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Hurricane Norman Discussion Number  31
NWS Central Pacific Hurricane Center Honolulu HI   EP162018
1100 AM HST Tue Sep 04 2018
 
Deep convection near the center decreased slightly, and a ragged,
banding eye emerged on geostationary satellite imagery this morning.
Subjective Dvorak current intensity estimates ranged from 3.5/55 kt
at SAB, to 4.0/65 kt at JTWC, to 4.5/77 kt out of HFO. While a pair
of SSMIS passes at 1510Z and 1602Z showed a well-defined eye, CIMSS
SATCON using these data yielded a 56 kt intensity estimate, and
CIMSS ADT was lower. Given the ragged presentation of the eye and
the abundance of inputs showing a weaker system, the initial
intensity will be lowered to 70 kt.

Norman continues to move toward the west (270 degrees), and the
forward motion has slowed to 12 kt. This westward motion with a loss
of forward speed will continue into tonight as Norman approaches a
weakness in the deep ridge sitting to the north and northeast. On
Wednesday, Norman will begin a turn toward the northwest as it
enters a break in the ridge, and a general northwest motion will
continue through day 5 as Norman interacts with a deep trough to the
northwest. The track was changed little from the prior forecast and
is in the middle of a tightly clustered guidance envelope through
the next 48 hours. While the guidance spread increases beyond 48
hours, all show the general northwestward motion of Norman, keeping
the system to the northeast of the Hawaiian Islands. The official
forecast continues to be near the middle of the guidance envelope 
near TVCE and HCCA.

Slight changes in the intensity of Norman are expected during the
next 36 hours. SSTs will remain marginal near 26.5 to 27 C, but as
the hurricane reaches the break in the upper level ridge on
Wednesday, the dynamical models are showing a brief increase in
intensity, possibly due to improved outflow. However, the
statistical models show a slight weakening during this time. By
Wednesday night, all guidance is in good agreement that Norman will
start to steadily weaken under the effects of increasing
southwesterly vertical wind shear, cooler SSTs, and a drier
surrounding airmass. This weakening will persist through day 5. The
intensity forecast follows similar trends of the prior advisory and
favors the dynamical guidance and is close to ICON.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  04/2100Z 19.8N 144.5W   70 KT  80 MPH
 12H  05/0600Z 19.9N 146.1W   70 KT  80 MPH
 24H  05/1800Z 20.0N 147.8W   75 KT  85 MPH
 36H  06/0600Z 20.7N 149.2W   75 KT  85 MPH
 48H  06/1800Z 21.7N 150.3W   70 KT  80 MPH
 72H  07/1800Z 24.3N 152.2W   55 KT  65 MPH
 96H  08/1800Z 27.0N 154.1W   40 KT  45 MPH
120H  09/1800Z 29.5N 155.6W   30 KT  35 MPH
 
$$
Forecaster Wroe
 
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