Oil rallied to a 13-month high as cold weather in Texas disrupted flows from America’s largest shale patch, exposing the fragility of global supplies amid sharp cutbacks by OPEC and its allies.

U.S. West Texas Intermediate futures surged as much as 2.5% and rose above $60 a barrel for the first time since January last year. Brent crude in London, meanwhile, rose above $63. Permian oil production has fallen by as much as 1 million barrels a day as temperatures fall to minus 1 Fahrenheit (minus 18 degrees Celsius) in Midland, Texas, the de facto capital of U.S. shale production, according to the National Weather Service Midland.

See also: Cold Blast in Texas Oil Patch Seen Leaving Barrels Trapped

The Arctic blast gripping the U.S. is threatening to crimp crude supplies and unleash a rush for everything from propane to heating oil, fuels that are used in mobile heating devices. In the past few weeks, harsh winter conditions across North Asia and parts of Europe have pushed some oil-product markets into a bullish backwardation structure. It comes amid a market that’s now “balanced” with prices reflecting the current state of play, said Russia’s deputy prime minister Alexander Novak.

There were signs that some traders were caught off guard by the cold snap in the U.S., with the nearest WTI futures contracts surging against the next month. That gauge — known as the prompt timespread — surged by 15 cents at the open, when on a normal day it would only move by one or two cents.

WTI has climbed about 16% since the start of February and Brent posted four straight weeks of gains after Saudi Arabia led the Organization of Petroleum Exporting Countries and its allies in deep output cuts this year, causing once-swollen global stockpiles to rapidly normalize. On the demand front, American oil demand surged with refiners processing the most crude since March in anticipation of a vaccine-driven boost in gasoline use this summer. Both moves underscore the recovery from the destruction that saw U.S. crude prices briefly turn negative in the depths of the pandemic last year.

“Energy prices have got another boost as a result of extremely cold weather in the U.S.,” said Warren Patterson, head of commodities strategy at ING Group. “This colder than usual weather will be around for much of the week, and so should continue to be supportive for both gas and oil prices, particularly if we do see significant supply disruptions.”

Rotating power outages were put into force in Texas as the state’s electricity grid operator grappled with surging demand amid a bitter arctic chill. The Electric Reliability Council of Texas, its grid operator, ordered transmission companies to reduce demand on the system after reserves dropped below 1,000 megawatts, according to an alert posted Monday on its website.

Elsewhere, potential strikes in Norway were adding to supply risks for Europe’s biggest producer. Equinor ASA warned that union action could lead to shutdowns of its Johan Sverdrup and Troll fields, two of the most significant in the North Sea. Security in the Middle East, meanwhile, was in focus after Saudi Arabian authorities blamed Yemeni rebels for an attack at an airport in the kingdom’s southern region.

See also: Saudis Say Attack From Yemen Set Plane on Fire at Airport

Still, concerns remain over the sustainability of crude’s rally. WTI futures’ 14-day Relative Strength Index remains well above 70 in a sign that the commodity is due for a pullback. Meanwhile, the threat of new virus strains and cases in parts of China may place a cap on what’s been a dramatic recovery in fuel consumption, with the International Energy Agency last week cutting its demand forecast for 2021 and describing the market as fragile.

“The cold snap, the additional Saudi cuts, the fresh U.S. stimulus promise are all helping,” said Vandana Hari, founder of Vanda Insights in Singapore. “But the single biggest factor — and one the doubters have probably been missing or disbelieving — is that Covid on a global level is in a retreat, for more than four weeks now.”