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Tropical Storm Olivia Discussion Number  44
NWS Central Pacific Hurricane Center Honolulu HI   EP172018
500 AM HST Tue Sep 11 2018
 
Hurricane Hunters from the 53rd Weather Reconnaissance Squadron 
spent most of the night flying through Olivia, sampling the core as 
well the periphery. Maximum flight level winds of 78 kt were 
observed in the northwest quadrant fairly early in the evening, but 
subsequent observations indicated that Olivia is well below 
hurricane intensity. The surface pressure was gradually rising, and 
the system exhibited tilt with height, with the low-level center on 
the south side of the cold cloud tops seen in satellite. 
Additionally, the observed wind field was rather asymmetric, with 
winds fairly weak in the southern semicircle. The initial intensity 
for this advisory is deemed to be 55 kt, and given current
satellite trends, this may be slightly generous.

The aircraft center fixes add confidence to the initial motion 
estimate of 270/9 kt. Olivia is being steered by a mid-level ridge 
centered to the distant northeast, but a mid-level ridge building
to the northwest of Olivia will induce a motion toward the west-
southwest later today. Once this occurs, this motion is expected to 
persist as Olivia crosses the Hawaiian Islands. The updated track 
forecast is very close to the previous, especially over the first
48 hours. In 48 to 72 hours, a low aloft is expected to develop
north of Olivia, and this development is expected to impart a
motion toward the west-northwest on days 4 and 5. The official
forecast is now near the center of the guidance envelope, and is
close to the multi-model consensus TVCE and GFEX. The along-track
differences between the GFS and ECMWF were reduced with the arrival
of the 06Z GFS, adding confidence to the short term forecast track,
which indicates a slightly faster speed of motion than the previous 
forecast.  

Moderate shear is expected to continue over the next 48 hours while 
SSTs increase to 28C. The shear is expected to induce slow
weakening as indicated by nearly all intensity guidance, with
interaction with island terrain potentially contributing to
weakening as well. After Olivia exits the Hawaiian Islands to the
west in 48 to 72 hours, a cold core low aloft is expected to develop
north of Olivia. By 96 hours the cold core low and Olivia become
vertically stacked, potentially resulting in Olivia becoming
post-tropical. The official intensity forecast indicates this will
occur by day 5, and closely follows IVCN through the bulk of the
forecast. 


Key Messages: 

1. Although Olivia is expected to move over the islands as a
tropical storm, it could still bring significantly worse impacts
than recent Hurricane Lane. Those impacts could include intense
flooding rainfall, damaging winds, and large and dangerous surf.

2. It is important to not focus on the exact forecast track 
and intensity when planning for Olivia. Regardless of the track 
that Olivia takes as it approaches the islands, significant impacts 
can be expected away from the center. In particular, the
mountainous terrain of Hawaii can produce localized areas of
strongly enhanced wind gusts and rainfall. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  11/1500Z 21.9N 151.1W   55 KT  65 MPH
 12H  12/0000Z 21.6N 152.9W   50 KT  60 MPH
 24H  12/1200Z 21.1N 155.3W   45 KT  50 MPH
 36H  13/0000Z 20.3N 157.8W   45 KT  50 MPH
 48H  13/1200Z 19.7N 160.2W   40 KT  45 MPH
 72H  14/1200Z 19.3N 164.7W   40 KT  45 MPH
 96H  15/1200Z 20.5N 169.5W   35 KT  40 MPH
120H  16/1200Z 21.6N 174.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Birchard
 
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