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Hawaiian Islands Synoptic Discussion and Guidance

572
FXHW60 PHFO 231317
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
317 AM HST Fri Apr 23 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 later today
through Saturday night as an upper low moves through from north to
south. Warm and humid conditions, along with increasing rainfall
chances, may return early next week as the trade winds diminish
ahead of an approaching front.

&&

.DISCUSSION...
The surface trough northwest of Kauai that disrupted the trade
winds earlier in the week will continue retreating westward and
away from the area today. Easterly trade winds have returned, and
will likely become breezy over the weekend as high pressure
builds to the north. Windward showers may increase later today
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 increase - especially through the
overnight and early morning hours. 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 today 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 approaches the region from the northwest. If this
scenario evolves, a land and sea breeze regime, along with warm
and humid conditions, will return and continue through the first
half of next week. Rainfall chances will trend up through this
time, with the best coverage setting up over interior and leeward
locations through the afternoon hours.

&&

.AVIATION...
A high pressure ridge north of the Hawaiian Islands will maintain
moderate trade winds today and tonight. 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 through Saturday. Instability
with this upper low will likely enhance shower activity mainly
over windward and mountain locations.

No AIRMETs are in effect. However mountain obscurations may be
needed for windward Big Island later this morning.

&&

.MARINE...
A surface high passing north of the islands today will maintain
fresh to locally strong trade winds through the weekend. A small
craft advisory is in effect to until 6 am HST Sunday for the
windier waters of Maui County and waters south of the Big Island.
Trade winds are expected to trend down and become southeast on
Monday, ahead of an approaching front from the northwest.

A northwest swell has peaked and will be subsiding slowly through
Saturday. A small west-northwest swell is tabbed to arrive late
Monday, peak Tuesday, then subside gradually through Wednesday.
The source of this swell is from a long lived distant typhoon. A
small short period northwest pulse is slated to arrive Wednesday
night and 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.

&&

.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