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TROPICAL STORM DANNY DISCUSSION NUMBER  20
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042015
500 AM AST SUN AUG 23 2015

Satellite imagery and data from a NOAA Hurricane Hunter aircraft
indicate that Danny has changed little since the previous advisory.
Although the center is partly exposed on the southern edge of the
deep convection, satellite images and data from the NOAA Hurricane
Hunters and an earlier Gulfstream-IV synoptic mission indicate that
Danny remains a well-defined and vertically deep system through at
least the 400-mb level.

Danny is moving westward and has maintained its previous motion of
275/13 kt. The track forecast reasoning remains basically unchanged
from the previous advisory, but the new forecast track is heavily
dependent on Danny remaining a tropical cyclone as it moves through
the Leeward Islands and the northeastern Caribbean Sea. The model
guidance, especially the GFS and ECMWF, made a significant
southward shift on the latest cycle due to those models weakening
the cyclone fairly quickly after 24 hours, with Danny becoming a
vertically shallow remnant low that then moves westward within the
easterly trade wind flow and into the Caribbean Sea by 36 hours.
Given that the 18-20 kt of south-southwesterly vertical wind shear
that is currently affecting the cyclone is not expected to change
during the next 48 hours, Danny is forecast to be a tropical cyclone
during that time and and not weaken as much of the global models
are suggesting. As a result, Danny is forecast remain north of the
GFEX and TVCN model consensus model tracks, and begin to make a
turn toward the west-northwest after 24 hours. The NHC track
forecast was nudged a little south of the previous advisory track,
mainly due to the slight southerly initial position.

As previously mentioned, the current environmental conditions that
Danny is experiencing are forecast by the SHIPS intensity model to
continue for the next 48 hours, which means that Danny should remain
a small and sheared tropical cyclone during that time. The official
intensity forecast still calls for Danny to weaken to a tropical
depression in 36-48 hours as it passes near or south of the Virgin
Islands and Puerto Rico. Danny is expected to become a remnant low
in 72 hours and dissipate by 120 hours, but interaction with the
land masses of Puerto Rico and/or Hispaniola could result in Danny
dissipating sooner than forecast.

Additional watches or warnings could be required for the Virgin
Islands and Puerto Rico later this morning if Danny does not weaken
as quickly as forecast or if the forecast track is not shifted
farther south on subsequent advisories.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 15.8N  56.0W   50 KT  60 MPH
 12H  23/1800Z 16.0N  58.0W   40 KT  45 MPH
 24H  24/0600Z 16.5N  60.5W   35 KT  40 MPH
 36H  24/1800Z 17.1N  63.1W   30 KT  35 MPH
 48H  25/0600Z 17.8N  65.6W   30 KT  35 MPH
 72H  26/0600Z 19.6N  70.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  27/0600Z 21.3N  75.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  28/0600Z...DISSIPATED

$$
Forecaster Stewart

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