By Victor Reklaitis and Sara Sjolin, MarketWatch

NEW YORK (MarketWatch) -- U.S. stocks traded flat to slightly negative in afternoon action Wednesday, keeping on track for solid yearly gains with just a couple of hours of trading left in 2014.

The Dow industrials are struggling to close out the year above 18,000 after topping that big round number for the first time last week.

The S&P 500 (SPX) was last down 3 points, or 0.1%, to 2,078 as the benchmark eyed an impressive yearly advance of 12.4%, while the Dow Jones Industrial Average (DJI) dipped by 5 points, or less than 0.1%, to 17,978. The blue-chip gauge, now up 8.4% for the year, clambered above 18,000 at the open Wednesday and was up by as much as 60 points for the session before slipping into the red.

The tech-heavy Nasdaq Composite (RIXF) was roughly flat at 4,778, looking at a 2014 gain of 14.4%. Bucking the negative trend, the Russell 2000 (RUT) added 3 points, or 0.3%, to 1,216, and the small-cap index is on track for a yearly advance of 4.5%.

There's no single factor behind the stock market's choppy action on Wednesday, and investors shouldn't read that much into its moves, according to Joseph Quinlan, chief market strategist at U.S. Trust.

"It's thin trading, and there's not much news out there," Quinlan told MarketWatch. Trading is light ahead of the New Year's Day holiday, with composite volumes for the New York Stock Exchange and Nasdaq far below their 30-day moving average, according to FactSet data.

On the economic front Wednesday, investors took in worse-than-anticipated readings for weekly jobless claims and Chicago-area business conditions. On the upside, a gauge of pending home sales rose 0.8% in November, topping expectations.

The year's advance has been driven largely by improving corporate earnings, steady gains by the U.S. economy and a still-accommodative Federal Reserve, even as the central bank is widely expected to begin raising interest rates in mid-2015. The S&P has averaged a yearly rise of 9.7% over the past two decades, so 2014's advance ranks as above average, albeit well below 2013's jump of 29.6%.

Both the New York Stock Exchange and Nasdaq are observing regular trading hours on Wednesday, closing at 4 p.m. Eastern. Markets will reopen on Friday.

See: When do markets close on New Year's Eve?

Movers and shakers: Southwest Airlines Co.(LUV) was among the S&P 500's best-performing stocks Wednesday. Airlines are benefiting from plunging oil prices, and crude was down again Wednesday.

NephroGenex Inc.(NRX) soared after the small biotech late Tuesday announced positive safety study results for a diabetic nephropathy treatment.

Meadowbrook Insurance Group Inc.(MIG) also gained after news that China's Fosun International Ltd. will buy the U.S. insurer for $433 million.

(Read more about the day's movers here: http://www.marketwatch.com/story/nephrogenex-mimedx-in-focus-for-wednesday-2014-12-30.)

Other markets: In Asia, most markets ended higher, although Australia and New Zealand finished in the red. The stock markets in Japan, South Korea, Indonesia, Thailand and the Philippines were closed for New Year's Eve.

Trading was upbeat but thin in Europe. German and Italian stock markets were closed.

The dollar (DXY) gained, with the greenback on track for a yearly advance of more than 12% and its best year since 2005, according to FactSet data. Oil prices slumped, while gold prices dropped.

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