[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 200841
TCDAT5

Hurricane Nigel Discussion Number  20
NWS National Hurricane Center Miami FL       AL152023
500 AM AST Wed Sep 20 2023

Nigel's overall satellite appearance has not changed much from the
previous advisory. Deep convection remains on the southern portion
of a thick band around the center of the system. Infrared imagery
shows that the eyewall has been opened on the northern side, but has
tried to fill in recently. There have been no microwave passes over
the system overnight to get a better idea of the overall structure
of Nigel. Subjective intensity estimates have remained fairly
steady, with a CI of 4.5/5.0 from TAFB and SAB, respectively. Given
these estimates and similar satellite appearance, the initial
intensity is held at 85 kt.

Nigel has about 18-24 hours before it is expected to move over
cooler sea surface temperatures and into significantly stronger
vertical wind shear. As a result, the latest NHC intensity forecast
shows Nigel remaining steady to slightly weakening in the short
term, followed by steady weakening through the remainder of the
forecast period. Although weakening is forecast, Nigel is expected
to be a strong extratropical cyclone in about 48 hours.

The system is moving toward the north at 14 kt, as Nigel rounds the
edge of a mid-level ridge over the central subtropical Atlantic.
Later today, the system is expected to turn northeastward and
accelerate in the flow of a deep-layer trough located over Atlantic
Canada. The track guidance remains in fairly good agreement in the
short term with only slight along-track speed differences. The NHC
track forecast is slightly faster than the previous one, and lies
near the consensus aids, but still not as fast as HCCA. As Nigel
reaches high latitudes, the extratropical low is forecast to
interact with a broader cyclonic circulation over the north
Atlantic, and it is possible that Nigel gets absorbed by this
broader circulation towards the end of the period. There is higher
track uncertainty towards the end of the forecast period due to this
interaction.


FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 34.4N  54.9W   85 KT 100 MPH
 12H  20/1800Z 36.7N  53.5W   85 KT 100 MPH
 24H  21/0600Z 39.8N  49.5W   80 KT  90 MPH
 36H  21/1800Z 42.8N  43.0W   70 KT  80 MPH
 48H  22/0600Z 45.7N  35.5W   60 KT  70 MPH...POST-TROP/EXTRATROP
 60H  22/1800Z 49.2N  28.4W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  23/0600Z 52.4N  24.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  24/0600Z 57.1N  23.7W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  25/0600Z 59.1N  20.0W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Kelly
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