UK GAS-Prompt prices soar on weak wind power output

June 19 (Reuters) - Prompt British wholesale gas prices soared on Monday morning, as expectations of lower output from the country's wind farms boosted demand for gas. * British within-day gas price up by 2.00 pence, or 6.5 percent, to 33.00 pence/therm at 0833 GMT. * Day-ahead gas price up 2.45 pence to 32.35 p/therm. * Traders said forecasts for low output from Britain's wind farms led to an increase in demand from gas-fired power generators, pushing prices higher and leading to an undersupplied system. * Gas system undersupplied by 3.2 million cubic metres (mcm) with demand forecast at 160.4 mcm and flows at 157.2 mcm/day, National Grid (LSE: NG.L - news) data showed. * Peak wind power generation is forecast at 1.6 gigawatts (GW) on Monday and 3 GW on Tuesday, National Grid data showed. * Wind power provided around 1 percent of Britain's electricity demand on Monday morning, while gas-fired power plants provided almost 50 percent, the data showed. * Weak wind output, and the closure of one of Britain's nuclear reactors for maintenance also led to an increase in prompt electricity prices. * The Dungeness-21 reactor went offline on Friday evening for maintenance, operator EDF Energy said in a market update. * British wholesale day-ahead electricity price up by 4.50 pounds to 40.00 pounds/ megawatt hour (MWh). * July gas contract up by 0.09 pence to 34.65 p/therm. * Day-ahead gas price at the Dutch TTF hub up by 0.02 euro to 15.20 euros per megawatt hour. * Benchmark Dec-17 EU carbon contract down by 0.01 euro to 4.87 euros per tonne. * Thomson Reuters (Dusseldorf: TOC.DU - news) analyst view: http://emea1.apps.cp.extranet.thomsonreuters.biz/cms/?pageid=united-kingdom-gas (Reporting by Susanna Twidale; editing by Jason Neely)