[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 211445
TCDAT3

Tropical Storm Franklin Discussion Number   4
NWS National Hurricane Center Miami FL       AL082023
1100 AM EDT Mon Aug 21 2023

There are no lack of cold cloud tops associated with Tropical Storm
Franklin this morning. The structure on satellite consists of a
large bursting pattern, with cloud tops as cold as -90C close to and
just east of the estimated center. However, it is unclear if any of
this deep convection has resulted in structural improvement to
the storm, with the most recent microwave imagery from a SSMIS F-16
pass at 1007 UTC showing a chaotic structure, with evidence the
center remains near the western edge of this large convective
shield. The latest subjective satellite intensity estimates form
TAFB is unchanged from earlier this morning. Thus, the initial
intensity will remain 45 kt this advisory, which is also close to
the latest D-MINT and D-PRINT estimates from UW-CIMSS.

Franklin does appear to be slowing down a tad this morning while
moving westward, with the latest estimate at 270/5 kt. The track
forecast in the short-term is a bit problematic, with a large spread
in model solutions even at 12 h, much higher than the climatological
spread. This initial uncertainty may be related intensity
differences in the global models, with some of the stronger guidance
suggesting the tropical cyclone might reform further to the north
and east over the next 12-24 hours. Thereafter, a large weakness to
the north of Franklin, produced by a large mid-ocean trough draped
across the Western Atlantic, should allow the storm to begin moving
northward. A mid-latitude trough reinforces this weakness to the
north in 60-72h, allowing the storm to begin moving northeastward.
The most significant change in the guidance this cycle is the
majority are quite a bit slower than the prior cycle, and the NHC
forecast track was slowed down a bit, but not as much as some of the
consensus aids.

Currently, Franklin is experiencing some moderate 15-20 kt westerly
shear, which has paused its intensification for the time being.
However, this shear is forecast to gradual decrease as it begins to
move more northward, providing an opportunity for the tropical storm
to strengthen as it approaches Hispaniola. Some weakening appears
likely once the system moves over Hispaniola, especially if the
storm moves over the highest terrain in the Dominican Republic. By
Thursday, Franklin should be back over open waters in the Atlantic
Ocean and has an opportunity to start strengthening again. However,
the environment does not appear to be especially pristine, with the
ECMWF showing more southwesterly vertical wind shear than the GFS.
For now, its assumed this shear will not prevent slow
intensification, possibly because the storm should be moving in the
same direction as the shear vector, and the NHC intensity forecast
will still show Franklin becoming a hurricane towards the end of the
forecast period. This forecast is in best agreement with the HFIP
corrected consensus approach (HCCA) but remains lower than some of
the more aggressive regional-hurricane models (COAMPS-TC, HWRF).


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  21/1500Z 15.0N  70.1W   45 KT  50 MPH
 12H  22/0000Z 15.2N  70.5W   45 KT  50 MPH
 24H  22/1200Z 15.7N  71.0W   50 KT  60 MPH
 36H  23/0000Z 17.3N  70.9W   55 KT  65 MPH
 48H  23/1200Z 19.2N  70.6W   45 KT  50 MPH...INLAND
 60H  24/0000Z 21.2N  69.9W   45 KT  50 MPH...OVER WATER
 72H  24/1200Z 22.8N  68.4W   55 KT  65 MPH
 96H  25/1200Z 24.4N  65.2W   65 KT  75 MPH
120H  26/1200Z 26.4N  63.7W   75 KT  85 MPH

$$
Forecaster Papin
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