It's become a familiar sight for denizens of cities up and down the Eastern seaboard over the past few days: Grocery shelves stripped of bread and milk, rationally or not, as people prep for a storm that's been described in increasingly apocalyptic terms throughout the week. If you shop on the way home from work, you might spend hours in lines to the back of the store -- but if you leave it until later, the pickings could be slim.

So what gives, America? Why can't the citizens of the most powerful nation on Earth buy the food they want when they want it?

The short answer is that planning for peak usage is hard, as anyone trying to handle customer service call volume at an airport during a snowstorm knows. But grocers do what they can.

One of the biggest in the D.C. area is Giant Foods, whose parent company Ahold also owns Stop & Shop in the Northeast. Giant has a whole team that, while not employing meteorologists like airlines do, monitors the weather and coordinates demand response.

"We start pre-planning pretty much immediately," says Giant spokesman Jamie Miller. In this case, it was Monday, when a large storm started to seem inevitable. "That involves coordination with our vendors who provide direct delivery to our stores, as well as increasing orders through our own supplies."

Every day, your average Giant store has 15 or 20 vendors who drop off loads of their main product, like Pepsi and Coke, Sara Lee, and H&S Bakery. A few days out, Giant asks them to bump up their deliveries for the days preceding a big weather event, especially for the things that people like to stock up on: Milk, eggs, bread, batteries, water.

Giant -- and most other grocery companies -- also has its own distribution centers in the suburbs, which take deliveries and supply stores as needed. On a normal day, the average store receives one trailer full of perishable goods and one of dry goods. On a pre-storm day, that delivery volume nearly doubles.

The problem is, you can't just keep unlimited amounts of food around to restock as items disappear. Most urban grocery stores don't have huge back rooms for storage, and you can't order a truck full of something on a few hours notice, because they have a schedule of other deliveries to make. Even if you had enough space off the sales floor to store backup supplies, it's risky to leave perishables out for very long. So once the trailers drop off their loads in the morning, that's all they've got to work with until 24 hours later, which can lead to shortages of key items like kale.

"It kind of get us through that day," Miller said on Thursday, following the small snow shower Wednesday evening that sent shoppers flocking to stock up early. "So this morning we’re recovering from yesterday, where stores were incredibly busy. By late morning, we were in pretty good shape."

Grocers also make contingency plans for what might happen during a storm, to make sure they can keep their doors open. Giant has backup generators to at least keep the registers working during a power outage, and dry ice to maintain refrigeration. With the Metro and bus systems shut down, Giant is planning to operate with skeleton crews if necessary built out of people who can get to work on foot. A big storm might interrupt those morning deliveries, but by that time, most people have already hunkered down anyway.

Of course, that becomes a bigger problem during a prolonged weather event like Super Storm Sandy. And Miller isn't making any promises about their operating status through the weekend. "It’s a fluid situation, we’ll have to see how things progress," he says.