With many markets ending the month near critical levels, it increases the level of uncertainty and may keep investors on the sidelines in September, writes MoneyShow's Jim Jubak.

One of the things I'm worried about as we head into September and October is that we're finishing August at kind of make or break technical levels for a lot of things; for treasuries, for currencies, for the US S&P. All of these are finishing August perched at levels that, if they broke through, people would go "Uh-oh," and that, in itself, is enough to make people feel uncertain, because we have a lot of other uncertainties.

If you're looking at the US dollar versus the yen, or you're looking at the yen versus the dollar, you've got the yen at 97, which has, sort of, been decent support. Is the yen going to break lower, which would be good for Japanese stocks, or stay right here? The dollar, in terms of the dollar index, again, not much in the way of direction, which makes people more and more nervous.

You have all these things, including the S&P, which is, sort of, near support from July. The question then, of course, is if it's goes through that, where is the next support? When you're in a situation like this, that's what people are asking. They're looking at the charts and not going, "Oh, so where is the resistance? How much further up can I go, so how much can I buy before I need to start worrying about hitting the ceiling?" They're worried about, "Okay, if I go through this support, and how much below where I am now is the next support?" They're starting to worry about really taking the loss and seeing that loss multiple.

That's the kind of nervousness that we don't need as we go into September and October, which is a nervous enough month. Technically, this market is looking increasingly nervous, increasingly poised, so that investors will ask, "Where is the next support, because it looks like we're going through this one." If that's the psychology that takes effect, as we go into September and October, that almost guarantees that we're going to see, at best, flat months, and the worst, the continuation of something. It looks like it might actually build into the first correction that this rally has seen in a long time. Actually, I think it may be good in the long term to rebuild the rally, but in the short term is going to be, well...painful.

This is Jim Jubak for the MoneyShow.com video network.