Don't blame stock losses on the weather, pro says

US economy slows on soft China data: Strategist

As spring continues to bloom, it will become more difficult for stock market observers to attribute Wall Street losses to the weather, BlackRock's Russ Koesterich said Monday on CNBC.

"It's been hard to sort of disentangle the economic trends from the weather and for a long time what investors had been arguing is that, 'Look, we know the data is soft. We know that January and parts of February were weak, but this was mostly weather related.'" said Koesterich, global chief investment strategist at BlackRock, the world's largest asset manager, as stocks fell sharply on Monday.

In late trading, all three major averages were in the red. On the Nasdaq, 70 percent of the stocks traded were lower, while about 56 percent of New York Stock Exchange-listed shares fell. Health care was down the most of the S&P 500 sectors, with the decline tied to bio-tech stocks, while the telecom and utilities sectors were positive on the day.

"I think some of the disappointment we're seeing today is the fact that some of the softness is not only to be blamed on the weather," Koesterich said on "Squawk on the Street." "As March got a little bit more normal, we're still seeing that many of the prints are coming in below expectations."

To Koesterich, Monday's losses were really not a surprise, especially considering soft economic data out of the China and the United States.

The flash Markit/HSBC purchasing managers' index had Chinese manufacturing contracting in the first quarter of the year, boosting hopes that China's government might take steps to stimulate the economy.

In the U.S., Markit said its preliminary read on March manufacturing activity slowed after nearing a four-year high last month. The financial data firm, however, said the rate of growth and the pace of hiring remained strong.

Looking ahead, Koesterich said Friday's personal income report for February will be the most important economic indicator in the days to come.

"The economy is getting better. The labor market is slowly healing. One thing, which has been missing? Income growth," Koesterich said, suggesting a strong personal income number would indicate economic growth.

—By CNBC's Drew Sandholm. Follow him on Twitter@DrewSandholm. CNBC's Giovanny Moreano and Reuters contributed to this article.