UK GAS-Unplanned storage outage sparks price rally

* Rough gas storage outage restricts supply

* Transmission network short, replacement supply kicks in (Updates throughout)

Feb 20 (Reuters) - British wholesale spot gas prices rallied on Friday after an unplanned outage at the country's biggest gas storage site restricted supply sharply at a time of elevated demand.

Gas prices for same-day delivery shot higher after Centrica (LSE: CNA.L - news) 's Rough gas storage site advised it would cut output by 5 million cubic metres (mcm) over the day, which translated into a much sharper reduction in real-time flows.

The price rose 1.6 pence per therm or 3.22 percent at 51.25 pence by 1630 GMT and wiping out Thursday's losses.

Gas prices for day-ahead delivery rose 0.7 pence per therm or 1.42 percent to 50.10 pence, while weekend prices gained just over a penny or 2.15 percent at 49.80 pence per therm.

The outage at Rough, which began at 1428 GMT, saw withdrawals plunge to lows of 21 mcm/day from highs of 35 mcm prior to the announcement.

To compensate, smaller storage sites at Holford and Hilltop ramped up output and imports into Shell (LSE: RDSB.L - news) 's Bacton terminal rose.

National Grid (LSE: NG.L - news) data showed physical gas flows into the country's transmission system at 286.2 mcm fell short of demand estimated at 288.5 mcm.

However, the system was still forecast to be over supplied on average.

Flows from the UK Continental Shelf are at around 125 mcm/day, in line with average flows on Thursday.

Norwegian supplies picked up after an outage at the Troll gas field ended early on Friday following reduced output on Thursday due to a compressor failure.

In the Netherlands, the day-ahead gas price at the TTF hub was up 0.20 euros at 22.35 euros per megawatt-hour. Prices for March delivery were 0.27 euros higher at 22.17 euros/MWh.

Dutch production was nominated at 1,515 gigawatt hours on Friday, down 100 GWh compared to previous days' nominations, signalling that actual output could fall on Friday, according to Thomson Reuters Point Carbon analysts.

The benchmark European Union carbon price inched down six euro cents to 7.35 euros a tonne. (Reporting by Oleg Vukmanovic and Nina Chestney, editing by William Hardy and David Evans)