Natural gas futures fell Monday as a break from high temperatures across much of the U.S. suggests weakening demand is ahead.

Natural gas for September delivery fell 10.5 cents, or 2.5%, at $3.955 a million British thermal units on the New York Mercantile Exchange, dropping below the important $4 mark. The benchmark contract has declined by nearly $1 since reaching highs just under $5 in early June, but only recently have prices flirted with $4.

Gas futures fell as the high temperatures that had plagued large swaths of the U.S. over the past month are beginning to moderate.

"The trend of warmer-than-normal conditions is breaking," said Matt Smith, an energy analyst at Summit Energy, in a research note. Last week, he noted, cooling degree days fell by 20% from the previous week. Cooling degree days are days when air conditioning use is higher.

Below-normal temperatures are now expected for the next two weeks, said Jim Ritterbusch, head of trading-advisory firm Ritterbusch & Associates. He said the market "is likely to remain subdued," and said storage injections could remain higher than average "for a few more weeks."

On Thursday, the Energy Department reported an injection of 25 billion cubic feet, well below the 35-bcf injection expected by analysts.

Demand for natural gas tends to fluctuate in the summer as the fuel is used to generate electricity for cooling needs.

"The fact is, the market has a sufficient supply of gas," said Gene McGillian, a broker and analyst with Tradition Energy. He cautioned however, that the end of August is when attention begins to turn in earnest to hurricane season in the Atlantic Ocean.

Investors are keeping watch on the tropics for any major storms that could halt natural gas output, though only a little more than 7% of U.S. gas production is from the Gulf of Mexico.

The National Hurricane Center is currently tracking three weather systems in the Atlantic Ocean. Two storms with under a 10% chance of becoming tropical cyclones in the next 48 hours are moving west, while Tropical Storm Gert is expected to turn east of Bermuda and head back out to sea across the North Atlantic.