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Hawaiian Islands Synoptic Discussion and Guidance

452
FXHW60 PHFO 161338
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 AM HST Wed Oct 16 2019

.SYNOPSIS...
A dissipating front approaching from the north, combined with a
developing upper trough, will support increased rainfall coverage
across the western half of the state today and tonight. 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 Saturday night and
Sunday.

&&

.DISCUSSION...
A Flash Flood Watch begins today 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 tonight. 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 Sunday. 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...
Moderate easterly or northeasterly trade flow will persist over
the next several hours as surface high pressure resides far to the
north. Today, an upper level low will continue 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 this
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 37N 160W,
about 1000 nm north of the area. This high will maintain locally
strong easterly trade winds over the coastal waters through at
least Wednesday night. A Small Craft Advisory remains in effect
for Maalaea Bay and the Pailolo and Alenuihaha Channels through
tonight. A weak front between the high and the main Hawaiian
Islands is keeping the winds from being stronger than they are.

A low aloft and the remnant of the front will bring a slight
chance of thunderstorms from late this morning through tonight.
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 series of northwest and north swells
will produce small surf through early next week. A series of south
swells will also 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 noon HST today through late tonight for
Niihau-Kauai-Oahu.

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

$$

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

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



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