U.S. stocks are set to close out August with the Dow Industrials (NYSEArca: DIA - News) dropping more than 4% on the month. If the economic numbers continue to indicate a possible double-dip recession however, stocks are likely to fall by a much greater amount in September.

Historically, it isn't crash-prone October when U.S. stocks have their worse performance, but September. Stocks are entering the month in a technically weakened state that began earlier in the summer. In July, all four major indices - the Dow Industrials, the S&P 500 (NYSEArca: SPY - News), the Nasdaq (NasdaqGM: ONEQ - News) and the Russell 2000 (NYSEArca: IWM - News) - began a bear market trading pattern when their 50-day moving averages fell below their 200-day moving averages (sometimes referred to hyperbolically as a death cross). This is not enough to confirm a bear market however. The 200-day moving average needs to also start moving down. This has happened on the Dow Industrials and the S&P 500 in the last few trading days. The 200-days on the Nasdaq and the Russell 2000 have been moving sideways for a week or more and should start dropping soon. The Dow Transportation Average also needs to have a 50-day 200-day cross to confirm the negative action on the Industrials. As long as there isn't a massive rally, this will happen today. So stocks will be entering September in a technically vulnerable condition.

If more negative economic reports that indicate the economy continues to deteriorate then take place, the mix could be combustible. More hints of a double-dip recession from jobs or manufacturing would be especially damaging. Housing numbers this fall probably won't affect the market as much because things simply can't get any worse (with the exception of housing prices, which still have a lot of room to drop). The bad news on housing from the summer - numbers worse than those at the bottom of the Credit Crisis - may have a delayed impact on stocks though. Jobs have been the perennial weak spot of the attempted recovery and numbers have continually been at recession levels for over two years. Worsening unemployment figures would not be viewed kindly by stock traders. Falling manufacturing numbers won't be either since manufacturing led the economy up from its bottom in the fourth quarter of 2008.

U.S. stocks (NYSEArca: VTI - News) may also be following Japanese stocks (NYSEArca: EWJ - News) down. The Nikkei dropped 325 points or 3.55% in its last day of August trade. It is now at 8824 and could easily test its Credit Crisis bottom, which is around 2000 points lower. U.S. investors need to watch the key 10,000 level on the Dow Industrials and 1000 on the S&P 500. Stocks moving and staying below these key points would damage sentiment severely. The only thing left at that point to hold up the market would be the Fed's liquidity injections. These might work until the election on November 2nd. If so, you may not want to own stocks later that week.

Disclosure: No positions

Daryl Montgomery

Organizer, New York Investing meetup

Daryl Montgomery is an independent contributor to ETFguide. His viewpoints do not necessarily reflect those of ETFguide or the ETF Profit Strategy Newsletter.