The dome at the summit has been expanding in diameter over much of the summer.
Should Cleveland send an ash cloud soaring above the Aleutian Islands, as it has in the past, key trans-Pacific air routes could be affected.
The volcano covers about half of the uninhabited Chuginadak Island.
The nearest community is Nikolski, an Aleut village with a population of 20, located 45 miles to the east.
Photo: Kim Yano - NOAA