Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of coal miner Alpha Natural Resources (NYSE: ANR) fell 13% today after the company cut outlook.

So what: Alpha is expecting to ship 102.5 million to 109.5 million tons this year, down from a 104 million to 112 million ton estimate previously provided. Management has cited lower customer demand of metallurgical coal in Asia and lower production at two of the company's mines for the shortfall.

Now what: Alpha isn't alone lowering expectations as it seems to be a trend for coal miners of late. Walter Energy (NYSE: WLT) announced a reduced guidance today, and we saw Patriot Coal (NYSE: PCX) lower expectations last week. I would take a cautious approach heading into third-quarter earnings with guidance falling across the sector, but there could be some good value if demand and production is expected to pick up in 2012.

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