Intense Hybrid Storm On The Way Update One

Tropical Storm Philippe Discussion Number   6
NWS National Hurricane Center Miami FL       AL182017
1100 PM EDT Sat Oct 28 2017

Philippe has a very non-classical structure for a tropical cyclone
this evening.  A combination of satellite imagery, radar data, and
surface observations show that the circulation center is elongated
northwest-southeast from southwest of Naples, Florida, to central
Cuba with at least three vorticity centers present in this area.
The estimated center position is a mean of the multiple vorticity
centers, with this position near the region of lowest pressure
suggested by the surface observations.  Currently, the primary deep
convection is located from the northwestern Bahamas southwestward
across Cuba into the Caribbean, with a smaller area of convection
over the southeastern Gulf of Mexico.  The initial intensity
remains 35 kt, based mainly on continuity from the previous
advisory.

The initial motion is a rather uncertain 015/21. A deep-layer
trough and developing surface low over the eastern United States
should cause Philippe to turn northeastward soon, followed by a
rapid northeastward motion across the southern end of the Florida
Peninsula into the Atlantic.  The cyclone should continue to move
around the large baroclinic system until it is absorbed after the
48 h point.  The new forecast track is a blend of the previous
forecast and the current guidance, and it is shifted to the north
of the previous track based mainly on the current initial position.

It is unclear how much additional strengthening Philippe can do as
a tropical cyclone, as the shear is increasing over the system and
water vapor imagery shows dry air entrainment in the southwestern
quadrant.  However, the upper-level divergence over the system is
very strong, and the large-scale models forecast deepening as the
center crosses southern Florida and the Bahamas.  In addition, the
increasing forward speed may increase the maximum winds as well.
Based on these factors, the intensity forecast calls for some
increase in strength during the next 24-36 h.  Philippe is likely
to merge with a frontal system associated with the eastern U. S.
trough in about 36 h, and the mid-latitude cyclone should
completely absorb Philippe after 48 h, if not sooner.


KEY MESSAGES:

1. Although the center of Philippe is now forecast to move across
the Florida Keys or extreme south Florida, most of the strongest
winds are expected to remain well east and southeast of the center.
However, tropical-storm-force winds, mainly in gusts, could occur
in brief heavy squalls across the upper Florida Keys and southeast
Florida overnight. For that reason, a tropical storm watch remains
in effect for these areas.

2. Regardless of the exact track of Philippe, the primary threat
from this system will be heavy rainfall that can cause localized
flooding across portions of Cuba, the Florida Keys, and South
Florida.


FORECAST POSITIONS AND MAX WINDS

INIT  29/0300Z 24.8N  82.1W   35 KT  40 MPH
 12H  29/1200Z 27.6N  78.9W   40 KT  45 MPH
 24H  30/0000Z 33.9N  74.0W   50 KT  60 MPH
 36H  30/1200Z 40.3N  69.6W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  31/0000Z 48.0N  67.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  01/0000Z...ABSORBED BY EXTRATROPICAL LOW

$$
Forecaster Beven