Ensemble Variability in Rainfall Forecasts of Hurricane Irene (2011)

As tropical cyclones (TCs) move from the tropics into the midlatitudes, they are often associated with extensive regions of heavy precipitation. This precipitation can lead to widespread, devastating flood events, as occurred with Hurricane Irene (2011) over the northeastern United States. Despite the high-impact nature of these events, there are relatively few studies that have explored the sensitivity of precipitation forecasts to model initial conditions beyond looking at the variability in TC track. The goal of this work is to understand what modulates precipitation forecasts over the northeastern United States during Hurricane Irene. This is accomplished by comparing members of the Global Forecasting System (GFS) ensemble prediction system initialized at 0000 UTC 27 August 2011 that forecast the largest precipitation totals (i.e. wet members) over the Catskill Mountains of New York State (where over 15” of rain were observed) against members that predicted the least precipitation (i.e. dry members). Preliminary results indicate that the amount of rainfall is tied to the storm track, with wetter members being on the western side of the track envelope. In addition, the wetter members are associated with more divergence aloft, possibly due to a greater amount of latent heat release within the Irene. This increase in outflow prevents an upstream trough over the Great Lakes region from moving to the east, which could provide a mechanism for steering Irene closer to the coast. To evaluate the role of horizontal grid spacing on the forecasts of Irene, 15 km Weather Research and Forecasting (WRF) model ensemble forecasts are initialized from the GFS ensemble forecasts. Unlike the GFS ensemble results, there is no systematic relationship between track and precipitation in the WRF forecasts. Instead, the wetter members feature a stronger cyclonic circulation, leading to increased confluence and frontogenesis over the Catskills.