[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 192044
TCDAT5

Hurricane Nigel Discussion Number  18
NWS National Hurricane Center Miami FL       AL152023
500 PM AST Tue Sep 19 2023

The satellite structure of Nigel has improved this afternoon.
Satellite images indicate a solid ring of deep convection surrounds
the large, 45 to 50-n-mi wide eye of the hurricane. The NOAA
Hurricane Hunters have conducted research missions into Nigel today
and provided helpful data to better assess the hurricane's structure
and intensity. The aircraft recorded peak 700-mb flight-level winds
of around 100 kt, which reduces to a surface intensity of 90 kt
using a standard reduction factor. However, peak SFMR winds from
both planes were around 75 kt, and dropsonde data indicate only
modest pressure falls to around 974 mb. Based on a blend of these
data, the initial intensity is set at 85 kt for this advisory. The
wind radii were adjusted slightly outward based on the aircraft data
and scatterometer data received just after the previous advisory.

Given its improved structure, additional near-term strengthening is
forecast as Nigel moves over 28C SSTs in a weak vertical shear
environment. By late Wednesday, the guidance indicates deep-layer
shear will increase while the hurricane quickly moves into a drier
environment over much cooler waters. So, the NHC forecast shows
steady weakening through late week that closely follows the latest
multi-model consensus aids. The global model fields indicate that
Nigel will complete its extratropical transition by 60 h, which is
reflected in this advisory.

It appears that Nigel has begun to turn more northward, and its
initial motion is now north-northwestward (330/12 kt). The steering
ridge to the northeast of Nigel will continue to slide eastward
through tonight. As a result, the hurricane is expected to turn
northward around the western periphery of this ridge. Then, Nigel
should turn northeastward and accelerate within the flow ahead of a
deep-layer trough moving across the northeastern United States and
Atlantic Canada. The track guidance remains well clustered around
this scenario, and the latest NHC track forecast was only nudged
slightly west based on the latest TVCA and HCCA aids. As Nigel moves
deeper into the mid-latitudes, the extratropical low is forecast to
become captured within a broader cyclonic circulation over the north
Atlantic on days 4 and 5, which introduces increased track
uncertainty late in the period.


FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 31.8N  54.7W   85 KT 100 MPH
 12H  20/0600Z 33.7N  54.9W   95 KT 110 MPH
 24H  20/1800Z 36.7N  53.3W   90 KT 105 MPH
 36H  21/0600Z 39.8N  49.3W   80 KT  90 MPH
 48H  21/1800Z 42.8N  42.9W   70 KT  80 MPH
 60H  22/0600Z 45.7N  35.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  22/1800Z 48.6N  28.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  23/1800Z 56.0N  20.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  24/1800Z 58.0N  21.5W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Reinhart
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