The eyes of the commodities market were on oil as US sanctions on Iran took effect. They should have been on natural gas, which soared as much as 8.89 percent on Monday for its biggest intraday gain since October 2016. Prices are now up about 20 percent for the year. The surge came as forecasts turned sharply colder for the eastern and central regions of the US One weather model shows the icy blast extending into Thanksgiving, according to Commodity Weather Group LLC, a pattern also seen in the winter of 2013-2014 when the “polar vortex” sent gas prices to a six-year high, according to Bloomberg News’s Naureen S. Malik and Christine Buurma. The rally underscores the market’s vulnerability to supply constraints as stockpiles enter the winter at a 15-year seasonal low. Demand has surged amid record exports and as power plants and industrial users burn more of the fuel at home and abroad. Though production from shale basins has climbed to an all-time high, gas bulls are betting that the output gains won’t be enough to meet heating needs. Money managers’ net-long bets on gas are hovering near a nine-month high, US Commodity Futures Trading Commission data show. Hedge funds are “going to go for what’s hot, and it’s natural gas,” Bob Yawger, director of the futures division at Mizuho Securities USA Inc., told Bloomberg News. The jump in natural gas prices may offset some of the good news for consumers from the recent drop in oil prices, which has resulted in the lowest gasoline prices since March.