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Tropical Storm Flossie Discussion Number  28
NWS Central Pacific Hurricane Center Honolulu HI   EP072019
1100 PM HST Sat Aug 03 2019
 
Flossie's satellite presentation is typical of a sheared tropical 
cyclone, with deep convection removed from the low level
circulation center (LLCC). In this case, westerly vertical wind
shear around 25 kt is keeping the convection displaced east of the
LLCC, which remains obscured by mid- and high-level clouds. The
initial intensity has been maintained at 40 kt for this advisory, as
the satellite appearance has changed little since an earlier ASCAT
pass indicating winds close to 40 kt, entirely in the northern
semicircle. This is higher than the Dvorak intensity estimates that
range from 1.0/25 kt to 2.5/35 kt from HFO/SAB/PGTW, but close to
UW-CIMSS SATCON. 

Flossie is tracking almost due west this evening, with an initial 
motion estimate of 280/11 kt. A low- to mid-level ridge north of 
Flossie has been supporting this motion for the past couple of
days, and forecast models are now indicating that this ridge will
remain stronger than originally anticipated in the short term.
Track guidance has once again shifted to the left in response, and
the official forecast follows suit, building on a trend that started
about 24 hours ago. 

HWRF/HMON and CTCI have been on the left side of the guidance 
envelope for several runs, taking Flossie very close to, or over, 
portions of the Hawaiian Islands Monday and Tuesday - albeit as a 
weak tropical cyclone. Other reliable models are now indicating a 
similar scenario, and the updated official forecast brings the 
center of Flossie over waters just east of the Hawaiian Islands as
a tropical depression. Given recent trends and average forecast
error, a Tropical Storm Watch may be required for portions of the
Hawaiian Islands on Sunday. The updated track forecast closely
follows the ECMWF guidance, and indicates a turn toward the
northwest after 48 hours as Flossie finally rounds the ridge, and
gets wrapped in the flow around a persistent deep-layer trough
northwest of Hawaii.

The gradual spin down of Flossie is expected to continue as it
draws closer to the Hawaiian Islands, despite SSTs gradually warming
to near 27C. Vertical wind shear is expected to increase to over 30
kt after 24 hours, leading to steady weakening, and eventually 
dissipation. The intensity forecast is mainly an update of the
prior forecast, and is close to the trends indicated by both the 
statistical and dynamical guidance. 

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  04/0900Z 19.1N 146.7W   40 KT  45 MPH
 12H  04/1800Z 19.3N 148.4W   35 KT  40 MPH
 24H  05/0600Z 19.4N 150.8W   35 KT  40 MPH
 36H  05/1800Z 19.9N 153.5W   30 KT  35 MPH
 48H  06/0600Z 21.0N 155.9W   30 KT  35 MPH
 72H  07/0600Z 22.9N 159.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  08/0600Z...DISSIPATED
 
$$
Forecaster Birchard
 
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