UK GAS-Prices firm on lower supplies from UK's continental shelf

* Gas system undersupplied by 20 mcm

* Withdrawals from Rough gas storage cut

March 4 (Reuters) - British prompt gas prices firmed on Friday on lower supplies from the UK's continental shelf against a backdrop of forecasts for cooler weather over the weekend.

Gas for immediate delivery rose 0.1 pence to 31.10 pence per therm by 0830 GMT, while day-ahead contract gained 0.1 pence to 30.35 pence/therm.

The gains were capped by expectations that temperatures and wind speeds were to rise later next week, reducing gas demand for heating and power generation.

"Through next week and into the following week, an unsettled pattern is expected to continue, with periods of rain and strong winds interspersed with spells of sunshine and showers," Britain's Met Office said.

Britain's gas system on Friday morning was undersupplied by around 20 million cubic metres (mcm), with demand forecast at about 309 mcm/day and flows at about 289 mcm/day, National Grid (LSE: NG.L - news) data showed.

The system was undersupplied mainly as a result of deliveries from the UK's continental shelf (UKCS) falling to 132 mcm/day from typically 140 mcm/day due to maintenance at PX's Teesside and Total (Swiss: FP.SW - news) 's Bacton Seal gas terminals, analysts at Thomson Reuters (Dusseldorf: TOC.DU - news) said.

Gas withdrawal capacity at Britain's biggest underground gas storage Rough, was also reduced by 30 mcm/day due to maintenance on Friday, its operator Centrica (LSE: CNA.L - news) said.

The loss of deliveries from the UKCS was partly compensated by higher imports from the Netherlands via BBL pipeline.

Supplies from Norway inched up by 3 mcm from the previous session, with the Langeled pipeline flows running near maximum capacity of 72 mcm/day, while deliveries from UK's liquefied natural gas (LNG) terminals remained steady at around 40 mcm/day.

Britain is expected to receive three new LNG cargoes up to March 10.

Further along the UK gas curve, the summer 2016 contract , fell by 0.25 pence to 27.10 pence/therm.

The Dutch day-ahead gas prices fell 0.02 euro to 12.48 euro/MWh.

In the European carbon market, front-year allowances fell 0.11 euro or 2.2 percent to 4.79 euro a tonne. (Reporting by Nerijus Adomaitis in Oslo. Editing by Jane Merriman)