Hurricane Nigel Discussion Number  24
NWS National Hurricane Center Miami FL       AL152023
500 AM AST Thu Sep 21 2023

Nigel is starting to feel the effects of increased vertical wind
shear this morning. Proxy-vis and infrared satellite imagery show
that the large, ragged eye from the past few days has become
obscured. An AMSR microwave pass at 0513 UTC confirmed that the
eyewall structure has collapsed, and a thick curved band on the
northern side is all that remains.  Vertical wind shear has the deep
convection displaced to the northern side of the system, within the
main banding feature, with cold cloud tops in the northwest
semicircle near -70 Celsius. Subjective satellite current intensity
(CI) values were 4.5 from both TAFB and SAB. Given the degradation
in the satellite appearance and these estimates, the initial
intensity is set to 75 kt for this advisory.

Nigel is moving swiftly to the northeast at an estimated motion of
50/ 26 kt. The system is currently moving within the flow ahead of a
mid-latitude trough off the northeast US Coast and Atlantic
Canada, with this quick northeast motion expected to continue
during the next day or two. Late Friday and into the weekend,
models show Nigel interacting with a larger polar low over the
North Atlantic. The NHC track forecast is very similar to the
previous advisory and remains near the consensus aids.

The hurricane is moving into a more hostile environment with
vertical deep layer shear forecast to further increase, and sea
surface temperatures along the forecast track continue to cool over
the northern Atlantic. Nigel is also entering a drier mid-level air
mass with relative humidities below 40 percent. Nigel should begin
to transition into an extratropical cyclone very soon, and this
process is expected to be complete in about 24 hours. The ECMWF and
GFS SHIPS guidance show extratropical transition occuring even
sooner than what is forecast. The official intensity forecast shows
gradual weakening, with Nigel becoming a powerful extratropical
cyclone on Friday.  By the end of the forecast period, Nigel will
become absorbed in the aforementioned larger polar low.


FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 40.6N  47.8W   75 KT  85 MPH
12H  21/1800Z 42.9N  42.7W   70 KT  80 MPH
24H  22/0600Z 45.9N  35.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
36H  22/1800Z 49.5N  27.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
48H  23/0600Z 53.1N  23.6W   50 KT  60 MPH...POST-TROP/EXTRATROP
60H  23/1800Z 56.8N  23.1W   50 KT  60 MPH...POST-TROP/EXTRATROP
72H  24/0600Z 58.3N  24.4W   45 KT  50 MPH...POST-TROP/EXTRATROP
96H  25/0600Z...DISSIPATED

$$
Forecaster Kelly