The big chill gripping the U.S. is heating up the natural-gas market. Prices climbed to a two-and-a-half-year high on Thursday as one of the coldest autumns in more than a decade has kept stoking demand for the heating fuel. The run-up has helped investors who bet that gas prices would rise once temperatures fell. But it is bad news for utilities, which use gas to fuel power plants but are prevented in many states from immediately passing higher prices along to consumers. If the rally is sustained, it will spell relief for producers, which have struggled with low prices for years as output has soared. As cold weather set in across the Midwest and Northeast in November, utilities have been forced to use more gas, sending prices up more than 26% since Nov. 1. The added demand has sapped supplies. Gas in storage dropped by 81 billion cubic feet […]