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Tropical Storm Philippe Discussion Number  48
NWS National Hurricane Center Miami FL       AL172023
500 AM AST Thu Oct 05 2023
 
Philippe's low-level structure continues to be quite broad and
diffuse this morning. This structure has made finding the center
position quite a challenge. An earlier 0513 UTC GMI microwave pass
confirmed this broad structure, though it did hint at a tighter
mesoscale feature tucked under the convection to the southeast of
the broader low-level rotation seen on 1-minute GOES-16 imagery. The
initial intensity remains 35 kt this advisory, in agreement with the
TAFB subjective Dvorak estimate, in addition to an earlier received
saildrone observation that had sustained tropical-storm-force winds
well to the southeast of the broad circulation center.
 
Because the center is difficult to pinpoint, the initial motion is
also uncertain, estimated due north at 360/12 kt. The storm's
current motion is a result of it being steered between a mid-level
ridge positioned to its east, and a mid- to upper-level low
positioned to its west. This pattern should persist, leading to
Philippe gradually accelerating towards Bermuda over the next 24-36
h. Interestingly, the guidance has quite a bit of spread this cycle
in the short-term, likely related to how the current broad and
diffuse circulation interacts with the trough to its west, with the
HAFS-A/B runs notably pivoting Philippe further west early on than
the remainder of the guidance. The NHC track forecast ops to favor
the simple consensus aids, though it is worth noting the HFIP
Corrected Consensus Approach (HCCA) is farther west due to the
influence of these HAFS tracks. I guess it is not all that
surprising to see Philippe throw one additional curve ball in the
track forecast given the extreme difficulty the track forecast for
this system has proven to be over the last couple of weeks. After
48 hours, the guidance actually comes back into better agreement,
showing a gradual bend north-northwest and northwest when Philippe,
now a post-tropical cyclone, reaches the coast of Atlantic Canada
or New England this weekend.
 
The intensity forecast presents its own challenges. Assuming that 
Philippe remains a distinct entity relative to the developing 
non-tropical low to its west, the storm could intensify while it 
interacts with this upper-level trough to its west. However, its 
unclear if this will prove to be a favorable trough interaction, or 
if this trough will ultimately just absorb the tropical cyclone 
without much increase in the winds. Regardless, after Philippe 
passes north of Bermuda, increasing baroclinicity will hasten its 
transition to an extratropical cyclone. However, FSU phase-space 
diagrams suggest the system could undergo a warm-seclusion-type 
transition, resulting in further deepening even after the system 
becomes extratropical. The post-tropical cyclone is then expected to 
weaken once it moves inland over Maine and/or Atlantic Canada. The 
NHC intensity forecast continues to lie near the middle of the 
guidance envelope.
 
 
KEY MESSAGES:
 
1. Tropical storm conditions are expected on Bermuda beginning
tonight, where a Tropical Storm Warning remains in effect. Heavy
rainfall will also begin to affect the island later today, which
could produce flash-flooding.
 
2. Philippe is likely to move over portions of Atlantic Canada and
eastern New England, likely as a post-tropical cyclone by this
weekend.  Regardless of Philippe's intensity or structure, interests
in those areas should monitor the storm's progress and be prepared
for the possibility of strong winds and heavy rainfall.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  05/0900Z 24.8N  65.9W   35 KT  40 MPH
 12H  05/1800Z 26.7N  66.0W   35 KT  40 MPH
 24H  06/0600Z 30.0N  66.0W   40 KT  45 MPH
 36H  06/1800Z 33.3N  66.0W   45 KT  50 MPH
 48H  07/0600Z 36.7N  66.2W   50 KT  60 MPH
 60H  07/1800Z 40.2N  66.7W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  08/0600Z 45.2N  67.8W   45 KT  50 MPH...POST-TROP/INLAND
 96H  09/0600Z 50.5N  76.0W   30 KT  35 MPH...POST-TROP/INLAND
120H  10/0600Z...DISSIPATED
 
$$
Forecaster Papin
 
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