[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 212049
TCDAT3

Tropical Storm Franklin Discussion Number   5
NWS National Hurricane Center Miami FL       AL082023
500 PM EDT Mon Aug 21 2023

Franklin is not an especially well-organized tropical cyclone this
afternoon. The NOAA reconnaissance mission has been sampling the
storm for a large part of the afternoon, and found a center
to the south of where the position was estimated earlier today. In
addition, the winds on the southwest side of the circulation are
more diffuse, and overall the circulation is a bit more elongated
than what the plane found yesterday at this time. Peak 850-mb flight
level winds were at 48 kt just to the northwest of the circulation,
which supports an intensity between 40-45 kt after using a standard
reduction. The initial intensity will remain at 45 kt for this
advisory, which is also in between the higher TAFB and lower SAB
intensity estimates.

The tropical storm has been meandering today, and my best guess at
an initial motion is a drift south of due west at 265/4 kt. Given
the current elongated nature of the surface circulation, the short-
term track forecast is tricky, ranging from the GFS on the east side
of the guidance envelope that has a center reformation to the
northeast, and the CMC and ECMWF which are on the western side of
the guidance envelope. The steering currents are also poorly defined
currently due to a large mid-oceanic trough in the western Atlantic
disrupting the flow pattern. Mid-level ridging eventually builds in
to the east of Franklin, which should ultimately induce a northward
motion by 24-36 h, and then northeastward between 48-96 h, as a
mid-latitude trough reinforces the weakness to the north of the
storm. The track guidance has slowed down again compared to the
previous cycle, prolonging the time the system will remain over the
Caribbean Sea before it moves over Hispaniola. The NHC track
forecast is once again slower than the previous cycle and has also
been nudged a touch westward compared to the previous one, but is
now a bit east of the simple and corrected consensus aids.

Given Franklin's current structure, I am somewhat skeptical there
will be a lot of meaningful near-term intensification, especially
while moderate vertical wind shear continues to disrupt the
convective structure. However, both the GFS- and ECMWF-based SHIPS
guidance indicate that the shear will briefly drop under 10 kt in
about 24 hours, and when combined with warm 29 C sea-surface
temperatures, it still seems prudent to show some intensification
before Franklin moves inland over Hispaniola. Significant land
interaction over the higher terrain will then lead to weakening,
which could be underdone here given the current track over some of
the most mountainous terrain. After Franklin emerges into the
western Atlantic, another round of southwesterly shear may slow the
rate of reintensification, and the latest NHC intensity forecast is
a bit more subdued than the previous one, once again electing to
follow the HCCA consensus aid. Given the complex intensity factors
mentioned above, this is a low confidence forecast.


KEY MESSAGES:

1. Heavy rainfall from Franklin is expected across portions of
Puerto Rico and Hispaniola through the middle of the week. The heavy
rainfall may produce areas of flash and urban flooding as well as
river rises and mudslides. Across Hispaniola, significant and
potentially life-threatening flash flooding is possible Tuesday into
Wednesday.

2. Franklin is expected to bring tropical storm conditions to
portions of the Dominican Republic and Haiti beginning on Tuesday
where Tropical Storm Warnings are in effect.


FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 14.3N  70.1W   45 KT  50 MPH
 12H  22/0600Z 14.3N  70.9W   45 KT  50 MPH
 24H  22/1800Z 15.3N  71.3W   50 KT  60 MPH
 36H  23/0600Z 17.2N  71.2W   55 KT  65 MPH
 48H  23/1800Z 19.1N  70.8W   40 KT  45 MPH...INLAND
 60H  24/0600Z 20.6N  70.3W   45 KT  50 MPH...OVER WATER
 72H  24/1800Z 22.2N  68.7W   50 KT  60 MPH
 96H  25/1800Z 23.6N  66.0W   60 KT  70 MPH
120H  26/1800Z 25.6N  65.5W   70 KT  80 MPH

$$
Forecaster Papin
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