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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180640

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Wed Apr 17 2019

A surface ridge will remain north of the islands through the
coming week. A trough aloft will move away to the east by Thursday
night as a ridge aloft builds over the area.


The 8 PM surface analysis shows a 1024 mb surface high about 650
miles north of Kauai near 31N163W. The high is forecast to move
northeast and strengthen through Friday, then remain nearly
stationary through early next week. A ridge extending southwest
from the high will remain north of the main Hawaiian Islands and
maintain locally breezy trade winds over the islands.

As of 8 PM, satellite loops and surface observations show mostly
cloudy skies over Kauai with partly cloudy skies elsewhere over
the main Hawaiian Islands. Radars show isolated showers moving
west over windward areas of the Big Island and Maui with mostly dry
conditions elsewhere. Little change is expected for the next few

The atmosphere east of the Big Island is somewhat unstable due to
a trough aloft. Satellite loops show an area of showery low clouds
about 150 miles east of the Big Island. These clouds produce an
increase in showers over the windward side of the Big Island late
tonight and early tomorrow morning. Some of the showers could
affect windward east Maui, but the showers should remain south of
the rest of the islands.

The trough aloft will move away to the east by Thursday night and
a ridge aloft will persist over the area through early next week.
Locally breezy trade winds will continue over the islands. The
trades will carry scattered showers over windward areas, mainly
during the night and early morning hours. Afternoon heating will
produce clouds and showers over leeward slopes of the Big Island
and Maui. Otherwise, leeward area will remain mostly dry.


Any leftover clouds and showers over Kauai now, are on the way out,
with drier conditions on tap for the rest of the night. Fair skies
elsewhere, currently, but do expect a slight increase in trade wind
clouds and showers in the coming hours. These clouds and showers
will bring brief MVFR conditions to the windward and mountain areas
to all of the smaller islands. Windward and eastern Big Island
could be the wettest overnight as a patch of loose clouds and
showers, located east of the Big Island on satellite, skirts those
areas late tonight. AIRMET Sierra for mountain obscuration is not
expected, but will be monitored closely. A late night shower or
two may develop along the Kona coast as well. Otherwise, VFR
conditions, will prevail over the rest of the area.

Trade winds will be trending locally stronger through the next 24
hours. The winds are too light now for AIRMET Tango, but likely
by 1600z.


Fresh to strong easterly trade winds associated with a strengthening
ridge north of the islands will fill in overnight and continue
through early next week. A small craft advisory has been issued for
the Pailolo and Alenuihaha channels, Maalaea Bay, and over the
waters around South Point of the Big Island through Friday where the
strong winds are expected. The advisory for these areas will
likely be extended to cover the weekend.

Surf along north and west facing shores will remain small through
Thursday, then rise Thursday night into Friday as a new north-
northwest (320-340 deg) swell builds down the island chain. This
source will be from a gale that was centered over the Aleutians near
the Date Line Sunday that quickly tracked south-southeast and
weakened Monday through Tuesday. After the peak on Friday, the
swell will quickly ease Friday night through into the weekend.
Outside of a small westerly pulse from a compact gale that has
developed east of Japan early next week, small surf is

Surf along south facing shores will remain small through the weekend
with mainly background south-southwest pulses moving through. An
upward trend is anticipated early next week, beginning as early as
Sunday night due to activity south to southeast of New Zealand over
the past few days. Guidance depicts this source peaking at Pago Pago
Saturday, which should correlate to above average south shore surf
continuing into midweek.

Surf along east facing shores will remain small through Thursday,
then rise slightly Friday through the weekend as the trades pick
up into the fresh category locally and just upstream. Expect this
trend to continue early next week.


Small Craft Advisory until 6 PM Friday for Maalaea Bay, the
Pailolo and Alenuihaha channels and waters south of the Big Island.



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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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