Extreme Events in a Tropical Paradise: An Anomalous Severe Convective Event in the Northern Bahamas

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Thursday, 6 February 2014: 4:15 PM
Room C101 (The Georgia World Congress Center )
Fiona Horsfall, NOAA/NWS, Silver Spring, MD; and B. C. Hagemeyer and A. Rolle

On November 17, 2012, a fisherman based on Grand Bahama Island in the Northern Bahamas was collecting his stone crab traps of Walker's Cay, the northern-most island in the Bahamian chain. A thunderstorm suddenly developed over the area, which is not uncommon, but in this case, his boat's Bimini top was destroyed by what he described as golf-ball sized hailstones that were followed by “slush.” Strong winds and the hail resulted in his boat being beached on the island. Walker's Cay is about 4°N of the Tropic of Cancer, where prevailing winds are the North East Trades that maintain a warm, humid environment with little temperature variation all year.

Convective activity in the Islands is typically the result of locally unstable air masses or is associated with frontal boundaries. Thunderstorms associated with frontal boundaries do not generally produce severe weather as defined by the National Weather Service because the air mass differential tends to be small at this latitude, and the abundance of warm water quickly modulates cold air masses. The Islands do not have any high terrain, so orographic lifting does not contribute to thunderstorm development; however, in the absence of these characteristics, there is still the development of occasional severe thunderstorms throughout the archipelago. A post-storm assessment confirmed that the National Weather Service identified the potential for thunderstorm development off the Florida coast on November 17, and detected a supercell hail event over Walker's Cay on its Doppler Weather Radar.

This talk will review the local climatology and the documented history of hail events in the Islands. We will present evidence of the supercell, review climate patterns at the time and assess their contribution to the event, and evaluate statistical significance of the event for its classification as extreme.