January is usually among the most expensive months to buy US natural gas. But next month is shaping up to be the cheapest thanks to unseasonal warm weather. Wholesale gas for January 2016 delivery has nosedived, dropping 6.3 per cent so far this week to just above $2 per million British thermal units. The price was the lowest for any January-delivered futures contract in 17 years, exchange records show. The crash is turning heads in energy markets because the US, where nearly half of all households heat with gas, is on the cusp of winter. An exceptionally warm start to the season has undercut forecast gas consumption, however. Mild temperatures across big northern cities such as New York and Chicago are one effect of the El Niño event that is under way in the Pacific Ocean. Globally, the year 2015 is likely to be the warmest on record, according to the World Meteorological Organisation. In October and November, the number of gas-weighted heating degree days, a yardstick for energy consumption, was about a fifth less than normal. The National Weather Service sees above-normal temperatures persisting to February across the continental US.