[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 280235
TCDAT2

Tropical Storm Philippe Discussion Number  19
NWS National Hurricane Center Miami FL       AL172023
1100 PM AST Wed Sep 27 2023

Deep convection has persisted around the eastern portion of
Philippe's circulation this evening, however there is not much
evidence of curved bands.  The low-level center may have reformed or
become slightly better defined just to the west of the main
convective mass since the previous advisory.  There has been no
recent microwave imagery to gain a better look at the cyclone's
structure and scatterometer appears likely to miss the center this
evening.  The latest subjective Dvorak intensity estimates were T3.0
(45 kt) and T2.5 (35 kt) from TAFB and SAB, respectively.  The
initial intensity is  held at 45 kt for this advisory, in deference
to the earlier scatterometer data and latest TAFB estimate.

The track forecast for Philippe remains quite complicated, and it is
very dependent on the future intensity and vertical depth of the
cyclone.  The current disorganized structure of Philippe and its
present environment of moderate southwesterly shear is likely to
inhibit strengthening over the next couple of days. The main
question is whether Philippe weakens and becomes more vertically
shallow or if it maintains some vertical coherence during that time.
If it becomes vertically shallow, Philippe is likely to move
generally westward or west-southwestward within the low-level
steering flow.  If it remains more vertically deep, the cyclone is
likely to move little as it remains to the southeast of a mid-level
ridge.  Another complicating factor is a disturbance (AL91) to
Philippe's southeast.  The latest GFS run moves that system close
enough to Philippe that binary interaction occurs between the two
systems. However this run appears to have initialized Philippe
stronger than it actually is, which may have led to the solution
where Philippe remains the dominate system.  Meanwhile, the
ECMWF weakens Philippe and keeps more separation between the two
systems, taking a weaker Philippe westward or west-southwestward.
The NHC forecast currently favors the latter scenario, but there is
an unusually large amount of uncertainty in the track and intensity
forecast.  In general, the guidance is a bit slower this cycle and
the NHC forecast has been adjusted accordingly.  The new track
forecast is not as slow as the consensus aids, but is a blend
of the previous official forecast and the latest consensus models.
It would not be surprising if additional adjustments to the track
forecast are required in subsequent advisories.

As mentioned above, moderate southwesterly shear and a slightly
drier airmass ahead of Philippe are likely to inhibit significant
convective organization of the cyclone at least during the next day
or two.  That is likely to cause some gradual weakening, and the NHC
intensity forecast follows that thinking and calls for the system to
weaken during that time.  However, there is still a lot of
uncertainty as to whether Philippe weakens and becomes a remnant low
later in the period, or moves little and remains a tropical cyclone.
The latest forecast maintains continuity from the previous few
advisories and calls for Philippe to become a remnant low, but this
is a low confidence forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 18.2N  54.5W   45 KT  50 MPH
 12H  28/1200Z 18.7N  55.5W   45 KT  50 MPH
 24H  29/0000Z 19.0N  56.6W   40 KT  45 MPH
 36H  29/1200Z 19.0N  57.6W   40 KT  45 MPH
 48H  30/0000Z 19.0N  58.6W   40 KT  45 MPH
 60H  30/1200Z 18.7N  59.4W   35 KT  40 MPH
 72H  01/0000Z 18.5N  60.3W   35 KT  40 MPH
 96H  02/0000Z 18.5N  62.1W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  03/0000Z 18.5N  63.5W   30 KT  35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brown
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