Thorough Discussion on Olivia 5AM

500 AM HST Wed Sep 12 2018

An explosive overnight development of thunderstorms in Olivia’s
eastern semicircle have since moved over the low level circulation
center (LLCC), despite westerly vertical wind shear near 25 kt.
Subjective Dvorak current intensity estimates ranged from 2.0/30 kt
to 2.5/35 kt this time around, but velocity data from the WSR-88D
on Molokai indicate 50 kt winds at 5000 feet, and 45 kt at 10000
feet. Using reduction factors typically applied to aircraft data,
this supports a surface wind estimate of 40 kt.

Olivia’s motion over the past 18 hours has been erratic, and
strongly modulated by the amount of associated deep convection. A
faster motion toward the west-southwest was observed when
convection dissipated yesterday, with a slower forward speed toward
the west-northwest observed with the deeper convection. This
is due to the vertical wind shear, with the exposed low cloud swirl
steered by low-level trade winds, while the deep convection allows
Olivia’s motion to be increasingly affected by the westerly flow
aloft. With the persistent overnight convective burst, the initial
motion estimate for this advisory is estimated to be 280/10 kt.

In the short term, Olivia is expected to move toward the west.
However, the expectation is that the deep convection will wane
later this morning, and Olivia will make a turn toward the
west-southwest by this afternoon. After emerging to the west of Maui
County this evening, the expectation is that terrain interaction and
increasing vertical wind shear will prevent deep convection from
persisting over the center. A motion toward the west-southwest is
then expected through 36 hours, with the assumption that the LLCC
will be intact after emerging to the southwest of Maui County. GFS
guidance indicates dissipation within 48 hours, while the ECMWF
carries a surface low through day 5. The official forecast splits
the difference, with Olivia devolving to a post-tropical remnant low
by day 3 before dissipation on day 5.

Key Messages:

  1. Flooding rainfall, high surf, and damaging winds are expected in
    the warning area. Significant impacts can occur well away from the
    center, especially when considering that the mountainous terrain of
    Hawaii can produce localized areas of strongly enhanced wind gusts
    and rainfall.

FORECAST POSITIONS AND MAX WINDS

INIT 12/1500Z 21.2N 155.7W 40 KT 45 MPH
12H 13/0000Z 20.6N 157.6W 40 KT 45 MPH
24H 13/1200Z 20.0N 160.4W 35 KT 40 MPH
36H 14/0000Z 19.6N 163.1W 30 KT 35 MPH
48H 14/1200Z 19.6N 166.0W 30 KT 35 MPH
72H 15/1200Z 20.5N 171.5W 30 KT 35 MPH…POST-TROP/REMNT LOW
96H 16/1200Z 21.0N 177.0W 30 KT 35 MPH…POST-TROP/REMNT LOW
120H 17/1200Z…DISSIPATED

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