Danny Feels the Effects of Shear

Danny 23 Aug 15 1845UTC
Tropical Storm Danny continuing its approach on the Northern Lesser Antilles Islands. The center of the cyclone (~15.5N, ~58.5W) is occasionally exposed between intermittent bursts of deep convection. GOES-floater RGB satellite imagery at 3:45pm local time.

Last night the strong vertical wind shear that was forecast to begin hindering Hurricane Danny’s intensification started to take its toll on the small cyclone. Visible satellite imagery before sunset last night showed the surface circulation pushing westward while the deep convection weakened and drifted northeastward leaving the center exposed – as a result Danny was not able to maintain hurricane status and has weakened to a tropical storm. Intermittent bursts of deep convection continue over the center of Danny, but the surface circulation is still exposed at times. The hope is that a weakened Danny will be able to bring beneficial rains to the northeastern Caribbean where unusually dry conditions have been the theme this summer.

Last night, Danny tracked to the left (south) of yesterday afternoon’s forecast track; likely under the influence of a more westerly low level steering flow rather than the deeper layered west-northwesterly steering flow. This means that it is now more likely that Danny will pass through, not north of, the Lesser Antilles Islands where tropical storm watches and warnings are in effect, and then the Greater Antilles for early next week. So the combination of strong vertical wind shear and resulting dry air entrainment combined with forecast land interactions along this more southerly track means that it is very likely that Danny will degenerate into a tropical wave and become diffuse over the mountains of Hispaniola by Tuesday night. This land interaction also reduces the chances for redevelopment past Hispaniola to near nil.

two_atl_5d0 (1)
National Hurricane Center 5-Day Tropical Weather Outlook. For the next five days: A tropical wave in the central tropical Atlantic (red ‘x’) has a high chance of becoming a tropical cyclone as it tracks through the red swath.

Elsewhere in the Atlantic, the tropical wave that was following Danny has maintained an area of deep convection and is showing signs of organization. This wave is much larger than Danny was but is expected to follow a similar track through the next five days. The National Hurricane Center is giving this wave a high (70%) chance of becoming a tropical cyclone by Tuesday afternoon, and a high (80%) chance of becoming a tropical cyclone by Friday afternoon as the wave slowly organizes on its quick west-northwestward track. Dry and dusty Saharan Air to the north of the wave combined with an encounter with the same area of strong vertical wind shear that Danny has struggled with are obstacles to this wave’s development through day five. Threat to land is small at this time and is at least five days out. Further, any threat to Bermuda is still low and would be at least 8 days out. This will allow plenty of time to track changes in the wave’s development and fine tune its track.

Monitor the National Hurricane Center and the Bermuda Weather Service forecasts for the latest official information regarding the track and intensity of Atlantic tropical cyclones and weather in Bermuda.


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