* Gas system undersupplied by 5 mcm
* Demand around 19 pct above seasonal norm
* Oil prices gain to highest levels since November
April 21 (Reuters) - British gas prices rose on Thursday morning on lower supplies from Norway and rising oil prices, while demand was well above seasonal norm due to cooler weather.
Gas for immediate delivery rose 0.9 pence or 3.2 percent to 29.00 pence per therm by around 0830 GMT, while the day-ahead contract gained 0.55 pence or 1.9 percent to 29.25 pence/therm.
Demand was forecast to be 240 million cubic metres (mcm) on Wednesday, some 19 percent above normal levels, due to cooler weather boosting demand for heating, data from National Grid (LSE: NG.L - news) showed.
Maximum temperatures in London and South East England are expected to fall to 11 degrees Celsius on Friday from 15 degrees on Thursday, and the weather will remain colder than normal next week, the UK's Met Office said.
Supply was expected to be 235 million mcm, leaving the gas system 5 mcm short.
Imports from Norway fell by 17 mcm to 74 mcm, mainly due to a drop in flows via the Langeled pipeline.
An unexpected outage at Norway's Kaarstoe gas processing plant reduced production capacity by 5 mcm/day on Thursday, gas system operator Gassco said.
Meanwhile, supply from UK's liquefied natural gas (LNG) terminals was expected to remain healthy at 35-40 mcm/day during the reminder of April, when two new tankers are confirmed to arrive from Qatar.
Gas contracts further along the curve gained ground as oil prices rose to their highest levels since last November on expectations of a sharp drop in oil production from non-OPEC countries.
Gas for May delivery firmed by 0.3 pence to 28.30 pence/therm, while the July contract was up 1.30 pence or 4.96 percent at 27.50 pence/therm.
In the Dutch gas market, the day-ahead price at the TTF hub rose 0.3 euro per megawatt hour (MWh) or 2.5 percent to 12.25 euro/MWh.
European front-year carbon allowances firmed 0.04 euro to 5.59 euro a tonne. (Reporting by Nerijus Adomaitis in Oslo; Editing by Nina Chestney)