NOAA National Weather Service Pacific Region

The tropical cyclone (TC) named Amos (2016) that impacted the Samoan Islands on 23 April 2016 was a particularly difficult storm to forecast. Both the intensity changes and the track of Amos represent a significant challenge for forecasters and this is briefly summarized in this report. Model forecasts initially indicated that the cyclone would track south of the Samoan Islands. However, the forecasts generally changed to a direct hit over Samoa as a Category 4 storm at approximately 0000 UTC 24 April based on model cycles initialized at 0000 UTC 23 April. TC Amos’ central pressure dropped from 983 hPa to 957 hPa between 0000 UTC 21 April and 0000 UTC 23 April. The models did not pick up on this rapid intensification until the intensification had already begun around 0000 UTC 21 April. The models also struggled to capture the rapid weakening of TC Amos due to vertical wind shear that began 0000 UTC 24 April as the cyclone continued to move north of the islands. Because of the initially ominous track forecasts for TC Amos to hit land, preparations for a Category 3 or Category 4 cyclone were underway in the Samoan islands and the population prepared for the worst. After the center of the storm moved north of the islands as a weaker storm than anticipated, the residents of the Samoan Islands were both surprised and relieved that the cyclone only gave a “glancing blow” to the islands and that the impacts were not as bad as originally feared. An in-depth evaluation of this particular tropical cyclone helps to shed some light on model deficiencies and can be used to help determine future model changes.