* UK wind power output seen peaking at 2.5 GW on Tuesday
* Rough gas storage to open for withdrawal by Nov. 1
LONDON, Aug 23 (Reuters) - Prompt prices for wholesale gas in Britain soared on Tuesday morning, as expectations of weaker output from the country's wind farms increased demand from gas-fired power plants.
Gas for immediate delivery was up 1.95 pence, or 7 percent, at 29.45 pence per therm at 0939 GMT.
Gas for day-ahead delivery was up 0.85 pence at 29.60 p/therm.
"Expected offtake from CCGT (combined-cycle gas turbine) generation has ramped up by around 15 percent from yesterday, in line with reduced output from wind farms," said Marcel Boonaert, head of gas trading at Wingas UK.
Wind power output in Britain was expected to peak at around 2.5 gigawatts (GW) on Tuesday and 2.4 GW on Wednesday, the data showed.
Britain's installed wind capacity is around 9 GW.
The September gas contract rose 0.75 pence to 30.00 p/therm.
Centrica (LSE: CNA.L - news) on Monday said 20 wells at its Rough gas storage site would be available for withdrawals from Nov. 1, having previously warned the facility could be closed throughout winter.
Although Centrica said injection at the site would remain unavailable, traders said expectations that injections could also start sooner than expected had buoyed the front-month gas contract.
Meanwhile, the winter 2016 contract slipped 0.05 p/therm to 40.60 p/therm as news of winter supply from Rough weighed on prices.
In the Netherlands, the day-ahead gas price at the TTF hub rose 0.4 euro to 11.45 euros per megawatt-hour.
In Europe's carbon market, front-year allowances fell 0.02 euro to 4.88 euros per tonne. (Reporting by Susanna Twidale; Editing by Dale Hudson)