A whale's mouth is the weird-looking effect of clouds when a gust front of a storm is passing over. The cold downdraft of air of the storm rushes outward and forward along the surface and lifts the warmer air in the direct vicinity of the storm over its condensation level. One gets to see the back side of this mini cold front and the inside of the gust front cloud. It looks somewhat like mammatus and other forms of turbulent cloud masses, sometimes showing very neat ordering. 

The picture was taken when a developing line of moderate to heavy rain showers, almost parallel in the upper airflow, slowly passed over. It was accompanied by a few rumbles of thunder, gusty wind of 38KT and some small hailstones as well. More pictures of this day to be found here