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: Alpha Natural Resources (NYSE: ANR) slipped closer to the brink of bankruptcy in March. The stock was down 16% due in part to an analyst saying the outlook for U.S. coal producers is "increasingly bleak" and that the sector is about to be swept up in a "wave of bankruptcies." Alpha Natural Resources stock is now down 77.9% over the past year on its way closer to zero.

So what: In mid-March, Macquarie Research warned that coal stocks were about to go under. This was after the firm's commodity team lowered its forecast for metallurgical coal prices by $5 per ton for 2015 and 2016. Furthermore, it warned that coal prices in the U.S. were decoupling from international coal prices, which meant even more downside on the horizon. That will force a "painful, but necessary, step to force rationalization on U.S. producers," according to Macquarie. Said another way, it doesn't see any other way out for coal producers but to go bankrupt, with Alpha Natural Resources a prime candidate for that fate. In fact, the firm lowered its price target on Alpha from $1.15 to just $0.60, suggesting there is much more downside to come.

Now what: It doesn't look good for U.S. coal stocks. Coal demand in the U.S. isn't going to pick up anytime soon as the economy remains sluggish and more coal-fired power plants are retired and replaced with facilities powered by cleaner-burning natural gas or renewables. Meanwhile, healthier global demand for coal hasn't materialized as quickly as expected, leading to depressed global seaborne coal prices. Once the first domino falls we could see several coal producers file for bankruptcy, with Alpha Natural Resources likely being among those pulled under.