381
FXHW60 PHFO 191351
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
351 AM HST Sat Apr 19 2025

.SYNOPSIS...
A somewhat wet weather pattern today as the state remains under
the influence of upper level troughing. The return of stronger
trades this weekend, along with daytime interior warmth within
unseasonably higher moisture and instability, will likely create
yet another day of island-wide showers. The highest rainfall
amounts will again be anticipated within higher terrain and over
windward mauka communities. A storm low will drop down northwest
of the state through the middle of next week. This will again
disrupt trade flow and introduce higher chances for island-wide
heavy rain and thunderstorm episodes.


&&

.DISCUSSION...
Widespread light to moderate showers across the entire Hawaiian
Island archipelago the last couple of days where many observation
sites, especially those windward and along higher elevation mauka
exposures, measured at least a quarter (to over a half) of an
inch of precipitation. The majority of the highest daily rainfall
fell over windward Kauai where the usual suspects such as Mount
Waialeale and the Wailua area picked up much more than their
neighbors. Many windward Ko`olau sites on Oahu received a good
shot of rain, a quarter to around an inch, since last night. Due
to the weaker nature of better shadowed leeward winds, many
interior wind-sheltered locales will experience return showers as
partial clearing will allow late morning heating to more
efficiently kick in.

Other than the copious amounts of measured widespread rain this
past 48 hours, 19/12Z data is still verifying that Hawaii is
located within an upper level trough axis and still resides
within a relatively unstable and moist air mass. Upper troughing
will continue to gradually fill in as ridging expands in from the
west, troughing lifting northeast this weekend. This morning`s 12Z
soundings have displayed this increasing stability in relation to
yesterday`s indices (e.g., lower CAPE values, lessened lower
level lapse rates) within steady column moisture (1.5 inch
precipitable waters). Another sign that stability is beginning to
take over is the near 6k ft inversion developing over Hilo; a
demarcation between lower southeasterlies and mid layer
southwesterlies. While we will remain in a somewhat unstable
situation this weekend, subtle warming of the mid to upper layers
will decrease the probability of experiencing repeat (significant)
rain events each subsequent day through Monday. There is still
ample instability within high moisture to have today`s forecast
calling for moderate to high probabilities of return showers, some
producing locally high rainfall, with isolated thunderstorms in
the windward waters. High resolution models indicate that very
small pockets of heavy rain and thunderstorms will impact the
islands through the day. Last night`s Winter Weather Advisory
will be allowed to expire this morning as temperatures will warm
above freezing shortly after 0900 HST. Moisture should still be
deep enough that, with below freezing temperatures on Big Island
summits tonight, ice accumulations and occasional light snow may
return overnight Sunday morning.

Strong high pressure system far north of the islands and a
weakening surface trough retreating to the west will maintain
light to moderate east to southeast winds the next few days. This
relatively weaker flow pattern may allow overnight drainage
(land) breezes to help to push any developing shower and/or storm
activity offshore. Showers caught up this east to southeasterly
steering flow will periodically move into and across windward
exposures, particularly during the overnight hours. More organized
cells will occasionally hang together across the higher terrain
and make it into leeward areas. Moderate easterly trades will
return as the surface trough pushes further away and dissipates
and allows high pressure to the north to exert more control of the
local wind behavior.

The next weather maker will come in the form of an closed-off upper
low that will sink down along and near 170W longitude and come
within 600 miles northwest of Kauai. The west northwest positioning
of this low will pull up a more moisture-rich air over the state
from the southwest. As heights begin to lower and mid to upper
layers cool from the west, enhanced instability within a moistened
air mass will likely induce another round or three of widespread
showers and thunderstorms. Ensemble guidance places the higher
probability for measurable precipitation along the upper trough
axis (centered on Maui County and points northeastward) and on
windward slopes this week. With the approach of either a broad
open wave trough or a closed low northwest of the islands next
week, the highest rain probabilities (or areas to receive more
frequent rain episodes) will be across the western half of the
state. Flooding concerns may be re-awakened next Tuesday through
Thursday as locally heavy precipitation falls upon saturated
soils.


&&

.AVIATION...
Land breezes tonight will accelerate clearing with a few SHRA
over windward and mauka locations. MVFR conds possible in any SHRA
otherwise VFR should prevail. Moderate trades return today but
will not be strong enough to eliminate the inland shower threat.
The resulting hybrid sea breeze pattern will focus moderate to
locally heavy SHRA and isol TSRA over western portions of the
smaller islands.

AIRMET Sierra is in effect for mtn obsc for Maui and Oahu.


&&

.MARINE...
High pressure far northeast of the state will hold steady
producing gentle to moderate tradewinds. An upper level
disturbance that brought scattered thunderstorms across the state
Thursday and Friday has moved east of the state and is producing
isolated thunderstorms near the eastern coastal waters and over
the eastern offshore waters. The potential for thunderstorms will
taper off over the eastern coastal waters later today and linger
over the offshore waters through Sunday. A deep low is expected to
develop northwest of the state Monday that will weaken the winds
to light to gentle strength and shift winds out of a more southerly
direction late Monday into Tuesday. The low is expected to
continue to develop and approach the far northwest offshore
waters Wednesday which could strengthen the southerly flow around
the state to more moderate speeds into Thursday and increase the
potential of thunderstorms.

A moderate, medium period north-northwest swell has shifted more
northerly and will continue to gradually trend down through
Sunday, producing below average surf along north and west facing
shores. A tiny, long period northwest swell may fill in Monday
into Tuesday before subsiding Wednesday, keeping surf from going
flat. In the long range, guidance does depict a gale forming
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 the weekend. East shore surf could trend down to tiny
levels by mid week as southerly winds take over.

A tiny, long period south swell will move through this weekend that
will provide a small boost in surf along south facing shores. A
larger, long period south-southwest swell is expected to fill in
Monday into Tuesday that could produce near to above average surf
along south facing showers.


&&

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


&&

$$


DISCUSSION...Blood
AVIATION...Walsh
MARINE...Almanza