Natural-gas futures are tumbling following fresh forecasts that suggest January won't be quite as cold as traders believed late last month when

A wave of predictions that a polar vortex will make this January the coldest in years sent natural gas prices up this week, and experts say it could test supplies in the U.S. and Europe.

Natural gas prices have surged nearly 25% over the last month. Energy experts attribute the rise to cold weather expected in January and geopolitical issues around the globe. The cold temperatures could make the month one of the coldest Januarys in over a decade.

The Urengoy-Pomary-Uzhgorod pipeline was built in the Soviet era to carry Siberian gas to European markets. It became the main conduit to Ukraine’s border with Slovakia from Siberia, passing through the town of Sudzha — which is now under the control of Ukrainian military forces — in Russia’s Kursk region.