* Dollar weakness helps limit U.S. crude price slide * Tropical waves eyed in Atlantic - NHC * Coming up: CFTC positions data, 3:30 p.m. EDT Friday NEW YORK, Aug 19 (Reuters) - U.S. crude futures fell on Friday in choppy trading, having recovered some losses after an early plunge below $80 a barrel as stronger refined products futures and a weak dollar offered some support. Brent crude briefly rallied more than $1, before slipping back as the weight of falling equities markets and continuing worries about slowed economic growth and Europe's debt problems keep investors cautious about prospects for oil demand. The S&P 500 stock index likely will extend its 4.5 percent decline of the previous session as concerns mount the U.S. economy may slip back into recession, stock futures indicated ahead of the Wall Street open. With no economic data or corporate earnings news expected, attention should shift to technical analysis, with last week's S&P low near 1,100 in the viewfinder. Analysts and brokers in the United States on Friday morning also said the possibility that tropical waves in the Atlantic could develop into threats to U.S. oil infrastructure next week might also be helping support oil products futures and to a lesser extent crude prices. FUNDAMENTALS * On the New York Mercantile Exchange, September crude fell 75 cents, or 0.9 percent, to $81.63 a barrel by 8:53 a.m. EDT (1253 GMT), trading from $79.17 to $82.33. * Nigeria has set its official selling price for Bonny Light and Qua Iboe crude oil at premiums of $2.50 a barrel to dated BFOE, trade sources said. * The restart date for Taiwan Formosa Petrochemical Corp's refinery is unclear as safety checks are still pending, a local government official said. * A tropical wave located about 900 miles (1448 km) east of the Lesser Antilles has a medium, 40 percent, chance of forming into a cyclone in the next 48 hours as it moves westward at 20 miles per hour (32 km/h), the U.S. National Hurricane Center said. * The center of Tropical Depression Eight was near the coast of Honduras , the NHC said. Tropical-storm-force winds are possible along the northern coast of Honduras and over the Bay Islands later Friday and on the coast of Guatemala on Saturday, the NHC said. * Libyan rebels could resume oil output at two large fields in the east of the country within three weeks in a bid to ease fuel shortages and potentially restore some supply to global markets. * An international body said it would mount an operation, probably by sea, to evacuate thousands of foreigners trapped in Tripoli by rebel advances that have put Muammar Gaddafi's capital under siege. MARKETS NEWS * The euro recovered early losses in thin trade, gaining momentum on stop-loss selling, but remained at risk of coming under renewed pressure from a darkening global economic outlook and worries the euro zone banking sector could face more funding pressures. The dollar index weakened. * Gold prices rallied more than 2.5 percent to a record high as investors sought refuge from a second day of hefty losses on the stock markets. * World shares sold off on the debt crisis and downbeat economies. * Copper steadied, helped by a weak dollar, although growing concerns about a slowdown in the United States and a debt crisis in Europe weighed on sentiment and capped further gains for the metal. UPCOMING DATA/EVENTS * U.S. Commodities Futures Trading Commission positions data due at 3:30 p.m. EDT (1930 GMT) on Friday. * NYMEX September crude contract expiration on Monday. 8:53 LAST NET PCT LOW HIGH CURRENT DAY AGO CHNG CHNG VOL VOL CLc1 81.63 -0.75 -0.9% 79.17 82.33 45,003 344,487 CLc2 81.83 -0.68 -0.8% 79.38 82.50 83,777 153,155 LCOc1 107.55 0.56 0.5% 105.06 108.34 113,471 260,626 RBc1 2.7947 0.0115 0.4% 2.7435 2.8192 2,359 54,460 RBc2 2.6765 0.0113 0.4% 2.6216 2.6981 3,114 48,587 HOc1 2.8912 0.0164 0.6% 2.8322 2.9053 5,134 47,893 HOc2 2.8990 0.0154 0.5% 2.8397 2.9130 3,990 20,764 * NYMEX crude oil for September fell 75 cents to $81.63 a barrel by 8:53 a.m. in volume of 45,003 lots.

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