Wall Street's favorite recession gauge has been flashing for 16 months, but the other key ingredient of a downturn is nowhere to be seen

• A key recession signal has been flashing for 16 months, but the other half of a downturn is missing. 
• Historically speaking, recessions follow an "unforeseen" shock to the economy. 
• There's nothing yet brewing that would send the economy into a tailspin akin to past recessions. 

The most closely watched gauge of a coming recession has been pointing to a downturn for almost a year and a half, with Wall Street scratching its head over the conflicting signals of the inverted yield curve and a persistently strong economy.

But the classic recession indicator is only one half of the recipe needed for a slowdown, and the other component is still missing, DataTrek Research wrote in a note this week.