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* High-pressure system may not clear until end of February
* Weather system's size makes it hard to forecast
By Yeganeh Torbati
LONDON, Feb 8 (Reuters) - The worst February cold spell Europe has seen in decades may last until the end of the month, leading meteorologists said, raising the prospect of an extended spike in European spot gas prices.
"We do have higher confidence in a change by mid-February, but not to milder weather," Leon Brown, a meteorologist at The Weather Channel in the UK, told Reuters.
"We expect a colder plunge from the Arctic with northerly winds as the blocking ridge declines eastwards to Russia, this time a blocking pattern developing to our west over the Atlantic. February will probably remain a cold month right to the end."
Cold polar air from northern Russia flanking an area of high pressure has prevented warmer weather from moving in across the Atlantic over Europe, plunging a wide swathe of the continent into sub-zero temperatures for much of the past 10 days.
Officials from the World Meteorological Organization (WMO), speaking at a briefing in Geneva this week, also did not rule out the possibility of cold temperatures lasting for the rest of February.
Omar Baddour, who coordinates the WMO's climate data monitoring programme, said there was a chance the pressure system might start lifting next week, but said it could remain until the end of the month.
A difference in pressure between Europe and the Arctic known as a "negative Arctic oscillation", part of the cause of the freezing weather, is expected to take two or three weeks to return to equilibrium, Baddour said, meaning there may be no early thaw.
Though the phenomenon of the high-pressure system itself is not unusual, the dramatic turn to below-normal temperatures after weeks of mild winter weather took experts by surprise, Brown said.
"It's actually quite unique and a bit baffling how this winter has developed," Brown said. "It's unusual for it to develop so suddenly and have it become a persistent block toward the end of January and February."
The cold spell is the strongest one to happen in the month of February in 26 years, said Georg Mueller, a forecaster at Point Carbon, a Thomson Reuters company.
"It was in 1986 when we had the last similarly severe cold weather (in February)," Mueller said.
The sheer size of the current Siberian blocking pattern has made it difficult to predict how it will move, Brown said.
"In this instance this big blocking of cold air ... seemed to influence the way the winds behaved rather than the other way around," he said. "We didn't expect the cold block to become so persistent and then move westward."
Even computer models are having trouble making forecasts for when the system will clear out of Europe, Brown said.
"Many of the computer model runs keep on trying to bring a breakdown about five or six days ahead, but they continuously backtrack and delay the pattern change," he said. "There have been a wide number of model solutions from six days out recently."
Already, the cold snap has driven UK gas prices up to their highest levels since 2006, hitting above 100 pence per therm on Tuesday, a surge of more than 15 percent .
Russia curtailed gas exports to Europe last week just as demand reached all-time highs, forcing countries like Italy to increase imports from Algeria and extract stored gas.
Protracted cold temperatures and increased domestic demand could force Russia to cut its exports to Europe again. (additional reporting by Tom Miles; Editing by Richard Mably)