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Hurricane Norman Discussion Number  30
NWS Central Pacific Hurricane Center Honolulu HI   EP162018
500 AM HST Tue Sep 04 2018
 
Norman has been holding its own during the night. The overall
satellite presentation has remained generally steady during the last
several hours, with some recent development of new deep convection
very close to the estimated low-level circulation center. A recent
1122Z AMSR microwave pass showed a rather well-defined eye.
Subjective Dvorak current intensity estimates came in at 4.5/77 kt
from PHFO/SAB, and 4.0/65 kt from JTWC. We have held the initial
intensity unchanged at 75 kt for this advisory.
 
The hurricane is now tracking due west, with an initial motion
estimate of 270/15 kt. Westward motion is expected to continue
today and tonight, with a considerable decrease in forward speed
as Norman nears the southwestern periphery of a strong subtropical
ridge centered to the northeast. Thereafter, the cyclone will
gradually turn to the west-northwest then to the northwest as it
moves around the end of the ridge and is increasingly steered by a
large deep-layered trough over the North Pacific. The reliable
guidance envelope shows very good agreement with this scenario,
including the location and timing of the northwest turn. The new
official forecast is shifted ever so slightly to the left through
the first 48 hours mainly due to the more westward initial motion,
and is simply an update of the previous forecast track thereafter.
This is in best agreement with the FSSE guidance, and is also very
close to the consensus HCCA and TVCE guidance.
 
The CIMSS initial vertical wind shear estimate was only 9 kt from
the south-southwest, and SHIPS guidance shows light wind shear
persisting for another 36 hours as Norman continues tracking around
the subtropical ridge. Norman is also now moving into an area of
slightly higher ocean heat content as per the current CIRA analysis.
Intensity guidance generally shows little change through the next
couple of days, with some of the guidance including CTCI and HWFI
showing an intensity increase between the 12- and 36-hour forecast
times. After 48 hours, a rapid increase in wind shear, a drier
surrounding airmass, and progressively cooler sea surface
temperatures should cause steady weakening. The new intensity
forecast has been adjusted upward slightly from 12 to 48 hours to
better match the guidance envelope, with a similar weakening trend
to the previous advisory thereafter.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  04/1500Z 19.9N 143.4W   75 KT  85 MPH
 12H  05/0000Z 19.9N 145.1W   75 KT  85 MPH
 24H  05/1200Z 19.9N 147.0W   80 KT  90 MPH
 36H  06/0000Z 20.4N 148.6W   80 KT  90 MPH
 48H  06/1200Z 21.2N 149.9W   75 KT  85 MPH
 72H  07/1200Z 23.6N 151.8W   55 KT  65 MPH
 96H  08/1200Z 26.5N 154.0W   40 KT  45 MPH
120H  09/1200Z 29.0N 155.5W   30 KT  35 MPH
 
$$
Forecaster Jacobson
 
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