(The opinions expressed here are those of the author, a columnist for Reuters.)

LONDON- It's that time of year when investors try to size up the economic, policy and political risks that will shape the 12 months ahead and outline the essentially unpredictable events that could knock their strategies and investments off course.

And 2019 is shaping up to be a year in which absolutely nothing can be ruled out, be they low-probability, high-impact "black swan" or slightly more predictable but still significant "grey swan" events.

President Trump impeached Brexit doesn't happen and Britain stays in the EU Trump fires Fed chief Jerome Powell Germany in recession The U.S.-China trade dispute resolved and markets soar Oil crashes to $20 a barrel

None of those are likely, all would have major implications for global markets. But given the febrile politics around the world and how mature the economic and market cycles are right now, absolutely anything goes.

Take Brexit. If Britain's departure from the European Union is delayed, postponed, dragged out for years or even scrapped all together, what would sterling do The chances are, it would rise considerably.

On the Brexit referendum night of 23 June, 2016, sterling was changing hands just under $1.50. Excluding a "flash crash" one night in October that year, it fell as low as $1.20 and this week found itself back below $1.25.

Brexit continues to weigh heavily on the pound. Any delay, or Britain somehow in fact remaining, could easily see the currency back above $1.40 in short order. And the odds on either, or on a second referendum, are shortening.

No Brexit or a soft Brexit could put Bank of England rate hikes back on the table, which could trigger a snap back in the sizeable short position that's been built up in sterling, certainly across the speculative trading community.

And what about monetary policy elsewhere President Trump has pilloried Fed Chair Jerome Powell, branding the central bank under his guidance "loco", "crazy" and "ridiculous". If Powell and Co continue to tighten policy next year, could Trump take the ultimate step and fire him

It's not all that outlandish: the Fed raises rates once too often, the yield curve inverts, the economy tips into recession, and the stock market crashes, forcing an even more incensed Trump to act.

That would unnerve investors, stoking uncertainty and volatility across all markets, and would probably lead to even lower yields and a deeper inverted curve on waves of safe-haven demand for Treasuries.

The 2s/10s U.S. yield curve last week got to within less than 10 basis points of inverting, a precursor of every U.S. recession over the past 50 years. It may well be different this time, which in itself would be a "grey swan" moment for investors.

A U.S. recession isn't on the cards for next year - that's expected to come in 2020, if at all - but a slowdown is. And it could be enough to help drag oil down to $20 a barrel, especially if Germany and the euro zone crater, too.

The European Central Bank's three-year, 2.6 trillion euro QE bond buying programme is ending, and the central bank says it is still on track to raise rates in the second half of next year. Not if there's a serious downturn, though.

In that scenario, Frankfurt's printing presses will be fired up again. But because of its capital key weightings and QE rules, the ECB is running out of bonds it can buy. Still, under Mario Draghi the central bank has shown nothing if not unprecedented flexibility and a willingness to go where it has never gone before.

A downturn in the United States and Europe wouldn't be good for oil. Remember the calls for $100 oil earlier this year They've been completely blown out of the water now following a 30 percent crash in barely two months, and at $60 now, Brent is just as close to $20 as it is $100. U.S. WTI oil is currently at $50.

Technically, oil is extremely fragile, so $20 is far more likely than a sharp rebound. Fundamentally, there's a glut of oil in the world now too, according to analysts at Nomura. And less than three years ago, crude was trading at a 13-year trough in the $20s.

There's no shortage of predictions and forecasts for next year from banks, brokers, traders and investors. Some serious, others less so. Below is a round up of black and grey swan calls from three banks, Saxo Bank, Nomura and HSBC. Some positive, some negative, all way out of consensus but all with the potential to pack a powerful punch for markets in 2019.

(By Jamie McGeever) ((jamie.mcgeever@thomsonreuters.com; +44 (0)207 542 8510;))