Pick a nice volumetric cloud setting or one that starts with the clouds set to cast shadows. The Preset sky called "Fall Wedding" comes to mind. Play with the atmosphere settings. Make the ambient shadow color darker. You can crank up the cloud cover a few percent to make more clouds and get the scene darker.

You can also use a negative infinite light at at between 2-5% behind your camera and low to the ground the ground to darken the undersides of the cloud. Remember Negatives reverse everything so if you want a blue cast to the clouds you should pick an orange brown color to use as the negative light. Positioning these will give you deep shadows but still leave shiny whites peeking through.