[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 182045
TCDAT5

Hurricane Nigel Discussion Number  14
NWS National Hurricane Center Miami FL       AL152023
500 PM AST Mon Sep 18 2023

The center of Nigel is embedded within a central dense overcast of
cold infrared cloud tops this afternoon. Recent GMI microwave images
do not show much improvement to the hurricane's inner core
structure, as it appears some drier air could be intruding into the
north side of the circulation. There is no eye feature apparent in
conventional satellite imagery at this time. The latest subjective
Dvorak estimates are T4.0 (SAB) and T4.5 (TAFB). Meanwhile, the
latest UW-CIMSS ADT and SATCON intensity estimates are 63 kt and 71
kt, respectively. A blend of these data support holding the initial
intensity at 70 kt for this advisory.

Despite seemingly conducive environmental conditions for
strengthening, Nigel's intensification appears to have been
disrupted, possibly by dry air entrainment related to some mid-level
shear. If the hurricane can solidify its inner core during the next
day or so, the very warm SSTs and relatively low shear should allow
for some strengthening in the near term. The NHC forecast has been
slightly lowered from the previous one but still shows Nigel peaking
near major hurricane intensity in 36-48 h. This prediction lies
above the latest intensity guidance, closest to HCCA and some of the
regional hurricane models (HAFS-A). Beyond 48 h, increasing
deep-layer shear associated with the upper trough and rapidly
cooling SSTs are forecast to induce steady weakening through the
rest of the period.

A mid-level ridge over the central Atlantic continues to steer Nigel
northwestward (320/10 kt). The hurricane is expected to turn
northward over the next couple of days while it moves around the
western periphery of this steering ridge. Then, an upper-level
trough moving across the western Atlantic is forecast to cause Nigel
to accelerate northeastward and move deeper in the mid-latitudes
through late this week. The NHC forecast remains near the center of
the tightly clustered track guidance, with a faster forward motion
shown later in the period in agreement with the TVCA and HCCA aids.
This takes Nigel more quickly into the mid-latitudes, and the latest
global model fields and phase diagrams suggest its extratropical
transition should be complete by 96 h.


FORECAST POSITIONS AND MAX WINDS

INIT  18/2100Z 28.0N  52.2W   70 KT  80 MPH
 12H  19/0600Z 29.2N  53.3W   80 KT  90 MPH
 24H  19/1800Z 31.0N  54.5W   90 KT 105 MPH
 36H  20/0600Z 33.4N  54.8W   95 KT 110 MPH
 48H  20/1800Z 36.2N  53.5W   95 KT 110 MPH
 60H  21/0600Z 39.1N  50.0W   85 KT 100 MPH
 72H  21/1800Z 42.1N  43.9W   75 KT  85 MPH
 96H  22/1800Z 48.0N  29.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
120H  23/1800Z 56.0N  18.0W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster B Reinhart/A Reinhart
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