[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 182032
TCDAT4

TROPICAL STORM DANNY DISCUSSION NUMBER   2
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042015
500 PM AST TUE AUG 18 2015

The convective cloud pattern of the tropical cyclone has continued
to improve since the previous advisory, including the development of
interlocking curved convective cloud bands and the formation of an
upper-level anticyclonic outflow pattern. Passive microwave images
indicate that the cyclone has a well-developed low- and mid-level
structure. The intensity has been increased to 35 kt based on a
Dvorak satellite intensity estimate of T2.5/35 kt from TAFB and a
UW-CIMMS ADT estimate of T2.5/35 kt. As a result, the system has
been upgraded to Tropical Storm Danny on this advisory.

The initial motion estimate remains 280/11 kt. There is no
significant change to the previous forecast track or reasoning. The
global and regional models remain in good agreement on Danny moving
west-northwestward toward a weakness in the subtropical ridge
located along 45W longitude for the next 72 hours. After that time,
however, there is some spread in the model guidance based on how
much and how soon the ridge builds back in to the north of Danny.
The UKMET retains the weakness in the ridge longer, taking the
cyclone northeast of the Lesser Antilles. In contrast, the ECMWF
model strengthens the ridge sooner, which drives Danny more westward
and considerably faster at 96 and 120 hours. The GFS poorly
initialized Danny this morning, and it is noticeably slower than
all of the available model guidance and, therefore, has been given
much less weight on this forecast cycle. The official forecast track
is faster than the consensus model TVCN due to the much slower GFS
model inducing a significant slow bias in the model consensus, and
is roughly a blend of the ECMWF, HWRF, and GFS-Ensemble mean
forecast solutions.

Atmospheric and oceanic conditions surrounding Danny are expected
to be favorable for slow but steady strengthening throughout the
forecast period. The only inhibiting factor appears to be dry
mid-level air located to the north and west of Danny occasionally
getting entrained into the circulation. However, the low vertical
wind shear regime that Danny will be migrating through should allow
the convective structure of the cyclone to steadily increase in
organization, which should enable the circulation to quickly mix out
any dry air intrusions. The official intensity forecast is similar
to but slightly lower than the intensity consensus model IVCN
through 96 hours, and near the LGEM intensity model at 120 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  18/2100Z 10.9N  37.5W   35 KT  40 MPH
 12H  19/0600Z 11.2N  38.9W   40 KT  45 MPH
 24H  19/1800Z 11.6N  40.4W   45 KT  50 MPH
 36H  20/0600Z 12.0N  41.7W   55 KT  65 MPH
 48H  20/1800Z 12.5N  43.0W   65 KT  75 MPH
 72H  21/1800Z 13.4N  46.0W   75 KT  85 MPH
 96H  22/1800Z 13.9N  49.7W   85 KT 100 MPH
120H  23/1800Z 14.5N  54.6W   85 KT 100 MPH

$$
Forecaster Stewart
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