Stocks are having their worst week in over a month. Here are the key levels to watch

The Dow, S&P 500 and Nasdaq are pacing for their worst week in 1½ months as concerns about the Trump administration's tariffs and the prospect of a trade war emerging weigh on the market.

Craig Johnson, chief market technician at Piper Jaffray, is highlighting several key levels to watch on the S&P 500 and Dow as both indexes on Thursday posted their worst days since early February. Here are his reasons why.

• On the S&P 500, 2,700 is the key "resistance" level to watch, while 2,650 is the key "support" level to watch.

• The 2,700 mark is just points above the S&P 500's 100-day moving average. The S&P broke below 2,650 heading into the market close, trading as low as 2,641.73. It was even lower in Friday's premarket.

• This break suggest we could see a retest of the S&P's 200-day moving average, at 2,584, in the coming weeks.

• Examining a chart of the Dow, the blue chips have failed to trade above its 50-day moving average in over 1½ weeks; if the index closes below its lows touched on March 2, at 24,217, then the next stop is its 200-day moving average, just below 23,500. It was at 23,840 in Friday's premarket.

Bottom line: The markets are having a volatile week, and Piper Jaffray's chief market technician is laying out key levels investors should watch in the Dow and S&P 500.

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Michael Santoli joined CNBC in October 2015 as a Senior Markets Commentator, based at the network's Global Headquarters in Englewood Cliffs, N.J.  Santoli brings his extensive markets expertise to CNBC's Business Day programming, with a regular appearance on CNBC's Closing Bell (M-F, 3PM-5PM ET). In addition, he contributes to CNBC and CNBC PRO, writing regular articles and creating original digital videos.

Previously, Santoli was a Senior Columnist at Yahoo Finance, where he wrote analysis and commentary on the stock market, corporate news and the economy. He also appeared on Yahoo Finance video programs, where he offered insights on the most important business stories of the day, and was a regular contributor to CNBC and other networks.

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