Generally the flashing ones are ordinary brightness ones. What you'll want are high brightness white and blue ones. I think for lightning, a strobing effect would look good, mabye alternate white then blue, very fast, and occasional pulses. So, what I think would be good is to have mabye a 14hz oscillator (555) to pulse the LEDs as a strobe, a NOT gate can give the alternate colour effect, use transistors to supply the current needed. To make the flashing random, a white noise generator is good, with a unit that samples the voltage of the waveform periodically and when it exceeds a set voltage, will trigger your LED array. I can draw a schematic for the 555 oscillator part, but i'm not certain how to go about the white noise random bit, someone else may know. I believe theres also a way of doing it with a PIC thats floating at one input :?: