Natural gas continued its decline on Tuesday, at one point falling below $2, the lowest level the commodity has seen since April 2012 — and according to Dennis Gartman, it's about to get a lot worse.
"Nat gas has a problem because right now with El Niño out there it looks like this year we're going to have a slightly warmer, probably even a very warm winter on the east coast and that's going to put demand down as far as nat gas is concerned," the editor of the Gartman Letter said on "Fast Money" Monday.
Some meteorologists are predicting the warmest winter in recent years, similar to past El Niño winters of 1982 and 1997, a winter weather phenomenon that could severely hit natural gas prices as supplies continue to rise.
Read MoreWhat El Niño really means for you
"The trend is clearly downward, the weather continues to be extraordinarily good – we're probably going to be one, two or three degrees warmer than last year," said Gartman. "And if that happens, as long as there's no shutdown continually in production you might put a 1-handle up on nat gas."
It's not just natural gas that Gartman believes will fall; the widely known investor sees the whole commodity complex going lower.
"We have continued to see fracking going on even though we have seen drill rigs go off the edge of the cliff. We're still producing, and as long as we're still producing you're going to get RBOB gasoline going down,crude oil continues to go down, nat gas continues to go down, it's still a bear market in energy and it's going to continue to go down until it stops," said Gartman.