[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 302043
TCDAT2

Tropical Storm Philippe Discussion Number  30
NWS National Hurricane Center Miami FL       AL172023
500 PM AST Sat Sep 30 2023

Philippe remains a sheared tropical cyclone. Its low-level center
has become increasingly separated from the deep convection today.
Visible satellite images and and fixes from the Air Force Hurricane
Hunters indicate the center is farther south and slightly east of
our previous estimates. In terms of Philippe's current intensity,
there were mixed signals in the aircraft data. The peak 850-mb
flight-level winds were only 49 kt, and a center dropsonde indicated
no change of the minimum pressure (999 mb) since the previous
flight. On the other hand, the SFMR retrievals showed winds of 45-50
kt outside of convection that were visually supported by the flight
crew, but a coincident dropsonde reported much lower near-surface
wind speeds. Given the conflicting data and less impressive
satellite presentation, the initial intensity is held at 45 kt.

The tropical storm has continued to move southwestward today, with
an initial motion of 215/4 kt. In the near term, the models suggest
Philippe will not move very much and could drift erratically through
early Sunday while Tropical Storm Rina passes to its north. Then,
there is general agreement that the storm will turn toward the
west-northwest and northwest around the western extent of a low- to
mid-level ridge over the central Atlantic. While there is still a
large spread in the track guidance during the 24-72 h time frame, it
is noteworthy that the 12z ECMWF has trended eastward this cycle and
shows Philippe passing farther east of the northern Leeward Islands.
The latest NHC track forecast is slightly farther south and west
early in the forecast due to the center relocation. However, the
updated prediction ends up near or slightly east of the previous
forecast at 36-72 h, in best agreement with the TVCA simple
consensus. By days 4-5, Philippe is forecast to accelerate northward
and northeastward between a central Atlantic ridge and a deep-layer
trough over the western Atlantic.

Only modest strengthening is forecast during the next 24-36 h while
Philippe contends with moderate to strong northwesterly shear. The
upper-level winds are forecast to become more conducive for
intensification in a couple of days or so, which should allow the
cyclone to become more vertically aligned and establish an inner
core. Thus, more strengthening is forecast at days 2-4 while
Philippe moves over the very warm SSTs of the central Atlantic,
which is supported by almost all of the intensity guidance. The NHC
forecast calls for Philippe to become a hurricane by 60 h with
continued strengthening thereafter. The peak intensity was raised to
bring the NHC forecast slightly closer to the latest HCCA and IVCN
aids at days 4-5, but it is noted that the regional hurricane models
are even stronger than the current NHC forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  30/2100Z 16.3N  56.5W   45 KT  50 MPH
 12H  01/0600Z 16.2N  56.7W   50 KT  60 MPH
 24H  01/1800Z 16.6N  57.4W   55 KT  65 MPH
 36H  02/0600Z 17.4N  58.1W   55 KT  65 MPH
 48H  02/1800Z 18.5N  58.9W   60 KT  70 MPH
 60H  03/0600Z 19.9N  59.5W   65 KT  75 MPH
 72H  03/1800Z 21.6N  59.7W   70 KT  80 MPH
 96H  04/1800Z 25.7N  59.0W   80 KT  90 MPH
120H  05/1800Z 29.5N  56.0W   80 KT  90 MPH

$$
Forecaster Reinhart
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