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Hawaiian Islands Synoptic Discussion and Guidance

876
FXHW60 PHFO 230135
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 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 near Kauai that disrupted 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...
Trade winds will strengthen across the state through Friday. VFR
conditions will prevail through midnight with brief/isolated
mountain obscurations over the windward slopes. Increasing clouds
and showers is anticipated for the windward side of the Big Island
after midnight as upstream moisture tracks in with the trade wind
flow. No AIRMETs are in effect, but may be needed for the Big
Island later on tonight.

An upper low will move south over Hawaii on Friday and may boost
clouds and showers around the state.

&&

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

A northwest swell is bringing moderate surf along north and west
facing shores. This swell will decline through Friday. A small
west northwest swell is possible next week with a source from a
long lived distant typhoon. A small short period northwest pulse
may arrive next Thursday.

Small, long period swell from the southern hemisphere will
maintain small surf along south facing shores. Expect east shore
surf to build to small to moderate heights over the weekend, and
then diminish Monday.

&&

.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...Powell
AVIATION...Foster
MARINE...Morrison

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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