By Scott DiSavino

May 19 (Reuters) - U.S. natural gas futures fell about 5% to a three-week low on Monday on a small increase in output over the last few days even though pipeline maintenance curtailed some gas flows from the Permian basin in West Texas.

That pipeline work in Texas trapped some gas in the Permian shale and caused spot prices at the Waha hub to turn negative for the fourth time this year.

Gas futures NGc1 for June delivery on the New York Mercantile Exchange fell 17 cents, or 5.1%, to $3.164 per million British thermal units, putting the contract on track for its lowest close since April 25.

That meant the front month has traded lower for four straight days, including Monday - the most sustained fall since late April. During those four days, gas prices have fallen about 13% so far.

Even though gas futures lost 12% last week, speculators boosted their net long futures and options positions on the New York Mercantile and Intercontinental exchanges by the most since March, taking them to their highest since April, the U.S. Commodity Futures Trading Commission's Commitments of Traders report showed.

Analysts said heating and cooling demand should remain low across much of the country in the coming weeks, allowing utilities to keep adding more gas into storage than normal for this time of year.

Gas stockpiles were already around 3% above the five-year (2020-2024) normal. EIA/GAS GAS/POLL

SUPPLY AND DEMAND

Financial firm LSEG said average gas output in the Lower 48 U.S. states fell to 103.9 billion cubic feet per day so far in May, down from a monthly record of 105.8 bcfd in April. The decline so far this month, however, was lower than expected on Friday.

Part of the reason for output reductions was maintenance on some gas pipes, including U.S. energy firm Kinder Morgan's KMI.N 2.7-bcfd Permian Highway from the Permian basin in West Texas to the Texas Gulf Coast.

Kinder Morgan said it will perform a turbine exchange at the Big Lake compressor station from May 13-26 that will reduce mainline capacity to around 2.2 bcfd.

Traders have noted the Permian Highway and other pipeline work trapped some gas in the Permian basin, helping spot gas prices at the Waha Hub NG-WAH-WTX-SNL in West Texas to drop from 94 cents per mmBtu for Friday to a negative $1.52 for Monday. That compares with an average of $1.83 over the prior seven days.

LSEG forecast average gas demand in the Lower 48, including exports, will drop from 99.0 bcfd this week to 94.6 bcfd in two weeks. Those forecasts were higher than LSEG's outlook on Friday.

The average amount of gas flowing to the eight big liquefied natural gas export plants operating in the U.S. fell to 15.1 bcfd so far in May, down from a monthly record of 16.0 bcfd in April.

The LNG feedgas decline so far this month was mostly due to reductions as a result of maintenance at Cameron LNG's 2.0-bcfd plant in Louisiana and Cheniere Energy's LNG.N 3.9-bcfd Corpus Christi plant under construction and in operation in Texas, and brief reductions at Freeport LNG's 2.1-bcfd plant in Texas.

The U.S. became the world's biggest LNG supplier in 2023, surpassing Australia and Qatar, as surging global prices fed demand for more exports, due in part to supply disruptions and sanctions linked to Russia's 2022 invasion of Ukraine.

Gas was trading at around $12 per mmBtu at both the Dutch Title Transfer Facility TRNLTTFMc1 benchmark in Europe and the Japan Korea Marker JKMc1 benchmark in Asia. NG/EU

(Reporting by Scott DiSavino; editing by Barbara Lewis)

((scott.disavino@thomsonreuters.com; +1 332 219 1922; Reuters Messaging: scott.disavino.thomsonreuters.com@reuters.net))

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For graphics on Baker Hughes rig counts, see: http://graphics.thomsonreuters.com/15/rigcount/index.html

For next-day SNL U.S. gas prices, see: 0#SNL-NG

For next-day SNL U.S. power prices, see: 0#SNL-PWR

For U.S. natgas price and storage polls, see: NGAS/POLL

For U.S. nuclear power outages, see: NUKE/

For U.S. Northwest hydro power report, see: NWRFC

For U.S./Canada natural gas rig count vs Henry Hub futures price, see: http://tmsnrt.rs/2eT9k44

For the U.S. natural gas speed guide, see: USGAS

For the U.S. power speed guide, see: USPOWER

To determine CFTC managed money net position add (NYMEX Henry Hub options and futures combined 3023651MNET) plus (ICE Henry Hub options and futures combined 3023391MNET divided by four) plus (NYMEX Henry Hub swaps options and futures combined 303565BMNET divided by four) plus (NYMEX Henry Hub penultimate gas swaps 303565CMNET divided by four)

NYMEX Henry Hub options and futures combined 0#3CFTC023651

NYMEX Henry Hub futures only 0#1CFTC023651

ICE Henry Hub options and futures combined 0#3CFTC023391

NYMEX Henry Hub swaps options and futures combined 0#3CFTC03565B

NYMEX Henry Hub Penultimate gas swaps 0#3CFTC03565C