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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 201335

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Wed Nov 20 2019

Trade wind weather will replace earlier unsettled weather as an
upper low moves east of the islands and building surface high
pressure dominates. Trades will strengthen and become windy
Thursday through the weekend. Clouds and showers will favor
windward and mauka areas, with leeward areas expected to get wet
from time to time as winds increase.


An upper low responsible for unsettled weather over the past few
days has weakened and moved east of the islands this morning.
Overnight soundings show our airmass remains moist and slightly
unstable, but satellite and radar show dramatically clearer skies
and diminished shower activity compared to last evening. The
gridded database was altered for decreased POPs through today,
with only isolated to scattered showers expected over windward
areas. The threat of flash flooding has departed and The Flash
Flood Watch for all Hawaiian Islands has been cancelled.

A 1028 mb high far north northwest of the islands is building
eastward as surface troughing over and north of the islands
vanishes. This will allow east northeast trade winds to gradually
build across the state, driving a transition from unsettled
weather within sluggish flow to a more typical trade wind weather
pattern. Clouds and showers will favor windward and mauka areas,
except for leeward areas of the Big island where clouds and
showers will increase during afternoons and early evenings.

Models show the departing upper low will be replaced by more
stable zonal flow aloft. Trade winds will strengthen as the surface
high builds in north of the area. Trade winds will become strong
by Thursday into the weekend as the surface high moves northeast
of Hawaii and strengthens. Windward and mauka showers will
continue, with leeward areas expected to see passing showers at
times due to the breezy winds.


Most of the shower activity has dissipated and that trend should
continue into the early morning hours of Wednesday. A more stable
atmosphere and a return to a moderate tradewind pattern are on tap
for Wednesday. Expect tradewind showers in the usual north through
east windward locations with passing MVFR conditions. The rest of
the state should have VFR conditions through this forecast period.

There are no AIRMETs currently in effect.


The weakening low aloft has begun to move more rapidly east,
while a weak surface trough near the islands has been moving west
and gradually dissipating. That means the atmosphere is becoming
more stable, and showers and thunderstorms have decreased markedly
across the area overnight.

Trade winds will gradually strengthen today, and then become
breezy to strong beginning tonight into the weekend. This will be
in response to strong high pressure far north and northeast of
the area as it becomes the dominant feature affecting the islands.
Also, combined seas are expected to exceed 10 feet in most
coastal zones by tonight or Thursday due to a building northwest
swell. Small Craft Advisories will be posted as warranted, with a
possibility of Gale Warnings in some channels toward the end of
the work week.

A new long-period northwest swell arriving this afternoon and
evening will require a High Surf Advisory (HSA) for exposed
shores of most islands through Thursday night. If peak swell
heights are larger than WaveWatch guidance, as they have been
recently, surf could approach the 25-foot threshold for High Surf
Warning along north facing shores. Another northwest swell
arriving this weekend may also require an HSA. Strong trade winds
will likely drive a steady increase in short-period wind waves
beginning tonight into the weekend, eventually leading to an HSA
for east facing shores. No other significant swells are expected.
See the latest Oahu Surf Discussion (SRDHFO) for details on swell


High Surf Advisory from 2 PM this afternoon to 6 AM HST Friday
for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

Small Craft Advisory from 10 AM this morning to 6 PM HST Friday
for all Hawaiian waters except Maalaea Bay-




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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