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Hawaiian Islands Synoptic Discussion and Guidance

742
FXHW60 PHFO 160641
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Tue Oct 15 2019

.SYNOPSIS...
A front approaching from the north, combined with a developing
upper trough, will support increased rainfall coverage across the
western half of the state Wednesday. Thunderstorms and heavy rain
will be possible mainly over Kauai and Oahu. Drier trade wind
weather will return Thursday and remain into Saturday. Increasing
humidity and showers are possible later on Saturday and through
the weekend.

&&

.DISCUSSION...
A Flash Flood Watch will begin Wednesday at noon as a front sags
down over Kauai and Oahu from the north, and an upper level low
brings unstable atmospheric conditions. The moisture at the
surface and instability aloft will bring the chance of
thunderstorms and heavy rain to the western end of the state. The
Watch remains in effect through Wednesday night. By Thursday
morning the moisture from the front is expected to push west of
the state, along with the upper level low, likely alleviating the
threat of widespread heavy rain. High pressure far north of the
state will maintain trade winds throughout the week so shower
coverage will focus over windward and mountain areas with less
rainfall expected over the leeward areas.

On Saturday, a weakening front over the Northwest Hawaiian Islands
may push the high pressure generating our trades far enough east
to shift the trade winds from the east-southeast. This slight
shift in wind direction may be enough to allow some tropical
moisture to flow over the eastern half of the state Saturday
afternoon and night. This would increase humidity and trade wind
showers mainly for the Big Island and Maui County. The upper level
forecast shows a stable atmosphere remaining overhead, so
widespread heavy showers are not expected at this time.

&&

.AVIATION...
Easterly or northeasterly trade winds will persist over the next
several hours as surface high pressure resides far to the north.
Tonight, an upper level low will begin to drop southward and
retrograde over the state. This feature will act to enhance -SHRA
activity along the windward slopes of all islands, then bring SHRA
and possibly +SHRA with TSRA to Oahu and Kauai by Wednesday
afternoon. Expect periodic MVFR conditions to be associated with
any of the passing storms.

No AIRMETs are currently in effect.

&&

.MARINE...
Surface analyses show a fairly strong 1029 MB high near 38N 164W,
about 1000 nm north of the area. This high will maintain locally
strong easterly trade winds over the area through at least
Wednesday night. An ASCAT pass around 10 AM Tuesday showed 25 kt
winds in the Alenuihaha Channel. ASCAT doesn't cover Maalaea Bay
or the Pailolo Channel, but winds speeds in those areas are
usually similar to those in the Alenuihaha Channel. We have issued
a Small Craft Advisory for Maalaea Bay and the Pailolo and
Alenuihaha Channels through Wednesday night. Winds over the area
might be stronger, but a weak front between the high and the main
Hawaiian Islands is weakening the pressure gradient a bit.

A low aloft and the remnant of the front will bring a slight
chance of thunderstorms Wednesday and Wednesday night. Showers
will subside and strong trade winds may become more widespread
starting Thursday night as the low aloft moves away. Winds may
slightly weaken on Sunday as a new front passing northwest of the
state weakens the pressure gradient.

Surf is expected to remain below advisory levels along all shores
through early next week. A small northwest swell from Hagibis
will peak Thursday. A small north swell is expected to peak Friday
and a new northwest swell may arrive next Tuesday night. A series
of south swells will continue, but they will all remain rather
small. Surf along east facing shores will then trend up toward the
end of the week as the trades strengthen.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Wednesday afternoon through late
Wednesday night for Niihau-Kauai-Oahu.

Small Craft Advisory until 6 AM Thursday for Maalaea Bay, Pailolo
and Alenuihaha Channels.

&&

$$

DISCUSSION...Morrison
AVIATION...Thomas
MARINE...Donaldson

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