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Tropical Storm Philippe Discussion Number  52
NWS National Hurricane Center Miami FL       AL172023
500 AM AST Fri Oct 06 2023
 
Philippe's structure this morning is consistent of a cyclone in the 
initial stages of extratropical transition. While the low-level 
circulation center appears better defined than yesterday at this 
time, most of the deep convection remains displaced well east of the 
center, a result likely caused by nearly 30 kt of SSW vertical wind 
shear. There is another cloud band of linear convection to the 
northwest of the center, but this appears more related to synoptic 
forcing caused by an upper-level cutoff low triggering non-tropical 
surface cyclogenesis west of Philippe. The interaction of this low 
with Philippe will likely dictate how quickly its completion of 
extratropical transition is. The initial intensity this advisory 
remains at 45 kt, which remains on the high end of the satellite 
estimates.
 
Philippe appears to be taking a jog to the north-northeast this
morning with some acceleration, estimated at 15/16 kt. The storm is
caught in the flow between the aforementioned cutoff low to its
west, and a amplified mid-level ridge to its east. An additional
complication is the non-tropical surface cyclone forming to the west
of Philippe that may also interact with it over the next day or so.
The end result of this interaction is that there might be a
short-term north-northeastward deviation in Philippe's motion, but
then pivots back north-northwest as the two systems undergo some
binary interaction with each other. The track guidance this cycle
shows this short-term deviation, but end up roughly along the
previous forecast track after 24 hours. The NHC track forecast is
thus only shifted some early on, showing a bit more of short-term
eastward bend before Philippe resumes a northward and then
north-northwestward track, close to the simple and corrected
consensus aids.
 
While environmental conditions are becoming increasingly hostile for 
intensification as a tropical cyclone, Philippe's winds may still 
increase slightly over the next 24-36 h as it gets some baroclinic 
enhancement from the aforementioned upper-level trough interaction 
with the system. This trough interaction is also likely responsible 
for initiating its extratropical transition, and the latest forecast 
now shows Philippe completing this process and becoming a 
post-tropical cyclone in 36 h. Given the current structure, this 
could occur sooner after passing by north of Bermuda. Another even 
more amplified trough should fully capture this post-tropical 
cyclone, and weakening is expected after Philippe moves inland over 
New England and Atlantic Canada on Sunday. Whats left of Philippe 
will be absorbed by a much larger mid-latitude cyclone that is 
expected to occlude over Quebec. The official NHC intensity 
continues to lie near the middle of the guidance envelope.
 
Based on the forecast for Philippe to become post-tropical
on 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 beginning on Bermuda and expected
to continue today, where a Tropical Storm Warning remains in
effect. Heavy rainfall is expected to affect the island 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 may produce isolated to scattered instances of urban and
flash flooding.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  06/0900Z 29.5N  65.3W   45 KT  50 MPH
 12H  06/1800Z 31.9N  64.9W   45 KT  50 MPH
 24H  07/0600Z 35.0N  65.6W   50 KT  60 MPH
 36H  07/1800Z 38.1N  66.4W   50 KT  60 MPH...POST-TROP/EXTRATROP
 48H  08/0600Z 42.3N  67.3W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  08/1800Z 48.1N  71.6W   35 KT  40 MPH...POST-TROP/INLAND
 72H  09/0600Z...ABSORBED BY LARGER EXTRATROPICAL CYCLONE
 
$$
Forecaster Papin
 
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