Tropical Storm Philippe Discussion Number  50
NWS National Hurricane Center Miami FL       AL172023
500 PM AST Thu Oct 05 2023

Philippe's convective structure is gradually evolving as it begins 
to run into an old frontal zone to its north.  A band of rain is 
developing over the northern part of the circulation along the 
old front and is beginning to move over Bermuda, while other deep 
convection extends east and southeast of the center.  The initial 
intensity is held at 45 kt based on this morning's aircraft 
reconnaissance data.  Another mission is scheduled into Philippe 
this evening.

The current motion is a little faster toward the north, or 360/12 
kt.  The track forecast reasoning has not changed.  Philippe should 
continue accelerating northward over the western Atlantic during 
the next 60 hours while moving between a mid-level high over the 
central Atlantic and a deep-layer trough off the southeastern U.S. 
coast.  On this track, the center will approach the coasts of Nova 
Scotia, New Brunswick, and eastern Maine Saturday night.  After 
that time, Philippe is expected to turn north-northwestward ahead 
of an approaching mid-latitude trough, and it is forecast to become 
absorbed by another area of low pressure over eastern Quebec late 
Sunday.  The NHC track forecast generally lies close to the TVCA 
and HCCA consensus aids during the entire forecast period.

Diagnoses from the SHIPS model suggest that southwesterly shear 
will increase to 30 kt or more over the next 12-24 hours.  Because 
of that, no intensification is anticipated in the short term.  
Extratropical transition is likely to begin on Friday as Philippe 
moves into and tightens the thickness gradient to its north, and 
that process could cause the storm to strengthen a little due to 
baroclinic influences.  This scenario is supported by the GFS and 
ECMWF global models, as well as LGEM, IVCN, and HCCA consensus aids. 
Extratropical transition is forecast to be complete by 48 hours 
(Saturday afternoon) as Philippe becomes fully frontal, but the 
intensity is unlikely to change much before the center reaches land. 
Weakening is forecast once Philippe moves inland, and it's likely 
that the strongest winds from the system will occur on the eastern 
side of the circulation, primarily over parts of Atlantic Canada.

Based on the forecast for Philippe to become post-tropical by 
Saturday, and after coordination with National Weather Service 
offices in New England and the Canadian Hurricane Centre, the 
current plan is for hazards in New England and Atlantic Canada to 
be handled via local non-tropical statements and not issue tropical 
watches or warnings.  We will continually assess this plan if the 
forecast evolves and tropical watches or warnings become warranted.


KEY MESSAGES:

1. Tropical storm conditions are expected on Bermuda beginning
early Friday morning, and a Tropical Storm Warning remains in
effect.  Heavy rainfall is expected to affect the island today into 
early Friday.  This could produce flash flooding.

2. Philippe is expected to move over portions of Atlantic Canada and
New England as a post-tropical cyclone this weekend.  Regardless of 
Philippe's intensity or structure, interests in those areas should 
be prepared for the possibility of strong winds and heavy rainfall 
and monitor statements from their local weather office.  The 
rainfall could produce isolated to scattered instances of urban and 
flash flooding.  


FORECAST POSITIONS AND MAX WINDS

INIT  05/2100Z 27.0N  66.2W   45 KT  50 MPH
12H  06/0600Z 29.2N  65.8W   45 KT  50 MPH
24H  06/1800Z 32.5N  65.7W   45 KT  50 MPH
36H  07/0600Z 35.5N  66.1W   50 KT  60 MPH
48H  07/1800Z 39.0N  66.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
60H  08/0600Z 43.8N  66.7W   50 KT  60 MPH...POST-TROP/EXTRATROP
72H  08/1800Z 48.6N  71.4W   40 KT  45 MPH...POST-TROP/INLAND
96H  09/1800Z...DISSIPATED

$$
Forecaster Berg