A weak signal is 1) an idea or trend indicating a change that may affect a business or organization; 2) a new and surprising fact; 3) a low likelihood threat or opportunity to an organization that is often scoffed at by people who "know"; 4) an issue with a substantial lag time before it will impact planning and society.

According to Ansoff (1979), weak signals of incipient changes can help managers avoid strategic surprises. He also notes weak signals progressively increase in strength as environmental signs become stronger (p. 187).

from http://www.mgtaylor.com/mgtaylor/jotm/winter97/wsrintro.htm
Ansoff, H. I., Strategic management, Wiley, 1979.