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Hawaiian Islands Synoptic Discussion and Guidance

153
FXHW60 PHFO 221335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Thu Aug 22 2019

.SYNOPSIS...
Trade winds will weaken briefly late Friday through early
Saturday, then return to the moderate to breezy range late
Saturday through the second half of the weekend. Clouds and
showers will favor windward and mountain locations, especially
through the overnight and early morning periods as pockets of
higher moisture move through. The exception will be over the
leeward areas of the Big Island, where afternoon and evening
clouds and showers form in response to the local sea breeze. Warm
and humid conditions will become a possibility Tuesday through
midweek as the trades trend down into the light and variable
range.

&&

.DISCUSSION...
The latest surface analysis showed a weakness within the
subtropical ridge due to an upper low and dissipating surface
trough several hundred miles north to northeast of the islands.
Despite this weakness, breezy trades are holding over the islands.
Clouds and showers are focusing over the typical windward and
mountain locations. Rainfall accumulations have remained light
over the past six hours (through 2 AM HST), ranging from a few
hundredths to a quarter of an inch.

The short-term guidance remains in good agreement and depicts the
trades trending down briefly Friday through early Saturday as the
aforementioned upper low and surface trough pass to the north
from east to west. An upward trend in windward shower coverage
can't be ruled over Kauai tonight into Friday as the moisture axis
associated with the trough moves through (PWs near two inches).
Elsewhere, mostly dry conditions will prevail with clouds and
showers favoring windward and mountain locations.

Moderate to breezy trades will return over the weekend as the upper
low and surface trough continue westward and high pressure builds to
the north. An increase in shower coverage will be possible late
Saturday into Sunday as a pocket of higher moisture moves through
from east to west. Model PWs within this pocket of higher moisture
climb into the 1.75 to 1.9 inch range over the Big Island late
Saturday, then to the smaller islands Saturday night into Sunday.

For the extended, guidance remains in decent agreement through the
first half of the week and shows a weakness developing within the
ridge as a broad upper trough and surface front pass to the
north. The pressure gradient is forecast to respond and weaken
over the state, which could be enough for a land and sea breeze
regime to become established Tuesday through midweek. If this
scenario evolves, clouds and showers will favor interior areas
through the afternoon and evening periods.

&&

.AVIATION...
Some trade wind showers during the pre-dawn hours will bring some
brief MVFR conditions to some windward locations of the smaller
islands. Otherwise, expecting VFR conditions to prevail. High
pressure north of the islands will maintain a relatively stable
trade wind flow. Cloud tops are reaching near 8000 feet. AIRMET
Tango is in place for mechanical turbulence to the lee of the
mountains. The AIRMET is expected to continue into tonight.

&&

.MARINE...
High pressure to the north will maintain mostly fresh trades
through Friday. The typical windy areas around Maui and the Big
Island will continue to see locally strong trades through
at least tonight and possibly into Friday. Thus, the Small Craft
Advisory (SCA) has been extended through tonight for the
typically windy waters around Maui and the Big Island. A trough
approaching north of the state Friday will briefly ease the trade
winds Friday night to below SCA levels. A new high building
northeast of the islands will then strengthen the trades back to
moderate and locally strong levels over the weekend, with SCAs
likely being required once again for the windier marine zones.

The High Surf Advisory for south facing shores has been dropped
and the south swell will continue to lower through the rest of the
week. A series of small long-period south and southwest swells
will move through Sunday through early next week. A larger southwest
swell is possible Tuesday through the second half of next week.

Small northwest swells will linger over the next few days
providing some out of season surf along north facing shores.
Breezy trades will continue to produce small choppy surf along
east facing shores over the next few days. An increase in wind
swell is expected late in the weekend as trades strengthen locally
and upstream of the state.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...M Ballard
MARINE...Kino

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



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