In my New England neighborhood, we call it a "French toast emergency": Every time a heavy snowfall or tropical storm is predicted, the local grocery stores are jammed with folks frantically buying lots of bread, milk, and eggs. If there's any chance that supplies might be disrupted, even if only for a few days, people run to stock up.

So I wasn't surprised by a recent USA Today report suggesting that buyers have been rushing to Toyota (NYSE: TM) and Honda (NYSE: HMC) dealers in search of Priuses and Fits. Unlike many of the Hondas and Toyotas sold in the U.S., Toyota's Prius and Honda's Fit aren't made here -- they're imported from Japan.

And Japan, as you may have heard, is having some trouble at the moment.

How big a deal is this?
Although some Japanese auto factories have managed to restart production, supplies of many cars are expected to be limited for a while. Toyota resumed limited production of three hybrid models, including the Prius, on Monday, but most of the giant automaker's 18 Japanese factories are expected to be idle until at least mid-April.

Only one of those plants, a factory in Miyagi that produces the small Yaris, was significantly damaged by the earthquake, and that plant could be idle for another month or more. But parts shortages and utility disruptions are expected to be a major problem for a while, even for plants far from the disaster.

  • Honda is expected to cut production at its U.S. plants by as much as 50% starting this week, due to shortages of parts from Japan. Meanwhile, two critical Japanese factories are shuttered until (at least) next week, and damage to a technology center in the quake zone may affect the company's product-development efforts.
  • Nissan has some of its Japanese plants operating (for the moment, at least) with parts on hand, but a critical engine plant was damaged. The company is exploring the possibility of producing the engines in one of its Tennessee plants and shipping them to Japan to replace lost production. The all-electric Leaf is back in production, though its plants continue to face rolling power blackouts. The company's four U.S. plants are expected to keep running at least until next week.
  • Toyota has been hit very hard. According to Automotive News, the company said on Tuesday that it was still assessing its parts supplies and inventories, and said darkly that "there may be a significant impact on our production capabilities." And while Priuses are being built in Japan, batteries for the U.S. version of the car may be in short supply for a while. In addition, the company has asked its U.S. dealers to restrict orders of more than 200 critical parts to preserve supplies as long as possible. But for the moment, the company's 13 U.S. vehicle and engine plants are up and running.

It's not just a Japanese problem: Because of the interconnected nature of the global auto business, even factories thousands of miles away from the disaster are feeling its effects.

  • Ford (NYSE: F) and Chrysler have run short on certain Japanese-made paints, and have told their dealers that certain colors are on indefinite hold. (Want a black F-150? Too bad.) The company plans to idle a factory in Belgium for several days next week in an effort to conserve parts.
  • General Motors (NYSE: GM) had to halt production of some trucks because of a parts shortage; this in turn led to some layoffs last week at a plant that makes engines for those trucks. Those workers returned on Monday, and production is restarting -- for the moment. Shifts have been cut at two plants in Europe and one in Korea because of parts shortages, but as of Wednesday those plants are open and running at partial capacity.

Those last two may sound relatively trivial. But things may be about to get a lot worse.

How bad? This bad.
According to a report from economy watchers IHS Global Insight, supply disruptions related to the Japanese disaster have already shut down 13% of global auto production. So far, 320,000 vehicles that should have been produced haven't been, resulting in millions in lost revenues for the (mostly Japanese) affected automakers.

But here's the thing: Many plants that rely on Japanese-made parts are up and running today, but they're drawing down existing inventories of those parts. If supplies continue to be disrupted, those inventories will run dry over the next few weeks. IHS predicts that as much as 30% of global auto production could be shut down by mid-May. That's 100,000 vehicles a day, and nearly all of the major global automakers could be affected.

Getting more specific than that is tricky, because for the most part, the automakers aren't talking about this problem -- nobody wants to let their rivals (or shareholders, or customers) know they might be vulnerable. One exception is Volvo, which according to Edmunds, said that as of March 21 it had only a 10-day supply of its Japanese-made climate-control and navigation components. Ten percent of Volvo's parts come from Japan.

Beyond that, the effect is still hard to predict. But if you own shares in any automaker, this is a story to watch carefully over the next few weeks. You can also click HERE to add the companies mentioned in this article to My Watchlist.

Fool contributor John Rosevear owns shares of Ford and General Motors. You can follow his auto-related musings on Twitter at @jrosevear. General Motors is a Motley Fool Inside Value recommendation. Ford is a Motley Fool Stock Advisor pick. The Fool owns shares of Ford. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.