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Hawaiian Islands Synoptic Discussion and Guidance

625
FXHW60 PHFO 220634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Sat Jul 21 2018

.SYNOPSIS...
A high pressure ridge far to the north of the Hawaiian Islands
will continue to keep the state in a moderate trade wind weather
pattern through next week with clouds and showers favoring the
windward and mountain slopes of each island. A few weak
disturbances developing in the deep tropics will bring higher
moisture amounts northward with periods of enhanced showers
affecting the islands through week. The first round of enhanced
showers will begin on Sunday and last through Tuesday morning.

&&

.DISCUSSION...
Two 1032 MB surface high pressure centers are far north of the
islands this evening, one in the Central Pacific and the second
high in the East Pacific, are helping to maintain a high pressure
ridge north of the Hawaiian Islands through the week. A cold
front moving south near the International Date Line will break
down the ridge northwest of the islands as the East Pacific high
builds. The break in this ridge will cause a slight shift in ridge
axis, causing the trade wind directions to shift slightly towards
the east with potentially a slight southeasterly component from
Thursday through Friday, yet it will not significantly affect the
moderate trade wind speeds. Three periods of wet weather are
forecast through the week as several weak disturbances developing
in the deep tropics will send plumes of higher moisture amounts
northward with several days of enhanced showers across the state
through the next seven days.

The first round of enhanced showers will arrive on Sunday with
higher low level moisture amounts producing increasing shower
trends across all islands through the day. Precipitable Water
(PW) amounts will range from 1.5 to 2.0 inches as this bands
passes slowly through the state over the next few days. This
evenings satellite imagery shows the leading edge of this deeper
moisture plume is moving westward roughly 200 miles east of Hilo.
Numerous showers will develop across Windward and Mauka areas with
periods of showers drifting over leeward sections as the deeper
moisture moves slowly through the state. More stable air will move
into the islands decreasing shower activity a bit on Tuesday and
Wednesday, with a brief period of higher trade wind speeds.

A second elevated moisture plume will drift across the state on
Thursday with PW amounts increasing back into the 1.5 to 2.0 inch
range causing numerous showers along windward and mountain slopes
with scattered showers over leeward areas from Thursday through
Friday with a similar flavor of wet trade wind weather forecast for
each island. More stable air moves back into Hawaii from the east
from late Friday into Saturday.

Longer range models hint at a third round of unstable moisture
and showers reaching the Big Island by Saturday afternoon and
spreading westward across the state through next Sunday. We will
continue to monitor these periods of wet trade wind weather
through the week. Stay tuned for updates as the weather pattern
slowly evolves over time.

&&

.AVIATION...
Locally breezy east winds will persist through the coming week.
Cloud and shower coverage will be relatively sparse through Sunday
morning. VFR conditions will prevail. There will be isolated MVFR
ceilings and visibility over east sections of the islands, but we
do not anticipate that AIRMET SIERRA will be required for MTN
OBSC. Clouds and showers will increase starting Sunday afternoon
as low level moisture spreads over the islands from the east and
MTN OBSC may become widespread enough to require AIRMETs.

&&

.MARINE...
Surface high pressure far north of the islands will continue to
produce locally breezy trade winds into early next week. A Small
Craft Advisory (SCA) remains in effect for Maalaea Bay and the
Pailolo and Alenuihaha Channels through Sunday night. The SCA
will likely continue for these waters, and be expanded to
include the waters south of the Big Island, starting Monday due
to a slight increase in trade wind speeds.

A new long-period south swell will continue to fill in through
early Sunday morning. This swell is expected to peak on Sunday,
and then gradually lower from Sunday night through early next
week. A High Surf Advisory remains in effect through Monday
afternoon for south facing shores of all islands. Otherwise,
only small background south swells are expected from mid-week
into next weekend. The trade winds will continue to produce
moderate short-period choppy surf along east facing shores
through most of next week.

&&

.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-
Leeward Haleakala-Kona-South Big Island-Big Island North and
East-Kohala.

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

&&

$$


DISCUSSION...Bohlin
AVIATION...Powell
MARINE...Houston

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