[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 301455
TCDAT2

Tropical Storm Philippe Discussion Number  29
NWS National Hurricane Center Miami FL       AL172023
1100 AM AST Sat Sep 30 2023

The satellite presentation of Philippe has not changed much this
morning. The partially exposed low-level circulation lies on the
western edge of a deep convective mass, as northwesterly shear
continues to affect the storm's stucture. Recent UW-CIMSS objective
estimates and subjective Dvorak classifications from TAFB and SAB
have generally held steady, and so the initial intensity remains at
45 kt for this advisory. The Air Force Reserve Hurricane Hunters
are scheduled to investigate the system later today.

The estimated initial motion of Philippe is southwestward (210/4
kt). Philippe is not expected to move much during the next 12-24 h,
although there is uncertainty even in the near term related to the
extent of the interaction between Philippe and Tropical Storm Rina
to its northeast. Most of the models show a slow and generally
westward motion during the next day or so, which is reflected in
the NHC forecast. Then, model spread increases regarding the timing
of Philippe's turn toward the northwest and north early next week.
Stronger model solutions (GFS, HAFS-A) depict an earlier turn along
the eastern side of the guidance envelope, while weaker models
(ECMWF, UKMET) show the storm making it west of 60W before turning.
Given the greater-than-normal model spread, the NHC forecast remains
near the TVCA and HCCA aids for this portion of the forecast, with
a slight westward adjustment from the previous prediction. By days
4-5, Philippe is forecast to move faster toward the north and
northeast between a mid-level ridge over the central Atlantic and a
deep-layer trough over the western Atlantic.

Given the sheared structure of Philippe, only minor intensity
fluctuations are expected during the next 12-24 h. Thereafter, the
shear is generally forecast to decrease on Monday and Tuesday,
although the magnitude remains at least somewhat dependent on how
far west Philippe tracks. Based on our latest track forecast, it is
assumed that the shear will relax enough to be conducive for some
strengthening over the 29-29.5C SSTs, and this prediction shows
Philippe becoming a hurricane by 60 h. Overall, the environment
appears reasonably favorable for continued strengthening through
days 4-5 as the cyclone moves into the subtropical Atlantic. The
updated intensity forecast has been raised during this period to
show additional strengthening, although it still lies below the
IVCN and HCCA aids, so further adjustments could be necessary.


FORECAST POSITIONS AND MAX WINDS

INIT  30/1500Z 16.9N  56.3W   45 KT  50 MPH
 12H  01/0000Z 16.8N  56.6W   50 KT  60 MPH
 24H  01/1200Z 17.0N  57.2W   50 KT  60 MPH
 36H  02/0000Z 17.6N  58.0W   55 KT  65 MPH
 48H  02/1200Z 18.5N  58.9W   60 KT  70 MPH
 60H  03/0000Z 19.7N  59.6W   65 KT  75 MPH
 72H  03/1200Z 21.0N  59.9W   70 KT  80 MPH
 96H  04/1200Z 24.9N  59.3W   75 KT  85 MPH
120H  05/1200Z 28.5N  57.0W   75 KT  85 MPH

$$
Forecaster Reinhart
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