Oil prices have surged to their highest levels since 2015 in recent weeks, driven by a combination of factors including global economic growth and the falling U.S. dollar.

But there are signs that the rally may be nearing a turning point. On Mad Money Tuesday night, Jim Cramer pored over the charts with colleague Carley Garner, co-founder of DeCarley Trading and the author of "Higher Probability Commodity Trading."

Cramer and Garner started by looking at a weekly chart of West Texas Intermediate crude, that includes the Commodity Futures Trading Commission's commitments of traders report. The report shows the net long or net short positions of three groups of traders: large speculators; small speculators; and commercial hedgers. Historically, when the large speculators amass a big enough position, one way or the other, it can indicate that the trend is about to change. The theory is that when everybody's long, there's no one left to buy; when everybody's short, there's no one left to sell.

As of last week, large speculators were holding the largest bullish position in the history of crude oil futures, just under 666,000 net long contracts. Similarly, commercial hedgers were also holding one of the largest oil hedges in history.

Sooner or later, the trade will have to unwind. And according to Garner, when that happens, you don't want to anywhere nearby.

If these speculators start liquidating, Garner suspects it could push crude back toward $50 a barrel. Possibly even lower if the fundamentals take a turn for the worse.

The weak dollar has been very good for oil prices since crude is denominated in dollars. A daily chart of oil versus the dollar index shows a strong correlation. As the dollar has fallen, oil's been surging. Garner says, however, that the dollar has now fallen to areas where it should be able to find a floor of support. If that happens, it will remove a major prop underneath the oil rally. 

Over the past 30 trading days, the dollar index and crude have settled in opposite directions roughly 90% of the time. But Garner points out that over the previous six months, the negative correlation was more like 50%. Garner thinks that makes a snapback in the dollar seem more likely. If the dollar bounces, oil goes down.

A weekly chart of the dollar index shows that even though the greenback has been making lower lows since 2015, it's always tended to find support as it approaches 90, which is where it is right now.

The weekly chart of crude oil futures is also problematic, according to Garner. Crude seems to be on the brink of a breakout, right under its ceiling of resistance, but Garner says breakouts tend to be the exception, rather than the rule. Meanwhile, the relative strength index, or RSI, an important momentum indicator, is above 70 for the first time since mid-2011. This is a classic sign that oil has gotten overbought, meaning it's come up too far too fast and is due for a pullback.

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