Hey there, I read this and thought i'd drop my solution here. It's not the best script, certainly could have been done better but it was a quick fix for what I needed and I have little time to spend perfecting it. I'm not going to explain it either, hope the code is useful regardless, good luck!

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class StormLightning : MonoBehaviour
 {
     [Header("TIME FOR LIGHT INCREASE")]
     public float time_lightening;
 
     [Header("TIME FOR LIGHT DECREASE")]
     public float time_darkening;
 
     [Header("LIGHT INTENSITY")]
     public float intensity;
 
     [Header("TIME UNTIL NEXT FLASH")]
     public float wait_time;
 
     [Header("RANDOMIZE FLASHES")]
     public bool flashIsRand;
 
     [Header("LIGHTNINGBOLT SPRITES")]
     public GameObject[] lightningBolts;
 
     private Light myLight;
 
     private float timeA_stamp, timeB_stamp, timeC_stamp, rand_wait;
     private bool timeA_stamped, timeB_stamped, timeC_stamped;
     private int rand_sfx, rand_bolt, prevBolt;
 
     void Start ()
     {
         myLight = this.GetComponent<Light>();
         intensity = 0.3f;
 
         timeA_stamp = 0f;
         timeB_stamp = 0f;
         timeC_stamp = 0f;
 
         timeA_stamped = false;
         timeB_stamped = false;
         timeC_stamped = false;
 
         rand_wait = 0f;
         rand_sfx = 0;
 
         prevBolt = 0;
         rand_bolt = 0;
         if(lightningBolts != null)
         {
             for(int i = 0; i < lightningBolts.Length; i++)
             {
                 lightningBolts[i].SetActive(false);
             }
         }
     }
     
     void Update ()
     {
         if(!timeC_stamped)
         {
             if (!timeA_stamped)
             {
                 if(!flashIsRand)
                     AudioManager.RunEnviroSFX(5, false);
                 else
                 {
                     rand_sfx = (int)UnityEngine.Random.Range(5, 8);
                     AudioManager.RunEnviroSFX(rand_sfx, false);
 
                     if (lightningBolts != null)
                     {
                         //Don't forget to disable the previous lightning bolt used.
                         if(lightningBolts[prevBolt].activeSelf)
                         {
                             lightningBolts[prevBolt].SetActive(false);
                         }
 
                         //Pick and set a lightning bolt sprite to display.
                         //It's attached script should render it invisible after
                         //a short display.
                         rand_bolt = (int)UnityEngine.Random.Range(0, 9);
                         prevBolt = rand_bolt;
                         lightningBolts[rand_bolt].SetActive(true);
                         lightningBolts[rand_bolt].GetComponent<LightningBolt>().flag = true;
                     }
                 }
 
                 timeA_stamp = Time.time;
                 timeA_stamped = true;
             }
             else
             {
                 //Create a fast increase in light intensity here.
 
                 if (!timeB_stamped)
                 {
                     if (Time.time < timeA_stamp + time_lightening)
                     {
                         myLight.intensity += intensity * Time.timeScale;
                     }
                     else
                     {
                         if (lightningBolts != null)
                             lightningBolts[rand_bolt].SetActive(false);
 
                         timeB_stamp = Time.time;
                         timeB_stamped = true;
                     }
                 }
 
             }
 
             if (timeB_stamped)
             {
                 //Here we want the light to descrease fast.
                 if (Time.time < timeB_stamp + time_darkening)
                 {
                     myLight.intensity -= intensity * Time.timeScale;
                 }
                 else
                 {
                     //Now we stamp the time as one lightning flash has occured.
                     //Only do this process again after the specidied wait time
                     //has passed.
 
                     if (!timeC_stamped)
                     {
                         //if(lightningBolts != null)
                         //    lightningBolts[rand_bolt].SetActive(false);
 
                         timeC_stamp = Time.time;
                         timeC_stamped = true;
 
                         if(flashIsRand)
                         {
                             rand_wait = (float)UnityEngine.Random.Range(1, 7);
 
                             //Don't forget to flag the logic of a lightningbolt display to false/disabled.
                             lightningBolts[rand_bolt].GetComponent<LightningBolt>().flag = false;
                         }
                     }
                 }
 
             }
         }
         else
         {
             //Here is the wait time before processing the logic that
             //creates a lightning flash.
 
             if(!flashIsRand)
             {
                 if (Time.time > timeC_stamp + wait_time)
                 {
                     timeA_stamped = false;
                     timeB_stamped = false;
                     timeC_stamped = false;
                 }
             }
             else
             {
 
                 if (Time.time > timeC_stamp + rand_wait)
                 {
                     timeA_stamped = false;
                     timeB_stamped = false;
                     timeC_stamped = false;
                 }
             }
 
         }
 
     }
 }