[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 250243
TCDAT2

Tropical Storm Philippe Discussion Number   7
NWS National Hurricane Center Miami FL       AL172023
1100 PM AST Sun Sep 24 2023

Another prominent convective burst has formed over Philippe (the
storm) since the prior advisory. However, indications from the last
received microwave imagery (a 2033 UTC F-16 SSMIS pass) suggest that
the large cirrus plume produced is mainly the product of a cluster
of cells in the down-shear quadrants of the storm, with little
indication of improved organization with the tropical cyclone. There
is also evidence of mid-level shear undercutting the outflow layer
to the west of the cyclone. Subjective Dvorak intensity
estimates from TAFB and SAB remain at T3.0/45-kt, and Philippe's
initial intensity this advisory is held at 45 kt, which is also
close to a mean D-MINT value associated with the above mentioned
microwave pass.

The intensity forecast for the next 2-3 days is tricky, as vertical
wind shear (especially mid-level shear under the outflow layer)
already appears to be keeping Philippe in check, and preventing the
deep convection from wrapping around the center. Neither the GFS or
ECMWF suggest this shear will abate much for at least the next 2-3
days. In fact the primary reason why the forecast was held steady
over this time span is that the cyclone will also continue
traversing anomalously warm 29C sea-surface temperatures, while
mid-level moisture stays about the same or increases some during
this time span. However, it would not be surprising to see some
weakening in the short-term either, as suggested by the
regional-hurricane models, particularly HMON and COAMPS-TC
forecasts. After 72 hours, assuming the storm begins to turn
poleward, there is some potential the storm start moving north of
this shear zone, where some gradual intensification could begin. The
intensity forecast is largely similar to the prior advisory, and is
closest to the HFIP Corrected Consensus Approach (HCCA).

Philippe continues to move west-northwestward at 290/12 kt. For the
next day or so, the guidance is in fairly good agreement on this
motion continuing as an extensive mid-level ridge centered north of
the cyclone remains in place. Beyond that time span, however, is
where the guidance diverges quite dramatically. While most of the
guidance agrees a large weakness will appear in the mid-level
ridging to the north of Philippe, whether or not the system is able
to turn northward into this weakness is largely a byproduct of how
vertically deep the cyclone is able to remain. Assuming the storm
remains at least somewhat vertically coherent, the track forecast
does show a turn northwest and then north-northwest by the end of
the 5-day forecast. However, the spread in the guidance at this time
period remains notable, with stronger solutions turning more north
or even northeast, while weaker (and solutions that dissipate
Philippe) maintaining a west-northwest heading. The NHC track
forecast continues to split the difference between these extremes,
and lies close to both the simple and corrected consensus aids. As
discussed previously, this remains a low confidence track and
intensity forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  25/0300Z 17.1N  43.3W   45 KT  50 MPH
 12H  25/1200Z 17.6N  45.0W   45 KT  50 MPH
 24H  26/0000Z 18.1N  47.3W   45 KT  50 MPH
 36H  26/1200Z 18.5N  49.2W   45 KT  50 MPH
 48H  27/0000Z 19.3N  51.0W   45 KT  50 MPH
 60H  27/1200Z 20.4N  52.7W   50 KT  60 MPH
 72H  28/0000Z 21.6N  54.0W   50 KT  60 MPH
 96H  29/0000Z 23.4N  55.9W   55 KT  65 MPH
120H  30/0000Z 25.5N  57.0W   55 KT  65 MPH

$$
Forecaster Papin
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