UK GAS-Prompt prices rise on weather, falling wind power, storage outages

* System undersupplied by about 18 mcm/day

* SSE (LSE: SSE.L - news) to mothball some withdrawal capacity at Hornsea

* Market waiting for update on Rough storage capacity

March 26 (Reuters) - British forward natural gas prices rose on Thursday as the system was left undersupplied because of cold weather and an expected demand boost from a drop in wind power.

Prices for next-day delivery rose 0.8 pence per therm to 47.9 pence by 1023 GMT, while the same-day contract gained by 0.65 pence to 47.8 pence per therm.

The gas network was undersupplied by about 18 million cubic metres (mcm), with total daily demand pegged at 271.2 mcm by National Grid (LSE: NG.L - news) , well above the seasonal norm.

Traders said that below normal temperatures, an expected drop in wind power output and storage outages have pushed prices higher.

"Prompt prices are up on a combination of factors, but we are not flying yet," one UK gas trader said, speaking just before a brief spike in the day-head price.

Peak output from wind power turbines was expected to fall by around 2 gigawatts on Friday from Thursday, meaning that gas-fired power plants will have to produce more electricity.

The price shot above 57 pence per therm around 1000 GMT due to a wrong trade and then dropped back to previous levels of about 47 pence.

SSE said on Thursday that it has decided to mothball a third of the natural gas withdrawal capacity at its Hornsea storage facility while the market awaits an update from Centrica on a potential reduction of Rough storage capacity.

If the reduction is confirmed, the price spread between summer and winter contracts will widen, one UK gas trader said.

In the Netherlands, the day ahead gas price at the TTF hub rose 0.23 euros per megawatt-hour (MWh) to 21.55 euros.

Further along the curve, British gas for April delivery was 0.8 pence higher at 47.5 pence per therm and the winter 2015 contract was up 0.97 pence at 52.4 pence per therm.

Deliveries from LNG terminals in Britain were nominated at 40 mcm per day, down from a two-week peak of 47 mcm, and were expected to fall further because of LNG plant maintenance in Qatar, analysts at Thomson Reuters Point Carbon said.

Saudi Arabia and Arab allies struck Shi'ite Muslim rebels seeking to oust Yemen's president on Thursday, raising concerns about tanker traffic.

The benchmark European Union carbon price were broadly steady at 7.03 euros a tonne.

(Reporting by Nerijus Adomaitis; Editing by David Goodman)