Dr. Hazelton presented a seminar titled “Examining Hurricane Dorian’s Early Intensification and Long-term Track Evolution Through an Ensemble of the Hurricane Analysis and Forecast System”.
Hurricane Dorian (2019) was one of the strongest hurricanes on record in the Atlantic basin, and caused significant devastation across the northern Bahamas as it stalled out as a Category 5 hurricane. Dorian was characterized by forecast uncertainty in several aspects. Soon after forming, the TC intensified more than expected near the Lesser Antilles, and ended up on the right side of most track forecasts during this early period. As the TC intensified into a major hurricane in the Southwest Atlantic, there was large forecast uncertainty about whether the TC would move across Florida or stall and turn north in the Bahamas (as it did eventually). In this study, we use an 80-member ensemble of the global-nested configuration of the Hurricane Analysis and Forecast System (HAFS) to examine the factors that led to the early intensification of Dorian near the Eastern Caribbean, and also the eventual long-term track with a stall over the Bahamas and offshore of Florida. The early intensification appeared to be driven by a combination of vortex-scale processes and large-scale evolution, with stronger ensemble members showing a small eyewall developing much earlier than in the weaker members, and placing the storm in a more moist environment with expansive upper-level outflow. For the analysis of the early intensification, the structure is compared with radar data collected from NOAA’s Hurricane Field Program, since Hurricane Dorian was one of the best-observed TCs on record, with flights tracking the storm from the Caribbean to the waters off of the Southeast United States. For analysis of the long-term track, the ensemble is separated into 4 distinct groups, including two that hit Florida (at different speeds), one that moved quickly north of the Bahamas, and one that slowed down significantly over the northern Bahamas (closest to reality). The factors leading to the different track outcomes are examined, including the possible role of the early rightward track deviation over the Caribbean, the evolution of the subtropical ridge over the Southwest Atlantic, and the strength of a shortwave trough over the United States. The analysis performed highlights the utility of high-resolution ensembles for understanding the complex structure evolution in TCs, and also shows how large ensembles can show key details of the large-scale features that steer TCs. This research also demonstrates how the developing HAFS system can be used as a high-quality research tool to better understand the physics of TC development and movement.
A recording of the presentation is available here.