Area Forecast Discussion
Issued by NWS Honolulu, HI

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239
FXHW60 PHFO 230616
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
816 PM HST Thu Aug 22 2019

.SYNOPSIS...
Trade winds will weaken late tonight through early Friday, then
return to the moderate to breezy range Friday night through 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 Wednesday 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 surface trough north to northeast of
the islands. Despite this weakness, breezy trades are holding over
the state. Clouds and showers are focusing over the typical windward
locations. Rainfall accumulations, however, have remained light
across the state over the past six hours (through 7 PM HST), except
for the leeward coast of the Big Island from Kailua-Kona to
Honaunau, where heavy showers and a thunderstorm formed and
produced up to 2.19 inches within a three-hour period.

The short-term guidance remains in good agreement and depicts the
trades trending down briefly late tonight through early Friday 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 hold 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. Although most of the shower coverage will focus
over windward and mountain locations, some will manage to spill
over into leeward locations with the gusty trades expected. Model
PWs within this pocket of higher moisture are forecast to 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 period, guidance remains in decent agreement
through the first half of next week and shows a weakness
developing within the ridge to the north 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...
A high pressure ridge north of the state will keep moderate trade
winds in the forecast. Mostly VFR conditions are expected over all
islands with isolated MVFR conditions in showers possible along
east slopes of all islands.

AIRMET Tango remains in effect overnight for moderate turbulence
south through west of mountain ranges. Decreasing wind speeds on
Friday will allow this AIRMET to drop in the morning hours.

&&

.MARINE...
High pressure to the north will maintain moderate to locally
strong trades through Saturday. A trough over the northern
offshore waters may briefly decrease the wind speeds tonight into
Friday morning, but confidence remains low. Thus, the Small Craft
Advisory (SCA) remains in effect through Friday for the typically
windy waters around Maui and the Big Island. The Small Craft
Advisory will likely need to be extended later on Friday. A new
high building northeast of the islands will then strengthen winds
to fresh to strong levels late in the weekend. The Small Craft
Advisory may need to be expanded into other zones depending on the
strength of the high over the weekend.

The current south swell will be on its way out on Friday with
mainly background energy expected over the weekend into early next
week. A larger southwest swell may arrive Tuesday and remain
through the 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 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Gibbs
AVIATION...Bohlin
MARINE...Kino



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