[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 040849
TCDAT2

Tropical Storm Philippe Discussion Number  44
NWS National Hurricane Center Miami FL       AL172023
500 AM AST Wed Oct 04 2023

Philippe remains a disorganized tropical storm this morning, yet
continues to produce heavy rainfall over the British and U.S. Virgin
Islands. This activity remains displaced well to the southeast of
the estimated circulation center, and appears to be more associated
with the storm's inflow interacting with a well-defined mid-level
circulation just north of St. Thomas, which is evident from radar in
Puerto Rico. Scatterometer data received just after the last
advisory showed values around 40 kt in this area of convection, and
the initial intensity will be held at 40 kt. This value is on the
higher end of the Dvorak intensity estimates, but does match the
most recent UW-CIMSS SATCON intensity estimate.

The earlier scatterometer data suggested that Philippe had slowed
down some, possibly due to the ongoing convection to its southeast,
but its latest motion appears to have resumed a more northwestward
motion at 325/8 kt. The track guidance remains in agreement about
the storm turning more northward later today, with gradual
acceleration as the mid-level ridge that has been impeding its
poleward progress gradually shifts eastward, and Philippe becomes
steered between a deep-layer low to its west, and the aforementioned
ridge to its east. Compared to the previous cycle, the guidance has
shifted a bit eastward over the first 2-3 days of the forecast
period, and the latest NHC track forecast was shifted a bit further
east early on, but not as far as the consensus aids. After Philippe
passes by Bermuda on Friday, it is forecast to interact with a much
more amplified trough swinging into the northeastern United States
this weekend. This evolution may result in Philippe's track
potentially bending back north-northwest on Saturday, approaching
Atlantic Canada and New England as it phases with the larger trough.
There still remains quite a bit of spread in the track guidance by
96 h due to this interaction.

The intensity forecast remains murky. Right now, the low-level
circulation of Philippe remains diffuse, and even though vertical
wind shear could potentially slacken some over the next 24 hours,
the cyclone appears to be in no state to intensify in the
short-term. Thereafter, Philippe may have an opportunity to take
advantage of some synoptic mid-latitude dynamics associated with the
initial upper-level trough positioned to the west of its
circulation. Thus, some gradual intensification is shown between
36-60 h. With that said, some of the model guidance (like the most
recent GFS run) shows Philippe instead being absorbed into this
non-tropical feature, though that solution is not shown at this
time. Regardless, increasing baroclinicity should also begin to
initiate extratropical transition after 48 hours, which is forecast
to be fully complete sometime in the 72-96 h forecast time frame.
The latest NHC intensity forecast remains near the middle of the
guidance envelope, and is also close to the HFIP Corrected
Consensus Approach (HCCA).

KEY MESSAGES:

1. Heavy rainfall from Philippe is expected to produce scattered
flash flooding across portions of the United States & British Virgin
Islands through today.

2. Gusty winds are likely to continue across portions of the
northern Leeward Islands and the Virgin Islands through early today.

3. The risk is increasing for tropical storm conditions to occur on
Bermuda late this week.  Interests on Bermuda should monitor the
progress of Philippe, and Tropical Storm Watches could become
required for the island later today.


FORECAST POSITIONS AND MAX WINDS

INIT  04/0900Z 20.5N  65.5W   40 KT  45 MPH
 12H  04/1800Z 21.4N  65.8W   40 KT  45 MPH
 24H  05/0600Z 23.3N  66.0W   40 KT  45 MPH
 36H  05/1800Z 26.0N  66.0W   45 KT  50 MPH
 48H  06/0600Z 29.0N  65.5W   45 KT  50 MPH
 60H  06/1800Z 32.0N  65.0W   50 KT  60 MPH
 72H  07/0600Z 35.3N  65.1W   50 KT  60 MPH
 96H  08/0600Z 43.1N  66.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  09/0600Z 51.5N  72.0W   30 KT  35 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin
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