GFS Storm Precipitation and Trajectories, 2021

Operational forecast models are necessary for the prediction of weather events in real time. Verification of these models must be performed to assess model skill and areas in need of improvement, particularly with different types of weather events that may occur. Despite the devastating impacts that can be caused by tropical cyclones (TCs) that undergo extratropical transition (ET) and become post-tropical cyclones (PTCs), these storms have not been extensively studied in the context of short-term weather prediction. This study completes the first analysis of the Global Forecast System (GFS) and a pre-operational version of the newly operational Hurricane Analysis and Forecast System (HAFS) models in forecasting the occurrence of ET and the rainfall associated with ET storms in the North Atlantic basin. GFS’s skill exceeds that of HAFS in forecasting the occurrence of ET, but HAFS tends to have lower track and rain rate errors in the fully tropical phase of ET storms’ life cycles. Both models simulate rain rates that are often too high near the storm center and fail to capture the larger area of moderate rain rates that greatly contributes to total rainfall accumulation. The discrepancies in rain rates between the models and IMERG could be attributed to the models’ tendency to keep storms too intense and too compact with an overly strong warm core, even throughout the ET process.

Methods
The data that support the findings of this study are available at the following URLs:

IMERG Final Run: https://gpm1.gesdisc.eosdis.nasa.gov/data/GPM_L3/GPM_3IMERGHH.06/

IBTrACS v4: https://www.ncdc.noaa.gov/ibtracs/index.php?name=ib-v4-access

ERA5 reanalysis: https://cds.climate.copernicus.eu/#!/search?text=ERA5&type=dataset

GFS data was accessed from the UCAR Research Data Archive: https://doi.org/10.5065/D65Q4TSG

TempestExtremes is publicly available on GitHub at:
https://github.com/ClimateGlobalChange/tempestextremes

ExTraTrack software is available at https://github.com/zarzycki/ExTraTrack