172
FXHW60 PHFO 182010
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1010 AM HST Fri Apr 18 2025

.SYNOPSIS...
An active convective pattern with mostly light winds will hold in
place today, with showers favoring the island interiors during
the afternoon and evening hours and along windward areas
overnight. Locally heavy rain and a slight chance of thunderstorms
is forecast through today as an upper low moves over the state.
Moderate trades are expected to return Saturday through the
weekend. Lingering moisture and instability will likely maintain
a rather showery pattern, even for leeward areas through the
weekend. A new storm system could bring another chance for heavy
rainfall and thunderstorms to the islands Tuesday and Wednesday.

&&

.DISCUSSION...
A cold core upper level low remains centered near Maui County
this morning and is expected to gradually push eastward through
the day today. The instability this upper level feature is
providing to the state combined with ample lingering moisture
(PWats over 1.6 inches as measured on this morning`s upper air
soundings) will continue to keep heavy rain in the forecast for
today.

Heavy showers have been impacting Kauai for a couple of hours
already this morning, with rain gages reporting up to 2 to 5
inches across interior and northern portions of Kauai in the last
6 hours. This persistent heavy rainfall has lead to flash
flooding, particularly near the Hanalei River Bridge, where
Emergency Management has reported flooding. High resolution models
are indicating that showers will continue to fall over interior
Kauai this morning which may further exasperate flooding issues and
will later develop over the interior portions of the other
islands by the afternoon as daytime heating induces sea breezes.
Any of these showers have the potential to produce localized heavy
downpours, and instability may be high enough to allow more
thunderstorms to develop. In addition, snow is expected across
the high elevation summits of the Big Island and the Winter
Weather Advisory has been extended through 6 AM Saturday for
elevations above 11,000 feet.

At the surface, a strong high pressure system to far north and
weakening surface trough retreating to west will bring light to
moderate southeast winds of the state today. As mentioned
earlier, this will allow sea breezes to develop and bring clouds
and showers to leeward and interior areas across the island chain
by the afternoon and then land breezes will help to push some of
this activity offshore overnight. In addition, showers caught up
in the southeast flow will periodically move into windward areas,
particularly during the overnight hours, and occasionally pass
over into leeward areas.

Over the weekend, moderate easterly trades will return as the
surface trough pushes further away and dissipates, allowing the
high pressure system to the north to take more control of the
local winds. Aloft, we`ll lose some of the instability as the
upper low opens into a trough as it weakens, but showers are
expected to stick around as moisture lingers. As a result, expect
a showery tradewind pattern through the weekend, with showers
occasionally spilling over to leeward areas.

A new, potent storm system will disrupt the tradewind flow
much of next week bringing light to moderate southeasterly winds
back to the area late Monday into Tuesday and another round of
potentially significant rainfall Tuesday through Thursday.

&&

.AVIATION...
Land breezes and some moisture shadowing within unstable ESE flow
is maintaining mostly quiet conditions from Maui through Oahu
this morning. Showers and thunderstorms mainly reside offshore.
Exceptions are Kauai where heavy interior showers have been
frequent as well as the SE facing slopes of the Big Island.
Similar to yesterday, diurnal heating will allow moisture to mix
inland leading to widespread moderate and isolated heavy showers
over island interiors. MVFR will be common during this time with
isolated pockets of IFR within heavier showers. A few
thunderstorms will also be possible.

AIRMET Sierra remains in effect for E thru S slopes of Kauai.
AIRMET Sierra may be needed for some or all remaining islands this
afternoon.

&&

.MARINE...
Light to moderate east-southeast winds will persist today due to
a broad surface trough located west of Kauai. Over the western end
of the islands, the light winds will allow a land and sea breeze
pattern to continue near the coasts. For the eastern end, light to
moderate east to southeast breezes, with localized fresh pockets,
will likely persist, particularly around the Big Island. Expect
spotty heavy showers and thunderstorms to continue today due to an
upper disturbance in the area. A return to moderate easterly
trades is expected over the weekend as high pressure builds to the
northeast. Guidance continues to show the trades becoming
disrupted and shifting to be out of the south early next week as
deep low pressure sets up nearby to the west.

Surf along exposed north and west facing shores peaked overnight
and will steadily lower through the remainder of the week, with
the swell direction shifting from northwest to north. On Sunday,
surf will dip below average, then remain so well into next week.
Guidance does depict a gale forming on Monday near the Kurils,
then tracking northeast to the western Aleutians by Wednesday. If
this materializes, Hawaii could experience an uptick in the
northwest swell next weekend.

Surf along east facing shores will remain below the April average
through early next week due to the lack of trades locally and
upstream.

Surf along south facing shores will remain small this weekend,
as a background south swell moves through. A larger pulse of
south-southwest swell may arrive early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM HST Saturday for Big Island
Summits.

&&

$$


DISCUSSION...Farris
AVIATION...JVC
MARINE...Bedal