I just flew a Piper Warrior today and I got confused with the instrument shown below, it's the one above the CDI and to the right of the altimeter.
The WX-10 System consists of a display, remote processor, and an antenna. This Stormscope offers thunderstorm information on a bright green CRT display. The more the dots on the display are clustered together, the stronger the storm activity. Up to 256 dots may be displayed. The WX-10 also has a Forward mode to concentrate the display and processing power on the airspace ahead of you.
See also: Wikipedia: Lightning detection § Aviation use