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Coal mining stocks have had a couple of bad years. Actually, since peaking in mid-2008, Central Appalachian thermal coal prices have dropped from more than $140 a ton to around $50 a ton. Cheap natural gas and fuel switching at U.S. power-generating plants get the blame. There now appear to be a few positive signs for the industry and another look at the mining stocks may offer up some opportunities.

First, natural gas prices are up from below $3 per million BTUs a year ago to around $3.60. As natural gas prices approach $4, coal once again becomes competitive. Second, even though gas prices have risen, coal inventories have only just begun to fall at U.S. power plants. As inventory levels drop, the plants will have to restock, and that should provide some support for coal prices.

Other factors contributing to a comeback are increasing demand from Japan and Germany, where nuclear power generation has been curtailed following the disaster at Fukushima and Germany’s decision to eliminate its nuclear power generation entirely by 2022. Demand for metallurgical coal used in steelmaking is expected to rise in emerging economies, helping boost the price for met coal from around $135 a ton back toward the 2008 level of $300.

Here’s our look at five top U.S. coal producers: Peabody Energy Corp. (NYSE: BTU), Arch Coal Inc. (NYSE: ACI), CONSOL Energy Inc. (NYSE: CNX), Alpha Natural Resources Inc. (NYSE: ANR) and Walter Energy Inc. (NYSE: WLT).

Peabody is the world’s largest coal miner and carries a market cap of around $5.2 billion. Shares closed on Monday at $18.67 and are trading around $19.00 early Tuesday morning. The consensus target price from Thomson Reuters is about $23, and the 52-week range is $14.34 to $29.84. The company’s dividend yield is 1.9%. The implied upside to the consensus target price is 23%, but that target is well below the 52-week high.

Arch Coal has a market cap of $1.1 billion, and the shares closed last night at $5.07. The stock is trading around $5.20, in a 52-week range of $3.47 to $8.86. The consensus target price is near $5.20, and the company’s dividend yield is 2.5%. Shares appear to be fully valued.

CONSOL Energy’s market cap is $7.9 billion, and shares closed Monday night at $34.71. The stock is trading at around $34.60, in a 52-week range of $26.25 to $37.39. The consensus price target is about $42.40, and CONSOL pays a dividend yield of 1.5%. The implied upside on the stock is about 22%.

Alpha Natural Resources has a market cap of about $1.5 billion, and shares closed last night at $6.60. The stock trades near $6.75, in a 52-week range of $4.78 to $10.74. The consensus price target is about $7.30, and the company does not pay a dividend. The implied upside is around 11%.

Walter Energy has a market cap of $983 million. Shares closed at $15.45 last night and the stock is trading at around $15.70, in a 52-week range of $9.88 to $41.32. The consensus price target is around $18.20, and the dividend yield is 0.3%. The implied upside on the stock is around 18%.

With the exception of Arch Coal, which is fully valued, the coal miners appear to offer some upside for price gains, with Peabody and CONSOL — the largest miners — offering the most potential. Peabody’s strength here is its Australian met coal operation, while CONSOL might be slightly better positioned to take advantage of restocking at U.S. power plants. A wild card remains the closing of older, dirtier coal-fired plants that are unable to meet the new clean air standards that are scheduled to take effect in 2015, which could drive down demand for thermal coal.

The coal miners already have done about all they can do to shore up their businesses. They have lengthened debt maturities, slashed discretionary capital spending, reduced marginal production and lowered costs. Now all they need is some help on pricing, but the cavalry has not arrived in force yet. Whether it arrives in time remains to be seen.