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Hawaiian Islands Synoptic Discussion and Guidance

031
FXHW60 PHFO 230610
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
810 PM HST Thu Apr 22 2021

.SYNOPSIS...
Moderate to breezy easterly trade winds will persist through the
weekend, with clouds and showers favoring windward and mountain
locations. Windward shower coverage could increase late Friday
through Saturday night as an upper low moves through from north
to south. Warm and humid conditions may return early next week as
the trade winds diminish in response to a front passing far to the
north.

&&

.DISCUSSION...
The surface trough northwest of Kauai that disrupted the trade
winds through the first half of the week will continue retreating
westward away from the area through Friday. Easterly trade winds
will continue to fill in from east to west as high pressure builds
to the north, establishing a stable trade wind regime into
Friday. Showers will favor windward and mountain locations, with
the best coverage likely through the overnight and early morning
periods.

Windward showers may increase late Friday through Saturday night
as an upper low drops southward into the area. Some showers could
reach leeward locations as the inversion weakens and the trades
become breezy. In addition to the increasing shower coverage, a
thunderstorm or two can't be ruled out over the Big Island slopes
through the afternoon hours Friday and Saturday.

Models show the upper low briefly drifting south of the state
Sunday through Sunday night, then lifting back through the area
and merging with an upper trough passing to the north Monday
through Tuesday. The subtropical ridge will weaken once again as
a front passes to the north. If this scenario evolves, a land and
sea breeze regime, with warm and humid conditions, will return
and continue through the first half of next week. Clouds and
showers will shift back over interior and leeward locations
through the afternoon hours.

&&

.AVIATION...
A high pressure ridge north of the Hawaiian Islands will maintain
moderate trade winds through Friday. Periods of mountain
obscurations are possible for windward slopes, especially over
the eastern slopes of Maui and the Big Island as cloud bands
drift through the region. An upper level low north of the island
chain will drift south across the state from Friday to Saturday.
Instability with this upper low will likely enhance shower
activity mainly over windward and mountain locations.

No AIRMETs are in effect, but may be needed for the Big Island
later tonight.

&&

.MARINE...
A surface high passing north of the islands tonight and Friday
will maintain fresh to locally strong trade winds through the
weekend. A small craft advisory is in effect for the windier
waters of Maui County and waters south of the Big Island. Winds
are expected to trend down next week as the high weakens northeast
of the islands.

A northwest swell has brought moderate surf to the north and west
facing shores. This swell has peaked and will be declining through
Friday. A small west northwest swell is possible Monday through
Tuesday of next week with a source from a long lived distant
typhoon. This swell will be followed by a small short period
northwest pulse slated to arrive Thursday.

Small surf will continue along the south facing shores into next
week due to a series of long period swell from the southern
hemisphere. The boost in the trade winds will raise the surf into
the small to moderate range along the east facing shores into the
weekend. The surf will trend smaller after Sunday as the trade
wind diminishes early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Gibbs
AVIATION...Bohlin
MARINE...H Lau

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



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