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

Stocks are having their worst week in a month and a half. Here are the essential levels to watch

The Dow, 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.