September is supposed to be the cruelest month for stocks, so it pays to take notice when the market bucks the historical pattern and closes the month in the black.

Although stocks have lost ground in the past two sessions, they’re nonetheless poised to finish September in positive territory. If the long-term trends hold true, this would be a favorable indicator for the fourth-quarter outlook.

Here are the numbers in a nutshell:

In the 33 calendar years from 1980 through 2012, the S&P 500 Index delivered a positive return in September on 16 occasions, and it fell in 17.

In the 16 years in which the index rose in September, it went on to gain an average of 5.91% in the fourth quarter — an annualized run rate of over 25%. Conversely, the average fourth-quarter gain for the 17 years in which the market closed lower in September was 2.71% — still positive, but well short of the 5.91% registered in the up years. (The full data set can be viewed here.)

In terms of absolute gains and losses, results in the fourth quarter have been close under each scenario. The market rose in the fourth quarter for 13 of the 16 years in which September was positive (81.3%), but it also rose in 13 of the 17 years in which September was negative (76.5%).

The reason why the average shows a more dramatic gap than the headline results is that in the three years in which the market registered its worst fourth-quarter returns — 1987 (-23.23%), 2000 (-8.09) and 2008 (-22.56%) — the downturn was preceded by a negative September. This indicates the numbers are skewed by a few outlier years.

But looked at another way, it could be said that the odds of a major decline in the fourth quarter drop considerably when the market finishes September in positive territory. Indeed, the losses were fairly benign in the three calendar years in which the market failed to gain ground in the fourth quarter after a positive September: -0.69% (1983), -3.82% (2007) and -1.01% (2012).

Assuming the market holds up in the month’s waning days, could this year be an exception to the long-term pattern? Absolutely. The market faces the three-pronged threat of a potential government shutdown (Oct. 1), the potential for the U.S. government to exceed the debt ceiling (mid-October) and two more Fed meetings to fuel renewed uncertainty about the fate of quantitative easing (Oct. 31 and Dec. 18).

However, while there is certainly no shortage of events to derail the market in the final three months of the year, the strong September might signal that stocks could very well overcome this latest “wall of worry” with minimal damage.