Their nightmare? A “big snow or ice storm at one of our hubs,” UPS meteorologist Randy Baker told CNNMoney.

Storms wreak a special level of havoc when they hit during the busy holiday shipping season.

Monday is expected to be the busiest day of the year for UPS. It planned to handle at least 34 million packages that day and a total of 585 million in December. FedEx and the U.S. Postal Service expected their busiest day of the season fell a week ago. [pullquote style=”left” quote=”dark”]“Normal airlines, if passengers don’t depart on their flight, they can actually roll to another flight,” said Steve Merchant,[/pullquote] the contingency manager at UPS. But UPS has a lot more riding on each individual flight. “A normal plane might be moving 230 passengers, whereas one of my planes might be moving 15,000 passengers in the form of packages.”

Forecasters at both UPS and FedEx sit in a control center alongside the teams that schedule crews, dispatch planes, develop contingency plans and keep the day-to-day operations flowing.

Their job is to look at both the big picture — say, a hurricane barreling towards the coast — and the conditions for individual takeoffs and landings.

“Occasionally we’re wrong and we hear about it,” Baker said.

That’s what happened last year, when many packages from Amazon weren’t delivered in time for Christmas. While badweather was a factor, UPS underestimated a last-minute surge in orders. Major companies like Amazon actually give UPS and FedEx projections about their volume.

FedEx’s team of 15 meteorologists provide “a crystal ball” as early as ten days out, Paul Tronsor, FedEx’s managing director of global operations, told CNNMoney. That’s a big help in keeping the fleet of 650 planes, 150,000 trucks and 300,000 employees nimble.

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The team runs forecasts on each key airport two or three times a day, FedEx meteorology manager Kory Gempler told CNNMoney.

So far, the weather this season has been “fairly tranquil,” said Gempler. But he’s watching a storm that could complicate travel and deliveries on Christmas Eve.

“The only thing I wish Kory (Gemper) could do is control the weather instead of just forecast it,” Tronsor said.