On the Track Forecast Predictability and Intensity−Outflow Relationship of Hurricane Joaquin (2015)

Hurricane Joaquin (2015) presented unique track forecast challenges for the U.S. East Coast, threatening land from Florida through Maine, but ultimately sparing the mainland. The real-time 11-member COAMPS-TC ensemble performed well in terms of depicting this uncertainty, and also produced a skillful intensity forecast. Utilizing the real-time ensemble, as well as some recent 30-member retrospective runs, we diagnose how uncertainty in the initial strength and position of a mid-latitude trough to the west, an anticyclonic wavebreaking event to the north, and a pseudo tropical transition event to the east collectively introduce uncertainty in the forecast for Joaquin. We also address the impact of uncertainty in the initial vortex on the forecast.

Additionally, the relationship between Joaquin’s outflow and two separate intensification events is examined. During the first event, outflow is predominantly directed equatorward, but strengthens steadily as the TC intensifies. A temporary weakening phase occurs as vertical wind shear increases, imposed by the approaching trough. However, outflow suddenly switches from equatorward to poleward as the TC begins interacting with this trough, coincident with a re-intensification. This suggests a favorable trough interaction. We compute lagged correlations between TC intensity and outflow strength, both with intensification leading outflow and outflow leading intensification, in order to better diagnose causality in the strengthening of the secondary circulation.