[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 201459
TCDAT4

HURRICANE DANNY DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042015
1100 AM AST THU AUG 20 2015

Danny's cloud pattern has continued to improve since the previous
advisory, and a 10 n mi diameter eye has become embedded in the
center of a small central dense overcast. Water vapor imagery
indicates that upper-level outflow has become more symmetrical. The
initial intensity of 65 kt is based on NHC objective Dvorak
classifications of T4.1-T4.2 using an eye feature.  Conventional and
microwave satellite images indicate that Danny is an unusually small
tropical cyclone.

Satellite fixes indicate that Danny is now moving west-northwestward
or 295/10 kt. The latest model guidance remains in very good
agreement on Danny moving west-northwestward for the next 48-72
hours toward a weakness in the subtropical ridge along 60W
longitude. A mid- to upper-level trough north of Bermuda is forecast
by the global models to begin lifting out to the north in 96-120
hours, which should allow the subtropical ridge to the north of
Danny to build back westward, forcing Danny on a more westward track
on days 4 and 5. The NHC official forecast track is essentially an
update of the previous advisory through 72 hours, but was shifted a
little north of the previous track after that in agreement with the
consensus model TVCN.

Danny's compact size makes it subject to significant fluctuations in
strength, both up and down, and such fluctuations are notoriously
difficult to forecast.  Low-shear conditions are expected to
continue for the next 36 hours or so, which argues for some further
strengthening during that time. However, the cyclone continues to be
surrounded by dry mid-level air, which could penetrate into the core
and disrupt the intensification process. By 48 hours and beyond, the
vertical wind shear is forecast to become southerly to southwesterly
and increase to near 15 kt.  As a result, Danny is expected to be
weakening as it approaches the Leeward Island.  The official
intensity forecast calls for a peak intensity somewhat higher and
earlier than the previous advisory, ending up close to the IVCN
consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT  20/1500Z 12.5N  44.8W   65 KT  75 MPH
 12H  21/0000Z 13.1N  46.0W   70 KT  80 MPH
 24H  21/1200Z 14.0N  47.6W   75 KT  85 MPH
 36H  22/0000Z 14.7N  49.4W   75 KT  85 MPH
 48H  22/1200Z 15.2N  51.5W   70 KT  80 MPH
 72H  23/1200Z 16.0N  56.4W   65 KT  75 MPH
 96H  24/1200Z 16.9N  61.7W   65 KT  75 MPH
120H  25/1200Z 18.0N  66.6W   55 KT  65 MPH

$$
Forecaster Stewart
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