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Hurricane Norman Discussion Number  29
NWS Central Pacific Hurricane Center Honolulu HI   EP162018
1100 PM HST Mon Sep 03 2018
 
Norman continues to weaken this evening. The final visible
satellite images of the day showed a cloud-filled eye, and cloud
tops have warmed further during the past several hours. Subjective
Dvorak current intensity estimates came in at 4.5/77 kt from PHFO,
and 4.0/65 kt from SAB/JTWC, while the latest CIMSS ADT estimate is
4.2/70 kt. A blend of these estimates supports lowering the initial
intensity to 75 kt for this advisory.

The hurricane has turned slightly to an almost due west track this
evening, and it appears that the anticipated decrease in forward
speed has begun. The initial motion estimate is 275/15 kt, which is
a few knots slower than the previous advisory. Westward motion with
a further decrease in forward speed is expected to continue through
the next 36 hours 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. Most of the reliable guidance has come into better
agreement with the location and timing of this turn. The consensus
guidance HCCA, TVCE and FSSE are tightly packed and in good
agreement with the previous official forecast through 48 hours, then
shift slightly to the right thereafter. The new official forecast is
very similar to the previous forecast, and generally lies very close
to the consensus aids in the middle of the guidance envelope.

The CIMSS initial vertical wind shear estimate was 10 kt, and SHIPS
guidance shows light shear continuing for another 48 hours as
Norman continues tracking around the subtropical ridge. The main
issue causing weakening for the hurricane at present appears to be
marginal sea surface temperatures and ocean heat content. CIRA
analysis shows that Norman should move into an area of slightly more
favorable OHC after about 12 more hours, and the guidance shows
intensity flattening out from 12 through 48 hours. Norman may
actually re-intensity a bit during that time period, but opted to
keep the intensity forecast level at 70 kt for this advisory.
Thereafter, a rapid increase in wind shear, a drier surrounding
airmass, and progressively cooler sea surface temperatures should
cause steady weakening. The new intensity forecast is similar to
the previous, and in good agreement with the consensus aids.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  04/0900Z 19.9N 141.7W   75 KT  85 MPH
 12H  04/1800Z 20.0N 143.7W   70 KT  80 MPH
 24H  05/0600Z 20.0N 145.9W   70 KT  80 MPH
 36H  05/1800Z 20.3N 147.8W   70 KT  80 MPH
 48H  06/0600Z 20.9N 149.2W   70 KT  80 MPH
 72H  07/0600Z 23.3N 151.3W   60 KT  70 MPH
 96H  08/0600Z 26.0N 153.5W   45 KT  50 MPH
120H  09/0600Z 29.0N 155.0W   30 KT  35 MPH
 
$$
Forecaster Jacobson
 
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