[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 300847
TCDAT1

TROPICAL STORM FRED DISCUSSION NUMBER   2
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062015
500 AM AST SUN AUG 30 2015

The convective cloud pattern of the tropical cyclone has continued
to improve this early morning with the development of a small CDO
feature and a tightly curved band in the western and southern
quadrants. A 0542 UTC SSMI/S microwave satellite image further
indicated that the convective band wraps almost completely around a
primitive mid-level eye feature. The initial intensity is raised to
35 kt based on a Dvorak classification of T2.5/35 kt from TAFB, and
this intensity could be conservative based on the impressive SSMI/S
satellite signature. This makes Fred only the fourth Atlantic
tropical storm to form east of 19W longitude in NHC's database.

The initial motion remains 305/10 kt. There is no change to the
previous forecast track reasoning. Fred is expected to move
northwestward toward a weakness just northwest of the Cape Verde
Islands within a deep-layer subtropical ridge. The mid-tropospheric
trough that is responsible for the weakness is forecast by the
global and regional models to shift eastward over the next 24-36
hours, which should allow the ridge to build back in, forcing Fred
on a west-northwestward to westward track after 48-72 hours. The
latest NHC model guidance is tightly packed about the previous
forecast track, so the new track forecast is essentially just an
update of the previous advisory, and lies close to the GFEX and TVCA
consensus model solutions.

Fred is expected to remain in favorable environmental and oceanic
conditions for the next 36 hours, characterized by vertical wind
shear less than 10 kt, mid-level humidity values greater than 70
percent, and sea-surface temperatures of 27.5-28 deg C. The main
inhibiting factor is decreasing instability ahead of the cyclone
after 24 hours. However, there should still be enough available
instability to support deep convection that will allow at least
steady strengthening through 36 hours to occur, and Fred could still
reach hurricane status before or while it moves through the Cape
Verde Islands in 36 hours or so. After 48 hours, southwesterly
vertical wind shear is expected to begin affecting the cyclone while
Fred is moving over sub-27C SSTs. These less favorable conditions
should combine to induce a gradual weakening trend. The NHC
intensity forecast is similar to the previous advisory and the IVCN
intensity consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT  30/0900Z 12.4N  18.9W   35 KT  40 MPH
 12H  30/1800Z 13.4N  20.3W   45 KT  50 MPH
 24H  31/0600Z 14.6N  22.1W   55 KT  65 MPH
 36H  31/1800Z 16.0N  23.9W   60 KT  70 MPH
 48H  01/0600Z 17.0N  25.8W   60 KT  70 MPH
 72H  02/0600Z 18.5N  29.9W   60 KT  70 MPH
 96H  03/0600Z 19.2N  34.0W   55 KT  65 MPH
120H  04/0600Z 19.8N  38.8W   50 KT  60 MPH

$$
Forecaster Stewart
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