The Citi U.S. Economic Surprise Index is heading straight south, dampening hopes for a broadening of S&P 500 performance leadership. The current trend suggests that even if we’re not in a technology bubble now, we might soon be. The economic surprise index gauges U.S. economic data releases, weighted by importance, relative to consensus forecasts. The current reading of -45 indicates that the majority of vital data released recently have significantly undershot economist expectations, a potential sign of fading U.S. momentum. A broadening of market leadership beyond the AI-related technology behemoths like Nvidia Corp., Alphabet and Microsoft Corp. will be, to…