Earnings season has only just begun, but there already has been a surge in companies raising their earnings-per-share forecasts for the full year as the windfall from tax legislation starts to take shape.

Earnings revisions for 2018 have "picked up meaningfully," Bank of America Merrill Lynch Global Research analysts wrote in a note Jan. 16. S&P 500 bottom-up earnings are now tracking above $150, which represents a 1% increase over the past week and a 3% increase since mid-December.

That puts Wall Street's estimates just 2% below BofA Merrill Lynch's average earnings forecast of $153 one week in. "We expect upward revisions to continue through earnings season, particularly as guidance trends have remained strong (which was true even prior to tax reform,)" analysts said.

BofA Merrill Lynch said bottom-up earnings on the S&P 500 were flat from last week to $34.82, representing an 11% expansion from a year ago.

Analysts noted that more important than results will be tax reform's impact on earnings, labor and spending. Investors ought to look for more clarity on those effects moving ahead, as some early reporters might not be certain just how impactful the tax legislation will be on results. "Some early details have emerged from the 20+ S&P 500 companies that announced one-time bonuses, where a third also announced increased capex and about half announced higher wages," BofA wrote.

Earnings season effectively started last week, with seven S&P 500 companies reporting fourth-quarter results. Including early reporting firms, that brings the total to 26 companies, or 9% of earnings. Big banks offered an optimistic start, beating earnings forecasts despite reporting GAAP results impacted by one-off charges related to tax reform.

According to BofA analysts, JPMorgan Chase & Co. (JPM) - Get Report "set a strong tone" for banks, posting a better-than-expected loan growth figure and high net interest margins despite worse-than-anticipated trading revenue. Citigroup Inc. (C) - Get Report reported an $18 billion fourth-quarter loss on Tuesday to write down the value of tax credits saved up following the bank's massive government bailout in 2008, but the bank's adjusted earnings beat forecasts.

Bank of America Corp. (BAC) - Get Report  and Goldman Sachs Group Inc. (GS) - Get Report  report Wednesday, Jan. 17, and Morgan Stanley (MS) - Get Report will report Thursday, Jan. 18. 

So far, 77% of companies beat on earnings, 85% beat on sales and 69% beat on both, analysts found, tracking near record highs for beating on earnings and for beating on both. This is the highest level of sales beats BofA has recorded in its data history.

Another 9% of earnings will be reported this week, bringing the total to 18%.

JPMorgan and Citigroup are holdings in Jim Cramer's Action Alerts PLUS Charitable Trust Portfolio. Want to be alerted before Cramer buys or sells JPM or C? Learn more now.

Banks must deliver this earnings season or else, says TheStreet's Executive Editor Brian Sozzi.

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