Placed above are IR images of clouding over the Darjeeling-Sikkim Himalaya of time 0530hrs and 1230hrs. The clouding is due to one of a series of WDs which has been affecting the western Himalaya first, then sweeping east towards us.
In Kalimpong there was continuous thunder and lightning from around 1330hrs till 1830hrs though the rainfall was only light. Darjeeling on the other hand was swathed in a blanket of hail. Read more here.
In this regard, I am placing the explanation to this weather event, below:
Darjeeling Sikkim Himalaya