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Hurricane Norman Discussion Number  39
NWS Central Pacific Hurricane Center Honolulu HI   EP162018
1100 AM HST Thu Sep 06 2018
 
Norman is weakening, but only slowly thus far. The center feature
is gradually becoming less distinct in visible and infrared
satellite imagery, but a 1717 UTC SSMIS pass shows a well-defined
eye feature with eyewall convection persisting around most of the
low level center. Despite a very gradual degradation in the
satellite presentation, the intensity estimates remain 5.0 from
PHFO, JTWC, and SAB, while the UW-CIMSS ADT had 102 knots. Have 
lowered the initial intensity to 95 kt for this advisory, as a blend
between the subjective and objective estimates.

The initial motion estimate is 315/8. Norman is headed toward a 
gradually digging upper level trough to the north of the main 
Hawaiian Islands. The track guidance is in fairly good agreement
with a continued northwest motion over the next 36 to 48 hours.
After 48 hours, forecasting whatever remains of Norman becomes
increasingly more challenging. Some of the guidance, like the ECMWF,
HMON, and HWRF, show Norman beginning to get picked up by the broad
circulation of a large low over the north Pacific near the date
line. Other models, such as the GFS and NAVGEM, have a shallower,
decoupled circulation that slows and then begins to track westward.
There is a very large guidance spread in the 96 to 120 hour time
frame, but by then Norman is expected to be a remnant low. Our
official forecast shows a bit slower motion in the later periods,
and remains close to the consensus guidance which is splitting the
difference in these two scenarios.

Norman will continue to experience rapidly increasing shear, 
expected to be near or greater than 40 kt within 24 hours. This 
should initiate a more rapid weakening trend. Although sea surface 
temperatures are marginal now, the circulation will be moving over 
cool sub-26C SSTs beyond 48 hours. The rate of weakening in this 
forecast is similar to the previous advisory. Norman is expected to
become a remnant low in 4 to 5 days, but this could happen even
sooner. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  06/2100Z 21.4N 150.9W   95 KT 110 MPH
 12H  07/0600Z 22.1N 151.7W   85 KT 100 MPH
 24H  07/1800Z 23.1N 152.8W   70 KT  80 MPH
 36H  08/0600Z 24.1N 153.9W   55 KT  65 MPH
 48H  08/1800Z 25.2N 155.0W   40 KT  45 MPH
 72H  09/1800Z 27.1N 156.7W   30 KT  35 MPH
 96H  10/1800Z 28.6N 158.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  11/1800Z 30.4N 159.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster R Ballard
 
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