The grind lower continues in the S&P 500 this week - as of Thursday's close, the big stock market index has ended eight consecutive trading sessions in the red, the first time the big index has done that since October 2008. And only the second time it's happened in the last two decades...

As plenty of people have already noted, so far, the declines have actually been pretty minimal. The S&P is only down 2.29% over that eight-day stretch.

But that performance stat doesn't tell the whole story. Once again, the big market averages are masking what's really going on behind the scenes. So, while the S&P isn't down much as a whole in the last eight days, the worst 20% of the big index is down 7% or more as of this writing.

That's a meaningful chunk of the market that's posted some awful performance in the last several trading sessions. And it makes an important point crystal clear: simply not owning the very worst performers could do more for your returns than owning the best ones as we continue down the final stretch of 2016...

To figure out which stocks to steer clear of, we're turning to the charts today for a technical look at five stocks that could be toxic for your portfolio in the month ahead.

For the unfamiliar, technical analysis is a way for investors to quantify qualitative factors, such as investor psychology, based on a stock's price action and trends. Once the domain of cloistered trading teams on Wall Street, technicals can help top traders make consistently profitable trades and can aid fundamental investors in better entry and exit points.

Just so we're clear, the companies I'm talking about today are hardly junk.

By that, I mean they're not next up in line at bankruptcy court - and many of them have very strong businesses. But that's frankly irrelevant to what happens to their stocks; from a technical analysis standpoint, sellers are shoving around these toxic stocks right now. For that reason, fundamental investors need to decide how long they're willing to take the pain if they want to hold onto these firms in the weeks and months ahead. And for investors looking to buy one of these positions, it makes sense to wait for more favorable technical conditions (and a lower share price) before piling in.

So, without further ado, let's take a look at five "toxic stocks" to sell.

Costco Wholesale Corp.

Up first on our list of toxic trades is Costco Wholesale  (COST) . It shouldn't come as a huge surprise that Costco isn't a name you want in your portfolio right now: this $63 billion membership warehouse stock has lost more than 10% of its market value since the calendar flipped to January, underperforming the rest of the S&P 500 in a big way in 2016. The bad news is that Costco could have even further to drop following a breakdown this week...

Costco has spent the last month and change forming a descending triangle pattern, a bearish continuation setup that signals the potential for lower ground ahead. The descending triangle is formed by horizontal support down below shares at $147.50, with downtrending resistance to the top-side. Basically, as Costco pinballs in between those two technically significant price levels, it's been getting squeezed closer and closer to a breakdown through its $147.50 price floor. That breakdown finally happened this week.

Relative strength, which measures Costco's performance versus the rest of the broad market, has been an extra piece of evidence against this stock in recent months. Costco's relative strength line has been making lower highs as shares have underperformed the broad market in 2016, and that downtrend in relative strength is still intact here, making this stock statistically likely to underperform going forward. Costco is a stock you don't want to own this fall...

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