Potential Tropical Cyclone Nine Forecast Discussion Number 3

By | September 13, 2019

Issued at 500 AM EDT Fri Sep 13 2019

000
WTNT44 KNHC 130858
TCDAT4
Potential Tropical Cyclone Nine Discussion Number   3
NWS National Hurricane Center Miami FL       AL092019
500 AM EDT Fri Sep 13 2019
Satellite imagery and surface observations indicate that the
tropical disturbance is currently comprised of a trough elongated
from west-southwest to east-northeast, with a vorticity center near
a loosely curved convective band at the eastern end of the trough.
While the system has become a little better organized since the
last advisory, it does not yet have a well-defined circulation
center.  The initial intensity remains 25 kt based on satellite
intensity estimates and earlier scatterometer data.
The disturbance is now moving a little faster, with the initial
motion of 315/5.  The system is to the south of a low- to mid-level
ridge over the southeastern United States, and it is to the east of
a mid- to upper-level low over the central Gulf of Mexico.  The
large-scale models forecast the ridge to shift eastward into the
Atlantic during the next 24-48 h, with a weakness developing near
the southeastern coast of the United States and the east coast of
the Florida peninsula.  The track guidance indicates that the
disturbance will move generally northwestward for 48 h or so,
followed by a turn to the north and eventually to the northeast as
the system moves through the weakness in the ridge.  There is some
spread in the guidance, with the GFS model taking a weaker system
into the Florida peninsula while the UKMET and ECMWF models show a
stronger cyclone farther offshore.  Overall, there has been an
eastward shift of the guidance since the previous advisory,
especially after 72 h.  The new forecast track is thus also nudged
a little to the east and now calls for the system to spend less
time over the Florida peninsula than previously forecast.  The new
forecast track is to the left of the center of the guidance
envelope and the consensus models, and additional adjustments may
be necessary once the circulation center becomes better defined.
The disturbance is currently in an environment of moderate
southwesterly shear and upper-level divergence caused by the
upper-level low and a shortwave trough extending from the low
eastward to the southeastern Bahamas. The shear should gradually
diminish during the next couple of days, which should allow the
disturbance to develop into a tropical cyclone and intensify.  The
large-scale models forecast the shear to increase after 72 h as a
mid-latitude westerly trough moves through the southeastern United
States and approaches the system.  The new intensity forecast is
similar to the previous forecast through 72 h, then it is stronger
than the previous forecast due to the forecast track keeping the
system more over water.
Key Messages:
1. The system is expected to become a tropical storm and bring
tropical-storm-force winds and heavy rainfall to portions of the
northwestern Bahamas today 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.
2. The system could bring tropical-storm-force winds to portions of
the Florida east coast over the weekend.  Residents there should
monitor the progress of this system and follow any advice given by
local officials.
3. Heavy rainfall is possible from Potential Tropical Cyclone Nine
across portions of eastern Florida into this weekend, which could
produce flash flooding.  There is greater uncertainty than usual in
heavy rainfall prospects farther north across the Carolinas.
4. Note that forecast uncertainty for these disturbances is
generally larger than for tropical cyclones, especially beyond 48-72
hours.
FORECAST POSITIONS AND MAX WINDS
INIT  13/0900Z 24.6N  75.2W   25 KT  30 MPH...POTENTIAL TROP CYCLONE
12H  13/1800Z 25.1N  76.2W   30 KT  35 MPH...TROPICAL DEPRESSION
24H  14/0600Z 26.0N  77.8W   35 KT  40 MPH
36H  14/1800Z 27.0N  79.3W   40 KT  45 MPH
48H  15/0600Z 28.2N  80.4W   45 KT  50 MPH
72H  16/0600Z 30.3N  81.1W   50 KT  60 MPH
96H  17/0600Z 31.0N  80.5W   50 KT  60 MPH
120H  18/0600Z 32.0N  78.5W   55 KT  65 MPH
$$
Forecaster Beven

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