307
FXHW60 PHFO 071350
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Tue May 7 2024

.SYNOPSIS...
A low pressure system moving into the Central Pacific basin north
of Hawaii will weaken the subtropical ridge and keep light to
moderate trade winds in the forecast this week. An unstable
environment will enhance shower activity across the Hawaii region
through the weekend with the highest rainfall amounts favoring the
windward and mountain areas. The potential for slow moving heavy
showers and thunderstorms are growing from Friday into the weekend
as a developing upper level low stalls out over the islands.

&&

.DISCUSSION...
Weak upper level troughing will continue to enhance trade wind
showers today. A low level cloud band, the remnants of an old East
Pacific cold front, will drift through the islands on Thursday as
an upper level trough digs into the Hawaii region from the north.
A combination of the low level cloud band and upper level forcing
from the upper trough will produce increasing wet weather trends
across the state from Thursday onward.

A weakening subtropical ridge north of the islands this week will
keep light to moderate trade winds in the forecast through at
least Sunday. Stronger winds aloft are creating warning level wind
speeds over the Big Island summits of Mauna Loa and Mauna Kea
this morning, these strong winds may continue above advisory
levels through tonight. A High Wind Warning was issued this
morning for the highest summits on the Big Island. Elsewhere in
the lower elevation areas, a hybrid sea breeze and trade wind
pattern will develop along terrain sheltered leeward areas. The
wind pattern changes again by this weekend, as an upper level low
sets up directly over the islands producing a surface trough
reflection over the islands and light to moderate east to
southeasterly winds.

A rather unstable weather pattern develops over the Hawaiian
Islands in the latest model guidance from Friday through Sunday as
the upper low deepens and stalls over the state. This pattern
change may bring the potential for slow moving heavy rain showers
and thunderstorms over some islands in the state. The heavy rain
and thunderstorm impacts with this upper low will vary greatly
depending upon the precise location where the low center sets up
relative to each island. If the upper low moves in north and west
of the island chain then the northwest islands of Kauai and Oahu
would be under a higher rainfall threat levels. If the upper low
sets up more over the south and east section of the state, then
Maui County and the Big Island will see higher threats for heavy
rain and thunderstorms. Rainfall coverage was increased and
isolated thunderstorms were added to the forecast for this end of
this week to account for more consistency in the run to run
medium range forecast guidance. The threat for flash flooding over
all islands is growing with time, however it remains too early to
pin down the highest flood threat areas with a Flood Watch product.

More island by island impacts will be revealed as the forecast time
period grows shorter, and the upper low track with weather impacts
evolve over time. Stay tuned.

&&

.AVIATION...
Moderate trade winds will prevail across the island chain today,
then ease up a bit tonight especially over the western islands.
Low clouds and showers will continue to favor windward and mauka
areas, with MVFR cigs/vsbys possible as showers move through. A
stray shower may spill leeward at times, but predominantly VFR
conditions are expected here.

AIRMET Sierra remains in effect for windward sections of Kauai and
Oahu. Conditions should improve here later this morning.

&&

.MARINE...
Fresh to strong trade winds will slowly ease through Wednesday as
a 1030 mb high 1000 nm north northeast of the islands moves off
to the northeast. A Small Craft Advisory (SCA) remains in effect
for the typically windy waters around the Big Island and Maui
County through this afternoon and continues to look like it may be
dropped by this evening  as winds ease to moderate to locally
fresh. A surface trough will form north of the state Wednesday.
As this feature drifts southward near or over the islands Thursday
into Saturday, the trade winds could ease further, and chances
for heavy showers will likely increase.

Near shore buoys are showing signs of fading energy in the 12 to
16 sec range suggesting the current south swell is slowly
beginning to fade today into Wednesday. Forerunners from a larger
and longer lived south- southwest swell will arrive Wednesday,
and surf is expected rise near, or to, the High Surf Advisory
level during the peak of the swell Thursday and Friday. This swell
will gradually decline through the weekend.

A small northwest swell of around 3 feet will peak today, then
decline Wednesday. Wind- driven waves will gradually decline to
around May average today, then drop to below average for the rest
of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 PM HST this evening for Big Island
Summits.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Almanza