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Hurricane Norman Discussion Number  32
NWS Central Pacific Hurricane Center Honolulu HI   EP162018
500 PM HST Tue Sep 04 2018
 
After being cloud-filled and ragged through most of the day, the eye
has become more distinct this afternoon, and outflow has improved
in the southern quadrant. At 2330Z, subjective Dvorak current
intensity estimates ranged from 3.5/55 kt at SAB to 4.0/65 kt at HFO
and JTWC. Given the improvement in the satellite presentation from
the Dvorak fix time and from a 2234Z CIMSS SATCON estimate of 66 kt,
the current intensity has been held at 70 kt, though the system
could be stronger. A pair of ASCAT passes from earlier in the day
were used to fine tune the wind radii. 

Norman continues to move toward the due west (270 degrees), and its
forward speed has slowed to 10 kt. Norman will continue to
decelerate tonight as it moves west toward a weakness in the deep
ridge sitting to the north and northeast. On Wednesday, Norman will
begin a long-awaited turn toward the northwest when it reaches a
break in the ridge aloft, and a general northwest motion will
continue from Wednesday night through the weekend as Norman
interacts with a deep trough to the northwest. The track was nudged
slightly left of the prior forecast and remains 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 through the
forecast. 

Norman is expected to remain a hurricane for the next couple of
days. While SSTs will remain marginal in the 26.5 to 27 C range,
dynamical models show a brief increase in intensity during the next
24 hours. This is likely due to slightly improved outflow as Norman
nears a break in the ridge aloft. However, statistical models show
little change or slow weakening. By Wednesday night, all guidance is
in good agreement that Norman will start to weaken under the effects
of increasing southwesterly vertical wind shear, cooler SSTs, and a
drier surrounding airmass. This weakening will persist through day
5, when Norman is forecast to become a remnant low. The intensity
forecast follows similar trends of the prior advisory and favors the
lower end of the dynamical guidance, closest to HMNI and ICON.

A couple of hours ago, the NOAA Gulfstream IV aircraft completed a
mission to sample the environment around Norman, with particular
interest in sampling the strength of the ridge to the north of the
hurricane. These data, which are greatly appreciated, will be
ingested into the 00Z forecast models that will be available
shortly.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  05/0300Z 19.8N 145.5W   70 KT  80 MPH
 12H  05/1200Z 19.9N 147.0W   75 KT  85 MPH
 24H  06/0000Z 20.2N 148.7W   75 KT  85 MPH
 36H  06/1200Z 21.1N 150.0W   70 KT  80 MPH
 48H  07/0000Z 22.2N 151.0W   65 KT  75 MPH
 72H  08/0000Z 25.0N 153.0W   50 KT  60 MPH
 96H  09/0000Z 27.6N 154.8W   30 KT  35 MPH
120H  10/0000Z 29.9N 156.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Wroe
 
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