A Strong trend is a movement affecting a phenomenon in such a way that its development in time can be predicted (Kuusi, 1999).
Reference
Kuusi, O. (1999). Expertise in the future use of generic technologies: epistemic and methodological considerations concerning Delphi studies: Helsinki School of Economics and Business Administration.
A Weak Signal is essentially a sign of change that is not yet generally appreciated. Weak signals are “imprecise early indications about impending impactful events” (Ansoff and McDonnell , 1990). A weak signal is a signal that is hardly perceivable at present but constitutes a strong trend in the future (Godet et al., 1994). Hiltunen (2006) describes weak signals as currently small and seemingly insignificant issues and events which have the potential to tell us about the future.
According to another view (Mendonça et al., 2004), the concept of weak signal analysis is information on the likelihood of events whose probability is estimated to be very low, but to which is attached a high uncertainty concerning the impact of those events and the trends that can develop afterwards, if any.
References
Ansoff, H. and McDonnell, E. (1990). Implanting Strategic Management, Second Edition. Prentice/Hall International Inc.An aspirational description of what technology the creative community envisages to use in the next 5-10 years