Trucking demand has improved as tonnage soared to a record high in February, thanks to a weaker than average January.

The American Trucking Associations’ seasonally-adjusted truck tonnage index—a measure for shipping activity in the U.S.—was up 7.2 percent in February from January, when the index was down 0.3 percent.

Compared to February 2015, the index rose 8.6 percent.

The increase in tonnage follows a month of bad weather when several shipments were held up.

“The strength [of February’s tonnage] was mainly due to a weaker than average January, including bad winter storms,” said Bob Costello, ATA’s chief economist. “[As a result] there was some catch-up going on in February.”

However, even with the increase, the trade group is worried.

“Normally, fleets report large declines to ATA in February tonnage, in the range of 5.4% to 6.7% over the last three years,” Costello said. “So, the small increase this year yielded a big seasonally adjusted gain. If March is strong, then I’ll get more excited.”

High inventory stocks, which increased again in January, are cause for concern and continue to weigh on truck freight numbers.

“We need those inventories reduced before trucking can count on more consistent, better freight volumes,” Costello said.

Trucking—an industry worth $700 billion in 2014—serves as an indicator for the U.S. economy, and represents 68.8% of tonnage carried by all modes of domestic freight transportation, according to the ATA.