166 
FXHW60 PHFO 060131
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
331 PM HST Sun Sep 5 2021

.SYNOPSIS...
Trade winds will decline to gentle to moderate strength tonight 
and Labor Day. While showers will continue to favor windward 
areas, a nearby upper level low will produce locally enhanced 
showers and allow a few showers to develop over leeward areas, 
especially on the Big Island during afternoon hours. Trade winds 
will strengthen during midweek as a more stable shower pattern 
returns.

&&

.DISCUSSION...
The atmosphere has become slightly unstable today as trade winds 
slowly declined. Trades are easing as surface high pressure far 
to the north weakens and an upper level low centered about 150 
miles northeast of Maui helps relax the local pressure gradient. 
The upper low is also weakening the local inversion and causing 
showers to be heavier than yesterday. This is more noticeable 
within a batch of low level moisture affecting the eastern half of
the island chain. As a result, windward areas of the Big Island 
and Maui have experienced more rainfall today, and spotty heavy 
showers have flared over leeward Big Island slopes north of 
Kailua-Kona this afternoon. The batch of moisture will clear late
tonight, but expect windward areas of the Big Island and Maui to 
remain somewhat wet, with heavy showers on leeward Big Island 
decreasing during the evening.

Trades will decline to gentle to moderate strength for Labor Day 
as the ridge to the north further weakens, and the lingering upper
low will maintain slightly unstable conditions. Low level 
moisture within the trade wind flow will likely be below normal, 
which suggests that showers will not be extensive over windward 
slopes. However, sea breezes over sheltered leeward areas may 
produce spotty afternoon showers, especially on the Big Island.

Trade winds will gradually strengthen Tuesday and Wednesday and
may become locally breezy next weekend. The upper level low will
be kicked away from the region by a ridge aloft, leading to a
stable weather pattern that will focus showers over mainly 
typical windward areas. 

&&

.AVIATION...
Lighter winds are expected over the next day or two due to the
weakening ridge north of the area. The lighter winds could lead to
some afternoon clouds and showers forming over some leeward areas
tomorrow afternoon. 

An upper level low northeast of the islands is helping to enhance
trade wind showers over the next day or so. The afternoon sounding
from Hilo shows the inversion has lifted to near 13K feet. This is
aiding in showers reaching interior and leeward areas. Anticipate
pockets of MVFR conditions mainly over windward and mountain 
area, although brief MVFR conditions can't be ruled out over 
leeward areas. 

AIRMET SIERRA was recently expanded to include all of the Big
Island, and the Haleakala slops of Maui. MVFR and isolated pockets
of IFR have been observed on the windward side of the Big Island,
while MVFR conditions are more likely over the remainder of the
AIRMET areas. Expect the AIRMET  to continue into the evening, 
with the leeward Big Island and Maui improving after sunset.

&&

.MARINE...
A ridge of high pressure far north of the state will weaken
tonight in response to a front passing north of the area. This 
will result in the trades gradually weakening through Monday, 
then remaining at moderate speeds with locally fresh breezes 
through most of the week. Winds are expected to remain below Small
Craft Advisory thresholds through the week.

A series of small long period south swells will maintain surf
heights along south facing shores near the seasonal average 
during the next few days, then gradually decline as we approach 
the middle of the week. Surf heights along northeast and east 
facing shores will trend lower over the next few days as the 
trade wind speeds decrease, lowering wave heights across the 
region. No significant swells are expected through the forecast
period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wroe
AVIATION...M Ballard
MARINE...Kino