[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 230924 CCA
TCDAT4

Tropical Depression Nine Discussion Number   1...Corrected
NWS National Hurricane Center Miami FL       AL092022
500 AM AST Fri Sep 23 2022

Corrected locations of convection in first paragraph.

Convection this morning has increased primarily to the west of the
area of low pressure we have been monitoring in the central
Caribbean Sea, currently passing by to the north of Curacao island.
The system already possessed a well-defined circulation for the last
12 to 18 hours, but it was only overnight that the ongoing
convective activity was able to persist long enough near the center
to be considered a tropical cyclone. The most recent satellite
intensity estimate from TAFB was up to T2.5/35-kt. However the
GOES-16 derived motion winds from the meso sector over the system
have only been 25-30 kt in the 925-850 mb layer on the northeastern
periphery of the circulation. In addition, buoy 42059 located to the
northeast of the center has been reporting winds up to only 27-kt.
The combination of all these data provide enough justification to
upgrade this system to Tropical Depression Nine, with the initial
winds set at 30 kt for this advisory. An Air Force Reserve
Reconnaissance mission will be investigating the system later this
morning to provide more in-situ information of the system's
structure and intensity.

The current motion right now is estimated to be off to the
west-northwest at 290/12 kt. In the short-term, the depression is
forecast to bend back more westward as a narrow east-to-west
oriented mid-level ridge builds in behind the weakness left behind
from Fiona. The current westward displaced convection may also tug
the broad center in that direction as well. However, after 48 hours,
this ridge will start to decay as a longwave deep-layer trough over
the eastern United States begins to amplify southward. This synoptic
evolution should then allow the cyclone to begin gaining latitude,
though the exact timing at which this occurs could be somewhat
related to the vertical depth of the cyclone and how it interacts
with a weak upper-level trough expected to be over the far western
Caribbean in the day 3-4 time-frame. The model guidance early on is
in fairly good agreement, but larger across-track spread begins to
take shape by 48 hours, with the GFS and its ensemble mean located
further south and west, and with the ECMWF and its ensemble mean
located further north and east. The initial track forecast has
decided to split the difference between these two model suites, and
lies fairly close to the consensus aids TVCN and HCCA. There is
still a healthy amount of uncertainty in the track forecast at the
day 4-5 timeframe.

Currently the structure of the depression is quite disheveled, with
the low-level circulation mostly exposed, with deep convective
activity displaced to its west-southwest. This structure is due to
25-30 kt of northeasterly 200-850 mb vertical wind shear caused from
the equatorward outflow channel of Fiona, resulting in significant
upper-level flow over the system. However, as the depression moves
westward, it will move out form under this unfavorable flow and into
a lighter upper-level northeasterlies. However, it may take time for
the low-level center to become better aligned with the convective
activity, and thus only slow intensification is forecast over the
next 24-48 hours. After that period, most of the guidance shows
environmental conditions becoming much more favorable as shear drops
under 10 kt and the cyclone is over the warm 29-30 C waters of the
northwestern Caribbean. Thereafter, potential land interaction with
Cuba, and the potential for an increase in southerly shear at
day 5 may cap off the intensity at the end of the forecast. The
initial NHC forecast shows the depression intensifying up to a
category 2 hurricane by the time it nears the coast of Cuba.


Key Messages:

1. Tropical Depression Nine is expected to produce heavy rainfall
and instances of flash flooding and possible mudslides in areas of
higher terrain in Aruba, Bonaire, and Curacao. Heavy rains are also
likely to spread into Jamaica and the Cayman Islands in the coming
days.

2. The depression is expected to approach Jamaica and the Cayman
islands as an intensifying tropical storm. Watches and warnings for
these locations may be required in subsequent forecast cycles.

3. This system is forecast to approach western Cuba and enter the
southeastern Gulf of Mexico by the end of the forecast period.
Interests in Cuba and those along the Eastern Gulf Coast of the
United States should closely monitor this system, though at this
juncture forecast uncertainty remains fairly high.


FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 13.9N  68.6W   30 KT  35 MPH
 12H  23/1800Z 14.4N  70.2W   35 KT  40 MPH
 24H  24/0600Z 14.7N  72.6W   35 KT  40 MPH
 36H  24/1800Z 14.8N  75.0W   35 KT  40 MPH
 48H  25/0600Z 15.5N  77.1W   40 KT  45 MPH
 60H  25/1800Z 17.0N  78.8W   55 KT  65 MPH
 72H  26/0600Z 18.9N  80.5W   65 KT  75 MPH
 96H  27/0600Z 22.6N  82.6W   90 KT 105 MPH...NEAR CUBA
120H  28/0600Z 26.0N  82.3W   95 KT 110 MPH

$$
Forecaster Papin
This weather bulletin was brought to you by:
www.nashvilleweather.net