Removing the abs() call should take care of the problem. With it in, the delta from light to dark and dark to light are always positive and greater than TRIGGER_THRESHHOLD, so each transition causes a trigger.
if (abs(newLightningVal - lightningVal) > TRIGGER_THRESHHOLD)
if ((newLightningVal - lightningVal) > TRIGGER_THRESHHOLD)
I'm making a assumption that the analog value for light is greater than the value for dark. If it's the opposite, just swap the values in the equation to this:
if ((lightningVal - newLightningVal) > TRIGGER_THRESHHOLD)