By the Numbers

Hot August may give way to cool September

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The is up 3.4 percent so far this month and on pace for the best August performance since the year 2000.

But here comes September, and if history is a guide, it may not be good news for investors.

Since 1950, September is the worst performing month for the index, losing on average more than negative 0.5 percent.

If you look just at the last 20 years, the negative trend has continued as September was the second worst performer, and as a result, second most volatile month.

Average volatility as measured by the VIX spiked up by more than 8.65 percent during the month.

After the financial crisis, the negative trend changed, and the S&P was up four of the last five years in September for an average gain of 2.2 percent. But what's troubling for stocks is the performance of September markets, after a powerful move higher in August.

S&P 500 stocks up each September

SymbolCompanySep-13Sep-12Sep-11Sep-10Sep-09Avg.
CERNCerner Corp14.225.813.8815.2921.2112.08
SNDKSanDisk Corp8.635.3610.1110.6222.611.46
AMZNAmazon.com12.472.440.4625.8214.9911.24
CELGCelgene10.456.054.111.937.157.94
PRGOPerrigo Co.1.775.642.512.6515.147.54
LBL Brands6.541.362.0413.4713.877.46
COSTCostco Wholesale3.442.344.5714.1410.597.02
TSCOTractor Supply8.523.571.8916.682.856.7
VFCV.F. Corp6.024.383.8114.814.136.63
AIZAssurant 2.385.821.7911.327.055.67
BMYBristol-Myers Sqibb10.822.245.483.951.764.85
IBMIBM2.556.471.728.941.324.2
AGNAllergan3.016.330.78.321.53.97
DDominion Res6.990.884.162.184.293.7
NINiSource4.824.680.090.235.152.99
TAT&T0.442.890.145.813.692.59
DUKDuke Energy1.980.025.713.081.612.48

Since 1928, there have been 22 instances when the S&P gained more than 3 percent in August.

In nearly three fourths of those instances the index gave back gains in September, losing an average 1.3 percent, and two-thirds of the time it was negative, it continued lower into the end of the year.

The table below provides S&P 500 returns in September and through the year when it closed up more than 3 percent in August:

S&P 500 September performance

Year S&P % Chg. Aug S&P % Chg. Sep S&P % Chg. Sep-EOY*
20093.363.579.26
20006.07-5.35-13.01
19943.76-2.69-3.41
19933.44-10.62
19873.49-2.41-25.08
19867.12-8.54-4.26
198410.63-0.350.34
198211.60.7617.68
19795.31-0.31-1.26
19723.45-0.496.27
19713.61-0.73.09
19704.453.4113.04
19694.01-2.5-3.61
19634.87-1.13.48
19513.93-0.091.93
19503.255.5910.91
Avg5.15-0.761.00

—By CNBC's Pradip Sigdyal and Giovanny Moreano. Follow Giovanny on Twitter: @giovannymoreano