Waipahu, Hawaii 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

017
FXHW60 PHFO 190733
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
933 PM HST Fri Apr 18 2025

.SYNOPSIS...
A relatively active weather pattern through Saturday as the state
will still remain under the influence of upper level troughing.
Rain will become widely scattered tonight as, under this weak wind
pattern, there will not be any particular inland area where
greater rainfall will focus. The return of stronger trades
tomorrow, along with daytime interior warming within lingering
moisture and instability, will likely maintain 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 rainfall
and thunderstorm episodes.

&&

.DISCUSSION...
Widespread rain across the entire Hawaiian Island archipelago
today where many observation sites measured at least a quarter to
a half of an inch of precipitation since sunrise. The majority of
the highest 24 hour rainfall fell over windward Kauai where the
usual suspects such as Mount Waialeale and the Wailua area picked
up between 7 to 10 inches. Many windward sites received a good
shot of rain, between 2 to 4 inches, since last night. Due to the
weak wind regime over the state, many leeward locales got in on
the action once heating kicked in late this morning. Numerous areas
picked up several tenths of an inch of rain today where higher
amounts were focused at higher elevation.

Other than the copious amounts of measured widespread rain this
past 36 hours, 19/00Z data still verifies that Hawaii is centered
within upper level troughing and resides within a unstable and
moist air mass (indices of greater than 1.5k to around 2k joules/k
most unstable CAPE, decent lower 3 km lapse rates within a 1.5
inch precipitable water air mass). Upper troughing will begin to
gradually fill in and lift northeast this weekend. 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 the near to short term (tonight through Saturday)
forecast calling for moderate to high probabilities of return
showers, some producing locally high rainfall and triggering
flooding, with possible isolated thunderstorms. High resolution
models indicate that small pockets of heavy rain and thunderstorms
will impact the islands from tonight through Saturday. Below
freezing temperatures at Big Island summits tonight has periods of
snow forecast through sunrise Saturday. A Winter Weather Advisory
has been extended through 6 AM Saturday for elevations above
11,000 feet to account for light ice accumulation and a snow
accumulations up to 6 inches.

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 this weekend. 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. Flooding concerns will be re-awakened
next Tuesday through Thursday as locally heavy precipitation falls
upon highly-saturated soils.


&&

.AVIATION...
Land breezes tonight will accelerate clearing with a few isol
SHRA over eastern portions of the state. Moderate trades return
early tomorrow, 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 remains in effect for mtn obsc for Maui.


&&

.MARINE...
High pressure far northeast of the state will hold steady through
Saturday while a weak surface trough west of the state continues
to move westward and dissipates. Expect a gradual return of east
to northeast trade wind flow tonight into Saturday with speeds
increasing to more moderate levels as the trough dissipates.
Meanwhile, a passing upper level low has triggered isolated to
scattered thunderstorms across much of the state earlier today but
has since started to dwindle this evening. The potential for
thunderstorms will still linger through tonight but confidence
will be low through the weekend as upper level support shifts east
of the state. In the long range, a deep low is expected to
develop northwest of the state Monday that will weaken the winds
to light to gentle strengthen 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 return the
potential of thunderstorms.

A moderate medium period north-northwest swell will gradually
trend down through the weekend producing below average surf along
north and west facing shores by Sunday. 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 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 the weekend. Surf trend down through much of next week,
becoming tiny 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 above average surf along
south facing showers.


&&

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


&&

$$


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