[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 190833
TCDAT5

Hurricane Nigel Discussion Number  16
NWS National Hurricane Center Miami FL       AL152023
500 AM AST Tue Sep 19 2023

Nigel is sending mixed signals this morning.  While the large eye of
the hurricane has become better defined and warmer, convection
around the eyewall has weakened (and AMSR2 microwave data show the
eyewall is open on the north side).  This convective disruption
appears to be due to persistent dry air and just enough shear to
entrain that air into the inner core.  Satellite intensity estimates
remain about the same as 6 h ago, so the initial wind speed is
unchanged at 75 kt. Both of the NOAA Hurricane Hunter P-3 aircraft
are scheduled to be in the hurricane late this morning, so we should
have a more precise assessment of Nigel this afternoon.

The hurricane still has another day or so to intensify while it
remains in light-shear and warm-water conditions, which could
facilitate mixing out the dry air that has plagued the internal
convective structure. However, it seems like changes with Nigel will
be gradual since it has formed a large eye (which tends to be more
stable).  Nigel should undergo a rapid extratropical transition from
48 to 72 h as it comes under the influence of a strong mid-latitude
trough.  Little change was made to the previous intensity forecast,
above the intensity consensus at short range and near the model
average at longer range.  SAR data from yesterday evening did
suggest Nigel that has grown in size, so that's reflected in the
current and near-term forecast wind radii.

The track forecast seems straightforward with Nigel, as the current
northwestward motion should bend to northward late today while the
hurricane moves along the western periphery of the subtropical
ridge.  Nigel should then move rather quickly northeastward through
late week due to fast flow ahead of a mid-latitudes trough dropping
out of the Canadian Maritimes, then northward as an extratropical
low this weekend as it is captured by the trough south of Iceland.
The new forecast is shifted a touch left to come into better
agreement with the consensus guidance.


FORECAST POSITIONS AND MAX WINDS

INIT  19/0900Z 29.6N  53.6W   75 KT  85 MPH
 12H  19/1800Z 31.2N  54.5W   85 KT 100 MPH
 24H  20/0600Z 33.8N  54.8W   90 KT 105 MPH
 36H  20/1800Z 36.7N  53.2W   85 KT 100 MPH
 48H  21/0600Z 39.6N  49.2W   80 KT  90 MPH
 60H  21/1800Z 42.5N  42.7W   70 KT  80 MPH
 72H  22/0600Z 45.0N  34.7W   60 KT  70 MPH...POST-TROP/EXTRATROP
 96H  23/0600Z 51.8N  21.8W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  24/0600Z 58.5N  21.0W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake
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