The S&P 500 is massively overvalued - and likely to plunge like it did last year, top economist David Rosenberg says

• David Rosenberg expects a repeat of last year's sharp decline in stocks as risks mount in markets.
• The economist sees several headwinds including Fitch and Moody's downgrades and Chinese deflation.
• The S&P 500 is way overvalued by historical standards and relative to other assets, Rosenberg says.

Get ready for a repeat of last year's slump in stocks, as investors wake up to the mounting risks in markets today, David Rosenberg has warned.