An Analysis of the Predictability of Tropical and Post-Tropical Ida's (2021) Heavy Precipitation

Hurricane Ida (2021) was the 6^th costliest tropical cyclone to impact the United States, causing over $82 billion in damages (NCEI). A significant portion of these Ida-related damages were the result of widespread heavy rainfall and flash flooding when the system was a tropical cyclone over the Gulf coast states and as a post-tropical cyclone over the Mid-Atlantic and New England states. Maximum storm total precipitation in these areas include 10.78 inches at Slidell, LA, 13.01 inches at Gulfport, MS, 7.19 inches at New York Central Park, and 8.44 inches at Newark, NJ. Of the 55 deaths in the United States directly attributed to Ida, 52 were attributed to storm surge and freshwater flooding.

Given the catastrophic effects of Hurricane Ida’s heavy precipitation, it is of interest to investigate and compare the mechanisms that were associated with these distinct areas of heavy precipitation and assess the predictability of the timing, location, and amounts of precipitation. Therefore, this study will investigate the synoptic-scale interactions and sensitivity of Ida’s heavy precipitation forecasts to the uncertainty in these features for forecasts leading up to Ida’s landfall in the Gulf coast states and its post-tropical impacts in the Mid-Atlantic and New England states a few days later. Ensemble-based sensitivity analysis will be used to assess the sensitivity of Ida’s precipitation forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble and the National Centers for Environmental Predication (NCEP) Global Ensemble Forecast System (GEFS) for both regions. The forecasts of the surrounding environmental features will be analyzed using the fifth generation ECMWF reanalysis (ERA5) dataset, while hourly precipitation observations will be taken from the NCEP Stage-IV analysis. In addition, the differences in predictability between Ida’s precipitation as a landfalling tropical cyclone and as a post-tropical cyclone interacting with the midlatitudes will be investigated. It is hypothesized that the precipitation in the Gulf coast states will be most sensitive to the low-level moisture transport, while the precipitation in the Mid-Atlantic and New England states will be most sensitive to interactions with a nearby upper-level trough and nearby baroclinic zone.