Bermuda subtropical storms

Abstract

This investigation focuses on North Atlantic subtropical cyclones which tracked within 100 nautical miles (185 km) of Bermuda from 1957 to 2005, identified through subtropical structural characteristics distinguished using Cyclone Phase Space, from the European Centre for Medium-Range Weather Forecasts 45-year reanalyses. The study assesses the characteristics of these hybrid storms that affect the Island, in order to aid the local forecaster. Reanalysis charts, surface analyses, local observations, HURDAT tracks, and satellite pictures, where available, were examined. This data shows that subtropical cyclones affecting Bermuda usually form in close proximity, to the south-southwest, over water of an average of 26 °C, under moderate vertical wind shear, with an upper trough lying to the west-north-west. They then move in a north-northeastward direction, intensifying quickly, but not often reaching a peak intensity of more than 26 m s−1. They generally have their beginnings along old baroclinic zones. September is the peak month of occurrence. A direct hit by a severe subtropical cyclone, producing locally observed winds of over 26 m s−1, appears to be a rare event. However, these storms are certainly a threat to the Island, particularly due to their lack of predictability, and conditions conducive to an incipient subtropical cyclone with potential to affect the Island should always be closely monitored.