A new lava dome is forming in the crater of Cleveland Volcano.  In response, scientists at the Alaska Volcano Observatory bumped the aviation alert level from yellow to orange on Tuesday.

It’s the fourth time since July that the Observatory has raised the alert.  Back in December, Cleveland had a minor eruption that sent up a 15,000 foot ash cloud and removed a lava dome that had been forming for months. 

Although the cloud dissipated quickly and didn’t affect aviation, it drew international attention to the remote volcano on uninhabited Chuginadak Island.  Cleveland lies on a major flight path and previous eruptions have disrupted air travel.

In a release about the growing lava dome the Observatory noted that “sudden explosions” with ash clouds “exceeding 20,000 feet” are possible at any time.

There’s no real-time monitoring network on Cleveland, but the Observatory does have an alert system that can detect the lightning that accompanies major ash clouds within minutes.