UK GAS-Prompt prices rise on lower wind output, supply outage

* Gas system 12.5 mcm undersupplied

* Peak wind output 1.3 GW

* North Morecambe outage extended to July 23

LONDON July 21 (Reuters) - British prompt gas prices rose on Thursday morning as demand from the power sector increased due to lower output from the country's wind farms and an outage that cut gas supply.

Gas for immediate delivery was up 0.65 pence at 35.65 pence per therm at 0853 GMT, while gas for day-ahead delivery was up 0.70 pence to 35.40 p/therm.

With (Other OTC: WWTH - news) supply flows at 174 mcm and demand expected to be 186.5 mcm/day, Britain's gas system was 12.5 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.

Thursday's demand was around 9 percent above the seasonal norm of 171 mcm, the data showed. Traders said low forecasts for output from wind farms had increased demand for gas from power generators.

Wind power output was expected to peak at 1.3 gigawatts (GW), down from around 3 GW on Wednesday, National Grid data showed.

Britain has an installed wind power capacity of almost 9 GW.

Meanwhile gas supply was reduced due to an ongoing unplanned outage at Centrica (LSE: CNA.L - news) 's North Morecambe gas terminal.

Flows have been cut to zero from 7.7 mcm since July 16 and the plant is not expected to resume operation until July 23, Centrica said in a market update on Thursday morning.

Gas prices further along the curve also edged higher, with the Winter 2016 gas contract up 0.32 pence at 45.95 p/therm.

In the Netherlands, the day-ahead price at the TTF hub was up 0.21 euro at 14.73 euros per megawatt-hour.

In Europe's carbon market, the front-year EU allowance price was up 0.03 euro at 4.72 euros per tonne. (Reporting by Susanna Twidale; editing by Jason Neely)