We had a fantastic lightening storm last nite, Aug Booms, low rumbles, actual lightening bolts (not the typical sheet lightening). Haven't had a good one in SW Puget Sound for years. a really good picture of an amazing set of lightning strikes.
CUMULONIMBUS INCUS at Mykonos, Greece - photo by Sfortis, via Wikipedia; a cumulonimbus cloud which has formed the characteristic flat, anvil-top shape. They can form into supercells resulting in severe storm phenomena such as tornadoes.