Not sure if it counts, but during the Civil War large parts of the Confederacy lived under famine conditions. The Union blockade, a relatively poor transportation system, and an agricultural system slanted towards cotton production (not to mention the expected disruption of fighting a war on its own soil) meant that food shortages plagued both the military and civilian populations.

Is not he Dustbowl the largest famine in U.S. history? I know it resulted in the largest North American migration since pre-historic times.

There was of course the weather of 1816 where volcanic eruptions in Indonesia caused crop failures world-wide. New England suffered intermittent freezing temperatures all summer which cut the grain crop to less than half, and many suffered hunger.

I'm not sure exactly how much hunger it takes to be a famine.

If such a thing happened today we we would be helped out by the transportation infrastructure that could move food from places that were less affected.