Arch Coal Inc.'s stock ACI, +1.75% tumbled 16% in afternoon trade, as a downbeat outlook for coal prices at J.P. Morgan kept pressure on the beaten down sector. The drop in Arch Coal's shares to below $5.50 comes less than two months after the company announced a 1-for-10 reverse stock split, aimed at raising the price of the stock so it can remain listed on the New York Stock Exchange. Shares of fellow coal producer Consol Energy Inc. CNX, -3.42% slumped 5.8% to pace the decliners within the S&P 500. The outlook for the U.S. thermal coal sector remains "subdued," said J.P. Morgan analyst John Bridges in a note to clients Friday, because the oversupply of shale gas is putting severe pressure on U.S. natural gas prices, which Bridges said he believes is "the biggest driver of U.S. thermal coal prices." He expects many coal producers to "struggle to avoid depleting their cash reserves on essential capital spending and finance costs." Shares of Arch Coal have plunged 70% year to date and Consol Energy's has tumbled 60%, while the S&P 500 Index SPX, +1.05% has lost 6.8%. The Market Vectors Coal ETF KOL, -0.14% was down 2.3% Friday, and has dropped 40% so far this year.