[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 130257
TCDAT4

Potential Tropical Cyclone Nine Discussion Number   2
NWS National Hurricane Center Miami FL       AL092019
1100 PM EDT Thu Sep 12 2019

An Air Force Reserve reconnaissance aircraft earlier this afternoon
found a closed but very broad cyclonic circulation along with
1000-ft flight-level and SFMR surface winds that supported an
intensity of 25 kt. Recent ship reports also support an intensity of
25 kt, although stronger winds could be present in some of the
fragmented convective bands located in the northeastern quadrant.

The initial motion is an uncertain 310/02 kt. The disturbance has
been meandering over the southeastern Bahamas the past several hours
due to southwesterly mid-/upper-level winds associated with an
east-west oriented shortwave trough impinging on the west side of
the circulation. A large upper-low located over the central Gulf of
Mexico that has produced this shortwave trough is expected to spin
out another trough on Friday that will move across South Florida and
the Straits, impinging on the disturbance and hindering any
significant northwestward motion. By late Friday and Saturday,
however, the upper-level is expected to have moved into the western
Gulf of Mexico, ending any negative effects in the disturbance. This
expected to result in the formation of a tropical cyclone with a
deeper vortex column that should get steered basically northwestward
in combined deep-layer southeasterly flow between the upper-low to
the west and a ridge to the northeast. The new NHC track forecast is
basically just an extension of the previous forecast track, albeit a
little slower, and lies down the middle of the widely divergent
guidance envelope, close to the various simple consensus models, and
to the right of the NOAA-HCCA consensus model.

The aforementioned complex steering flow pattern will also play a
major role in the intensification process. The two shortwaves
troughs are expected to hinder significant convective organization
for the next 24-30 hours due to their localized increase in the
vertical shear and intrusions of dry air. By 36 hours and beyond,
however, those negative factors are forecast to abate, resulting in
the disturbance moving underneath an axis of strong upper-level
difluence and speed divergence near or just to the east of the
Florida east coast. This more favorable upper-level outflow regime
should allow for convection to develop and organize fairly quickly
around the center before the system reaches the Florida east coast,
which should result in the formation of a moderate tropical storm.
Although the center is forecast to be inland at 72-96 hours, the
center is expected be close enough to the coast to keep
tropical-storm-force ongoing over the coastal waters through 120
hours. The official intensity forecast is identical to the previous
advisory and follows a blend of the consensus models HCCA and IVCN.

Key Messages:

1. The disturbance is expected to become a tropical storm and bring
tropical storm force winds to portions of the northwest Bahamas
within 36 hours. Note that forecast uncertainty for these
disturbances is generally larger than for tropical cyclones,
especially beyond 48-72 hours.

2. The system is expected to bring tropical-storm-force winds and
heavy rainfall to portions of the northwest Bahamas on Friday and
Saturday. Significant storm surge is not expected in the northwest
Bahamas from this system. Residents there should follow any advice
given by local officials.

3. The system could bring tropical-storm-force winds and rainfall
to portions of the Florida east coast over the weekend. Residents
there should monitor the progress of this system.

FORECAST POSITIONS AND MAX WINDS

INIT  13/0300Z 23.8N  74.5W   25 KT  30 MPH...POTENTIAL TROP CYCLONE
 12H  13/1200Z 24.6N  75.6W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 24H  14/0000Z 25.5N  77.2W   30 KT  35 MPH...TROPICAL CYCLONE
 36H  14/1200Z 26.5N  78.8W   40 KT  45 MPH
 48H  15/0000Z 27.5N  80.2W   45 KT  50 MPH
 72H  16/0000Z 29.8N  81.8W   40 KT  45 MPH...INLAND
 96H  17/0000Z 30.8N  81.4W   40 KT  45 MPH...INLAND
120H  18/0000Z 31.8N  79.8W   45 KT  50 MPH...OVER WATER

$$
Forecaster Stewart
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