Extreme Rainfall and Intensification Mechanisms in Hurricane Fiona (2022)

Hurricane Fiona’s (2022) heavy precipitation over Puerto Rico almost surpassed the record set in 2017 by the catastrophic Hurricane María. Hurricane Fiona had peak accumulations of 823 mm and caused devastating flooding on the island as an intensifying category 1 hurricane for a period of 72 hours. Radar and satellite data show that a persistent rainband was the primary cause of the rainfall totals. This study will encompass an overview of the mechanisms affecting Hurricane Fiona’s intensification and the associated precipitation as it interacted with vertical wind shear. We hypothesize that enhanced potential vorticity displaced from the inner core by vertical wind shear was a primary contributor to the heavy rainfall production, resulting in both isentropic uplift and boundary layer convergence in the downshear rainband. Additional enhancements of the rainfall occurred over Puerto Rico’s high terrain by orographic effects. To analyze the rainfall mechanisms we use the recently developed Hurricane Analysis and Forecast System (HAFS-B) model in concert with observations from the Puerto Rico NEXRAD radar data and aircraft data collected as part of the joint Office of Naval Research Tropical Cyclone Rapid Intensification (TCRI) Departmental Research Initiative and the NOAA Advancing the Prediction of Hurricanes Experiment (APHEX). Preliminary results support the hypothesis that heavy rainfall is associated with interactions between enhanced potential vorticity and moist southerly flow in the downshear rainbands. Strong vertical motions and rainfall continued even after the eyewall was located well to the northwest 24 hours after landfall in Puerto Rico. The physical mechanisms related to the extreme rainfall and their relationship to the concurrent intensification of Fiona in the presence of moderate vertical wind shear will be discussed.