparel giant Dick’s Sporting Goods (DKS) declined to comment. But Ira Rosh, a divisional merchandise manager at sporting goods superstore Paragon Sports in New York City says the repeated winter storms have delayed shipments and affected the retailer’s ability to refill shelves in a timely manner.

Similarly, Fanny Familia, a manager at City Sports on Fifth Avenue in New York, says that certain top brands, like North Face, have been selling out of her store very quickly. When that happens, Familia simply directs customers to another lower-tier brand that is still in stock.

Carolyn Hamilton, the owner of the 11-store ski-and-sport chain Peter Glenn, says her Hot Chillys supplier had some difficulty getting orders into her stores on time due to fabric shortages. The retailer has stores in Florida, Georgia, and Virginia, and Hamilton says she expects this year’s long underwear sales to be 20% stronger than last year. Hamilton is already in the process of filling orders for next winter, but doesn’t expect demand for warm base layers to be as strong.

“You really never know though,” Hamilton says. “We had no idea we were going to have this type of season this year.”