|Start:||July 17, 2019 ||Observed|Description:
On July 4, 2019 (AKDT; 06:51 July 7, 2019, UTC), the Alaska Volcano Observatory raised the Aviation Color Code to YELLOW and Alert Level to ADVISORY at Semisopochnoi, due to an increase in seismicity persisting at least eight hours. At the time of the color code change, no explosive activity had been detected on the Adak infrasound array and clouds obscured satellite views of the volcano.
On July 18, AVO raised the color code to ORANGE and the volcano alert level to WATCH, citing "Elevated seismicity that began yesterday has increased overnight. A stronger tremor signal recorded at 23:39 AKDT / 07:39 UTC also produced an infrasound signal on an infrasound array 260 km (161 miles) east on Adak island. This event likely produced ash emissions, and at the current level of continued unrest similar events could occur with little to no warning. The meteorological cloud deck has been around 10,000' (3000 m) asl over Semisopochnoi and no ash signals have been detected above this height. A small plume extending 18 km (11 miles) from the Cerberus vent was visible in satellite data from yesterday, but did not contain an ash signal."