While pre-election polls overwhelmingly give Hillary Clinton the advantage ahead of tonight's result, with Reuters reporting "Clinton has 90 percent chance of winning: Reuters/Ipsos States of the Nation", many banks and brokers - still hurting from the Brexit calamity - aren't taking chances. Some of the more prominent industry names, such as Morgan Stanley, JPMorgan and Goldman Sachs are bracing for potential tumult on financial markets after Tuesday's U.S. election should Trump win.

As the outcome of the most bitterly fought U.S. presidential elections starts to roll out by Wednesday in Asia, the regional markets will be the first to trade on the results. Reuters writes that as a result, Asia-focused banks HSBC and Japan's Nomura Holdings Ltd are among institutions boosting staff levels, while others are raising the margin requirements for trading to cope with a possible spike in volume or volatility.

In the United States, Morgan Stanley told staff to consider using stop-loss orders if the result causes trading volumes and volatility to spike. MS also told advisers in its wealth management unit to prepare for election-related conversations with clients and pointed them to relevant pieces of research. Most banks have already scheduled early Wednesday morning calls to institutional clients to guide them on what next steps will be in either outcome.

Meanwhile, traders expect the S&P to swing by about 2% in either direction on Wednesday based on the price of S&P 500 index options. Options on the PowerShares QQQ Trust Russell 2000 ETF, are pricing similarly large swings before the week is out.

Some banks are projecting a more extreme drop in the event of a victory for Republican Donald Trump, with Citigroup Inc estimating that a Trump victory could trigger a 3 percent to 5 percent sell-off for the S&P 500.

That said, the reaction may be more muted should Hillary be the winner as the vast majority of market participants expect: the "market pretty much told you who was going to win today," one capital markets official at a major bank who was not planning any extraordinary staffing measures told Reuters. Another official at a rival bank said Monday's 2.2 percent rally in U.S. stocks had lowered Wall Street's collective angst over the election from "DEFCON 4 to DEFCON 2," referring to the U.S. Defense Department's levels of alert.

Brokerage Nomura said in a report on Monday the election was the largest "known unknown" markets have had to contend with since the global financial crisis. It said a Trump victory would likely lead to a more than 6% drop in Asian equities. The election could result in both higher trading volumes and higher volatility as it's a bigger event for investors in this region than Brexit, said Stephane Loiseau, head of Societe Generale's Asia Pacific cash equities and global execution services. Ironically, both of these are urgently needed by banks to prop up flagging equity trading revenues, as such for many trading desks a Trump victory would be quite welcome.

Nomura plans to increase front office, back office, and technology staff, "ensuring we have a clear escalation system for handling decision-making", and possibly raising risk limits."

Chris Weston, chief markets strategist at IG Markets in Melbourne, said his firm had raised margins on U.S. indices and some dollar trades to 1 percent from 0.5 percent but doesn't expect to see "a massive collapse or spike".

In Asia and Europe, HSBC will bolster staff on trading floors in major hubs including London and Hong Kong to deal with client requests, said a person with direct knowledge of the plan. "Around any high profile, potentially market-moving event, it is not unusual for some trading desks to increase staffing levels," said a spokesman for Europe's biggest bank.

No U.S. stock exchange plans extraordinary measures to cope with potential market volatility, exchange officials told Reuters on Monday. This may change if early voting reflects that a Hillary victory is not a foregone conclusion.

Central banks are also prepared for any contingency. The Swiss National Bank stands ready to intervene in the currency markets if the outcome of the U.S. election triggers a rush into the safe haven currency, a member of the central bank's governing board said on Monday. Andrea Maechler declined to comment on the outcome of the election or how it would affect the Swiss franc or financial markets but said that "the uncertainties are large," she said in an interview with Swiss TV to be broadcast on Monday evening.

"What I can say is just like with the Brexit vote, we are ready. My team is prepared and the Swiss National Bank will be there, if necessary, to intervene to stabilize the currency markets."

We are confident all other central banks are making similar arrangements.

Still, while banks are taking measures, few are overly worried that a Brexit outcome will repeat itself. And yet, according to Nomura FX strategists, the USD/JPY is showing an “eerie” parallel with the lead up to the June 23 Brexit vote, after which it plunged to 99.02 on the unexpected result. "Markets appear to price in 90% chance of a Clinton win, so a Trump victory would have a huge market impact that could exceed that of Brexit" says Yunosuke Ikeda, head of FX strategy for Japan at Nomura

“The current USD/JPY levels and market situations are coming to resemble those seen before the June U.K. referendum."

But what is the signal that banks will be most focused on to determine if consensus will be wrong - again - and Brexit-like chaos is set for a repeat appearance?

According to counterparties, Deutsche Bank - which likewise has a base case that Clinton will win - has advised its sales and trading team to immediately report to their trading desks should Trump wins Pennsylvania and Michigan. This means shortly after the polls in these two states close at 8pm, trading will be either promptly die down or will just be getting started.

So should Trump be seen as leading and/or winning in those two key battleground states, expect all volatility hell to break loose.