Forecasts signal warmer temperatures in many states six to 15 days from now, a bearish development for natural gas.

Natural-gas stockpiles are 8% above last year’s levels and 6.2% above their five-year average for this time of year.

Natural-gas prices fell Thursday, extending a recent November slide with moderate temperatures sweeping across the U.S. and limiting demand for the power-generation fuel.

Most actively traded futures for January delivery dropped 5.7% to $2.675 a million British thermal units, bringing their tumble for the month to about 23%. Prices had surged to nearly two-year highs at the end of October as traders wagered that frigid weather late in the year would support demand, but forecasts in recent days have been milder than anticipated for much of the country.

That is a bearish development for natural gas because demand typically rises late in the fall and early in the winter when more people turn on their heaters in their homes. Nearly half of all U.S. homes are primarily heated with natural gas, according to the Energy Information Administration, so investors normally enter the colder months preparing for demand and price increases.

As this month’s selloff shows, a sudden change in weather forecasts can quickly unwind those wagers. Recent forecasts now signal warmer temperatures in many states six to 15 days from now.

“November’s mild weather has flipped the script,” Gelber & Associates analysts said in a recent note.

Front-month natural-gas futures for December delivery were down 6.2% at $2.544 a million British thermal units Thursday.