For the second quarter, S&P 500 earnings are expected to grow just 2.6 percent, according to Wall Street analysts estimates compiled by Thomson Reuters.

But projections for third-quarter earnings growth, on the other hand, are seen at 8.4 percent, with fourth-quarter growth seen at 12.8 percent. And investors are banking on those numbers to get them past what may be a rocky few weeks.

"It looks like the slow growth rate of S&P earnings has troughed and is poised to begin rising again after three successive, low, single-digit quarters," said Mike Jackson, founder of Denver-based investment firm T3 Equity Labs.

For the bulls, it had better work out that way. The S&P 500 <.INX><.SPX> on Thursday notched an all-time closing high, with the consensus for the S&P to hit 1,700 by the end of 2013, according to a Reuters poll.

Should those brighter second-half earnings expectations be cut, it could hurt the rally.

The lion's share of 2013 earnings growth is expected to come in the second half of the year. The last two quarters of 2013 are expected to account for more than 70 percent of the year's growth, Thomson Reuters data shows, with about 52 percent of total earnings per share forecast to come in that period.

The economic growth rate is going to be crucial in all this. Most economists so far have underestimated how slow the U.S. recovery has been. The U.S. government recently cut its reading of first-quarter growth to 1.8 percent, and some economists are predicting the second quarter will be below 1 percent.

U.S. economic growth is expected to pick up to an annualized 2.3 percent rate in the current quarter and 2.6 percent in October-December, according to a Reuters poll.

Rising U.S. home prices, the boom in U.S. oil and gas production, and a better jobs picture have all made the outlook for the U.S. economy look better than in many other places, particularly in comparison to weakness in Europe, China and other major emerging markets.

Even political gridlock in Washington and automatic government spending cuts have been less of a threat as the fiscal picture has improved.

Heavy monetary stimulus from the U.S. Federal Reserve has helped the stock market at a time when S&P 500 earnings growth has been lackluster, averaging 5 percent for the last four quarters. Revenue growth has averaged just 0.9 percent, according to Thomson Reuters data. Second-quarter revenue growth is expected to come in at 1.5 percent.

MISSING SALES GROWTH

Six of the 10 sectors in the Russell 2000 are forecast to earn more than their usual percentage of earnings in the second half. Information technology is the most skewed, with 55 percent of earnings expected in the second half, compared with a bit more than 51 percent historically, Credit Suisse analysts said.

In addition to IT, Credit Suisse mentioned: materials, energy, consumer staples, consumer discretionary and utilities

Companies that are expected to see more than 50 percent of their 2013 earnings in the second half of the year could be most at risk if year-end growth fails to materialize, they said.

Among those are Dell (>> Dell Inc.), where forecasts are for 54 percent of earnings in its second half compared with a usual 49 percent, household products maker Procter & Gamble (>> The Procter & Gamble Company), with 57 percent versus 55 percent, and drugs maker Bristol Myers Squibb (>> Bristol Myers Squibb Co.), at 53 percent compared with a typical 49 percent.

"In the absence of a better economic climate and with flat revenue growth, earnings expectations will likely have to be trimmed in the second half," said Mark Luschini, chief investment strategist at Janney Montgomery Scott in Philadelphia.

Current revenue forecasts don't suggest strong gains between now and year-end. Second-quarter sales growth is forecast at 1.5 percent, with third-quarter growth at 3.6 percent and fourth quarter at 1.4 percent, based on Thomson Reuters data, but analysts said those numbers may be too conservative.

"Mid-2013 is an inflection point for both GDP and EPS growth," Goldman Sachs analysts wrote in a research note this week, adding they expect "limited revisions" to second-half earnings estimates.

(Reporting by Caroline Valetkevitch; Editing by David Gaffen and Leslie Gevirtz)

By Caroline Valetkevitch