This weather pattern has caused extreme conditions, including the warmest Alberta winter in 90 years and bursts of cold in Texas, affecting drillers in unusual ways.

Most industry experts have already adjusted to the fact that the rise of the shale boom has created a new seasonality in oil markets, with harsh northern weather typically curtailing supply due to road closures or frozen fracking fluids. But this past winter has skewed that pattern thanks to the strongest El Niño conditions in nearly two decades, experts say.

Many will turn their attention to U.S. government data, searching for clues as to whether it is a turning point for an industry that has held up surprisingly well throughout a 19-month slump that has slashed crude prices by nearly 75 percent. One of the most closely-watched reports in the global oil market due out later on Monday may show the first significant monthly output decline in oil since the shale boom began six years ago.

A number of companies, like Hess Corp (>> Hess Corp.) and Continental Resources (>> Continental Resources, Inc.), have slashed budgets for the year, but declines in output have been smaller than expected.

In North Dakota, preliminary state data showed a 3 percent drop in December output versus the month before - similar to the preceding January, and the winter before that. With temperatures on average more than 8 degrees above normal in December, the warmest since 2011, the decline is more likely a fundamental shift rather than a seasonal anomaly.

"Looking at the weather, we had relatively few days where there was too much wind and (there was) no precipitation in November. It was a very dry month, and it was not a cold month," Lynn Helms, director of North Dakota's Department of Mineral Resources, told reporters on a call last month.

In Texas, it was the opposite with some producers walloped by a cold spell in late December. Output took a hit, but has rebounded, according to PointLogic Energy, which uses pipeline flow data to estimate oil production.

This has left many analysts guessing as to whether the shale oil output has finally reversed course after a six-year surge, a potential signal of the end of the oil rout.

"Weather is just one of a whole host of contingencies that can disrupt supply," said Michael Cohen, head of energy commodities research at Barclays. "You have to assume normal weather and when normal weather doesn't occur, then you have to adjust for it."

MODELING HEADACHES

To be sure, a spell of warm or cold weather is not going to disrupt the broad trend that is already underway. Oil prices languishing near a decade low of $30 a barrel have already started the output decline.

Jonathan Garrett, a principal analyst at Wood Mackenzie, said that economics in the Bakken region appear to be more "challenging."

Even stalwarts are taking drastic action. North Dakota's biggest producer, Whiting Petroleum Corp (>> Whiting Petroleum Corp), said it would suspend all fracking and spend 80 percent less this year amid low oil prices.

But colder or warmer weather may affect the timing and speed of the production drop.

In December, oil producer Apache Corp (>> Apache Corporation), which pumps around 93,000 barrels per day (bpd) from the Permian basin, had "some fairly significant weather downtime," President and CEO John Christmann said in an earnings call on Thursday.

Occidental Petroleum (>> Occidental Petroleum Corporation) said earlier this month that winter weather hurt production by 1,300 barrels of oil equivalent per day in the fourth quarter.

PointLogic data show that output dropped by nearly 300,000 bpd at the end of December - but then rebounded by some 220,000 bpd since then.

In Western Canada, conventional drillers step up operations during the winter months, when the ground is firm enough to support their heavy rigs, before withdrawing in April.

But this year, they began packing up weeks ago, according to data from Baker Hughes , deterred not only by the tumbling crude prices but by 60 degree temperatures that have triggered an early spring melt.

MODELING THE WEATHER

This year's El Niño injected new complexity into the already dark art of interpreting and modeling the U.S. shale boom and bust, which has repeatedly confounded experts who rely on data that is incomplete, late or both.

Most gave up long ago on data tracking the number of rigs drilling for oil. Once thought a good proxy for output, the rig count has fallen by half since its mid-2014 peak while oil production has barely ebbed as drillers managed to get increasingly more barrels out of every new well.

The U.S. Energy Information Administration last year revamped its monthly production data - released two months in arrears, with December data due on Monday - in order to improve its accuracy.

And it has begun to incorporate the effects of seasonal weather into its monthly short-term energy forecasts, a key metric for oil traders, just as it anticipates a certain level of hurricane-induced outages on Gulf of Mexico output.

Still, that always occurs after a weather-related disruption and it does not have survey data or state administrative data to measure the impact, according to John Staub, who heads the EIA's oil and gas exploration and production team.

(Reporting by Catherine Ngai; editing by Jonathan Leff and Diane Craft)

By Catherine Ngai