Natural gas futures posted the biggest drop in six months as forecasts for cooler U.S. weather signaled reduced demand from power plants.

A burst of heat in the Midwest and East over the next five days will give way to seasonal readings from Sept. 7 through Sept. 16, said Commodity Weather Group LLC. Gas stockpile gains have outpaced the five-year average for 19 consecutive weeks as the U.S. experienced the coolest June through August since 2009, based on population- and energy-weighted demand, according to Commodity Weather.

“There’s some heat, but forecasts don’t look too extreme,” said Tom Saal, senior vice president of energy trading at FCStone Latin America LLC in Miami. “With weaker weather maybe some people are expecting the storage injection to get back into triple-digits.”

Natural gas for October delivery slid 17.5 cents, or 4.3 percent, to settle at $3.89 per million British thermal units on the New York Mercantile Exchange, the biggest one-day percentage decline since Feb. 26. Volume for all futures traded was 33 percent above the 100-day average at 2:55 p.m. Gas has dropped 8 percent this year.

Forecasts turned hotter for this week in California, the Midwest and the East Coast, while readings were lower for mid- September, said Commodity Weather in Bethesda, Maryland.

New York

The high temperature in New York City today was expected to be 92 degrees Fahrenheit (33 degrees), 12 above normal, before dropping eight days later to 71, according to AccuWeather Inc. in State College, Pennsylvania. Power plants account for 31 percent of U.S. gas demand.

Gas stockpiles in the lower 48 states probably rose by 70 billion cubic feet last week, Tim Evans, an energy analyst at Citi Futures in New York, said in an Aug. 29 note to clients.

Inventories totaled 2.63 trillion cubic feet as of Aug. 22, 17 percent lower than the five-year average for the period. The deficit narrowed from a record 55 percent in late March, when storage levels fell to an 11-year low.

“The lack of demand and the flows into storage should be enough to press prices lower, at least one more time,” possibly testing $3.25 per million Btu, John Kilduff, partner at Again Capital LLC and editor of the Energy OverView newsletter in New York, wrote today.

A tumble to those levels will be a “value point” because stockpiles will be rapidly consumed with winter demand, said Kilduff, who said he likes the March-April spread as a long-term play.

Donald Murry, an economist at C.H. Guernsey & Co. in Oklahoma City, expects an inventory gain of 61 billion cubic feet. The five-year average increase for the period is 56 billion.

The premium for March gas futures to April deliveries narrowed 6 cents to 21.5 cents at 2:56 p.m., the least for this time of the year since 2007. The spread marks the end of winter stockpile declines and the start of storage increases during warmer weather.