Wharton School professor Jeremy Siegel marveled Thursday at the stock market's strong start to 2020, but said he is concerned it may not last.
"I'm just a little worried that this is becoming a momentum-driven market at this point," Siegel said on CNBC's "Fast Money: Halftime Report." "You don't know how far it will go. Certainly no sign of a break right now, but I think we people jumping on the train here and are going to ride it on a narrow path until they feel they've got some good gains."
The market's run is reminiscent of January 2018, Siegel said, when the S&P 500 posted, at the time, its best monthly performance since March 2016.
However, stocks plunged sharply in February before erasing their losses in the spring and summer months and returning to record highs. The recovery only lasted until the fall, at which point markets again fell sharply and reached a nadir on Christmas Eve 2018.
That collapse at the end of 2018 set up a powerful 2019 rally, which saw the S&P 500 gain nearly 29% for its best annual performance since 2013.
The index reached another record high on Thursday and is up more than 1% to start 2020. This is in spite of a turbulent start to the year in geopolitics as tensions between the U.S. and Iran have flared.
Siegel said the market's rally is taking place in a generally favorable environment that includes low interest rates, appropriate liquidity from the Federal Reserve's repo operations and the improved standing of President Donald Trump.
That does not mean the market is not vulnerable, though, Siegel said.
"I'm a little bit worried that if it continues much longer that something will puncture it and people will get off the train," he said.
Siegel said he doesn't think long-term investors need to significantly adjust their strategies, "but I don't see this sort of run continuing certainly throughout 2020 and maybe not even through the quarter."
Short-term traders with a tight range, on the other hand, should play the market, Siegel said.
"But as we saw what happened in January 2018, boy, when you get off that train it falls quickly," he said.