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Hawaiian Islands Synoptic Discussion and Guidance

738
FXHW60 PHFO 201943
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
943 AM HST Sat Oct 20 2018

.SYNOPSIS...
Trade winds will gradually strengthen through Sunday night. An
upper trough across the main island chain may trigger
thunderstorms and bouts of heavy rainfall across Maui County and
the Big Island through today and possibly Sunday. Expect locally
strong trades to return by Monday and persist through the middle
of next week. Showers riding in on the trades will favor windward
and mountain areas. Shower frequency will increase overnight,
then diminish in the morning.

&&

.DISCUSSION...
The axis of an upper trough extends across Molokai this morning,
setting up a contrast between clear and rather dry conditions
across Kauai and Oahu, and cloudy and rather wet conditions across
the Big Island and the islands of Maui County. Overnight
soundings show the contrast as well, with the Hilo sounding
depicting 1.92 inches of PW versus 1.53 inches at Lihue. Both
soundings showed an unstable airmass across the area. Satellite
loop reveals light to moderate surface trade flow across local
waters, with the stronger trades residing near Kauai. Radar
indicated patches of moderate to heavy showers persist over water
just northeast of the Big island, while weaker showers reside over
windward slopes of Maui. Persistent instability associated with
the upper trough and high PW will increase the chances of
thunderstorms and locally heavy rainfall through today, prompting
us to keep the Flash Flood Watch active through this afternoon.
The watch area may have to be trimmed but extended into Sunday as
the upper trough moves slowly eastward.

Models show the upper trough should pass east of the islands by
Monday. This will allow ridging to move in aloft which should
restabilize and dry out our airmass, resulting in clearer skies
and fewer showers. Trade wind speeds should rebound as well
beginning later today and continuing through early next week,
possibly creating Small Craft Advisory conditions for the windier
channels by Monday.

Until then, expect afternoon convective flare-ups across portions
of Maui County and the Big Island with the possibility of
thunderstorms and locally heavy rainfall through today. Weather
across Kauai and Oahu will be clearer and drier, with a mix of
windward cloud cover and showers and the addition of mauka cloud
cover and showers this afternoon. The pattern will shift to favor
windward clouds and showers as trade winds strengthen. A typical
trade wind pattern of clouds and showers will spread eastward to
Maui County and the Big Island as the upper trough moves away.
Models show an area of increased moisture embedded within trade
flow may reach the islands Tuesday. This would increase trade
shower coverage from midweek onward through the forecast period.

&&

.AVIATION...
A sharp northeast to southwest trough aloft over Molokai will
move southeast slowly. AIRMET TANGO is in effect for turbulence
aloft associated with the trough. Layered middle and high clouds
associated with the trough cover Maui and the Big Island of
Hawaii. AIRMET ZULU mentions the possibility of light icing in
these clouds. The trough is making the atmosphere unstable over
Maui County and the Big Island, producing a chance of heavy
showers and thunderstorms. AIRMET SIERRA is in effect for mountain
obscuration on Maui and the Big Island.

A surface ridge northwest of the main Hawaiian Islands will build
east. Light to moderate east northeast trade winds will become
moderate to fresh as the ridge builds. VFR will prevail over Kauai
County and Oahu but the trade winds will push a few showery low
clouds over windward areas.

&&

.MARINE...
High pressure building northwest of the marine area will lead to a
gradual increase in the trades through the weekend. Winds should
remain below Small Craft Advisory (SCA) levels through Sunday,
with moderate to locally strong trade winds potentially requiring
a SCA for the windier zones around Maui and the Big Island
developing Sunday night and continuing through much of next week.

A persistent upper-level trough over the area will keep
conditions unstable through the weekend, bringing the potential
for heavy showers and thunderstorms, particularly over the waters
surrounding Maui and the Big Island.

The latest nearshore NDBC and CDIP buoys show an incoming, long
period south swell continuing to move in this morning. Combined
seas are around 4 feet with an 18 second dominant period at the
Lanai and Barber's Point buoys at this time. This swell will
continue to build today, peak tonight and Sunday, before lowering
slightly for Monday. A High Surf Advisory remains in effect for
south facing shores of all islands through 6 PM Monday. Re-
enforcing south swells will keep south shore surf elevated through
much of next week, with surf heights again potentially rising to
advisory levels late next week or next weekend.

Elsewhere, surf heights will remain below advisory levels through
the middle to latter part of next week. A moderate short period
north-northwest swell is expected to hold through tonight, then
gradually drop Sunday through early next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai-
Lanai Makai-Kahoolawe-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Leeward Haleakala-Kona-South Big Island-Big
Island North and East-Kohala.

Flash Flood Watch until 6 PM HST this evening for Molokai-Lanai-
Kahoolawe-Maui-Big Island.

&&

$$

DISCUSSION...Powell

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

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