Aircraft weather radar systems cannot detect turbulence. It can occur anytime: on a clear cloudless day as well as in severe weather conditions. This is why flight attendants ask you to fasten seat belts as soon as you take your seat.
Conventional weather radar cannot detect wind speed or turbulence directly; it only detects solid and liquid objects above a threshold size.
Weather radar is merely detecting the presence of water. It is the pilot interpreting the picture, who is really detecting the turbulence.
Learn more about turbulence watching the video: Turbulence: what pilots should know?