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Hawaiian Islands Synoptic Discussion and Guidance

260
FXHW60 PHFO 210156
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
356 PM 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 tonight 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 the Alenuihaha channel
this afternoon, setting up a contrast between clear and rather dry
conditions from Kauai to Molokai, and cloudy and rather wet
conditions across the Big Island and Maui. Latest soundings show
the contrast as well, with the Hilo sounding depicting 1.91 inches
of PW versus 1.30 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 that earlier patches
of heavy showers and isolated thunderstorms across and near Maui
and the Big Island have decreased over the past few hours.
However, persistent instability associated with the upper trough
and high PW will maintain the chance of thunderstorms and locally
heavy rainfall through tonight, prompting us to extend the Flash
Flood Watch until 6 am Sunday morning. The threat of flash
flooding should decrease dramatically as the upper trough passes
east of the islands.

Models show the upper trough should pass east of the islands by
late Sunday. 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 tonight and continuing through early next week,
possibly creating Small Craft Advisory conditions for the windier
channels by Monday.

Until then, expect infrequent overnight convective flare-ups
across portions of Maui County and the Big Island with the
possibility of thunderstorms and locally heavy rainfall through
tonight. Weather from Kauai to Molokai will be clearer and drier,
with a mix of windward cloud cover and showers and the addition
of mauka cloud cover and showers during the afternoon Sunday. 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 the Alenuihaha
Channel 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 the Big Island of
Hawaii and waters southeast of that island. 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 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 from Kauai
to Lanai but the trade winds will push a few showery low clouds
over windward areas.

&&

.MARINE...
A upper-level trough over the area will keep the potential for
heavy showers and thunderstorms mainly for waters around the Big
Island into Sunday morning.

High pressure building northwest of the area will lead to a
gradual increase in the trades through early next week. 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 Sunday night and continuing through much of next week.

Nearshore NDBC and CDIP buoys show combined seas of around 4 to 5
feet (mostly south swell) with an 18 second dominant period at
the Lanai and Barber's Point buoys this afternoon. This swell will
maintain 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 during the second half
of next week.

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 through late tonight for Molokai-Lanai-
Kahoolawe-Maui-Big Island.

&&

$$

DISCUSSION...Powell

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