Wailupe Beach Park, Hawaii Weather Forecast Discussion

123
FXHW60 PHFO 261627 CCA
AFDHFO
Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
627 AM HST Fri Sep 26 2025
.SYNOPSIS...
Low pressure brings a threat for Flash Flooding to Kauai today with
localized heavy showers possible elsewhere. Moderate trades
accompanied by a typical distribution of clouds and showers
returns by Sunday.
&&
.DISCUSSION...
Deep layer cyclonic flow established across the island chain a
consequence of the closed low positioned north of Kauai at press
time. 00z GFS suggests 500mb temperatures around -14C at the center
of the attendant mid-level cold pool while observations of lightning
data generally indicate that thunderstorm activity is confined
within the -10C isotherm, or where lapse rates are about
-6.5C/km or steeper. This places the western end of the island chain
just outside the southern periphery of the most favorable
environment for a greater coverage of thunderstorms. Satellite
imagery does indeed indicate the bulk of convection clustered well
north of the islands with shallower and more scattered convection
over and around the islands themselves. This is a consequence of
weakly convergent flow in the boundary layer, evident by the flow-
parallel lines of shallow convection (tops ~15kft) that have
developed from Oahu westward. Even in the vicinity of Kauai, where
the surface trough resides, boundary layer convergence remains
lackluster and has been hard to come by. Isolated thunderstorms in
the Kauai Channel around 09z/11pm HST do, however, offer confidence
that convection has potential to intensify if low-level can get its
act together.
Water vapor imagery indicates a rather potent shortwave pivoting
around the parent low toward Kauai this morning. Guidance has a
reasonably good handle on this feature, though appearances suggest
it may be a bit stronger than modeled. This makes it hard to
disregard the HRRR which has, in recent runs, backed boundary layer
winds to SEly ahead of the surface trough more substantially than
the global models. This in turn increases low-level convergence over
Kauai this morning yielding what is likely the best opportunity for
heavy rainfall within the Flood Watch area. The best superposition
of mid-level forcing and low-level convergence looks to fall in the
5am-2pm HST time frame. Isolated thunder likewise remains in the
forecast for Kauai. Further east, the strengthening southerly
gradient may overwhelm the sea breeze signal over Oahu which is the
difference between organized afternoon heavy rainfall (similar to
yesterday) or poorly organized and transient showers. PoPs
hesitantly hint at scattered to numerous showers mainly over the
northern half of the island where convergence will be maximized.
Thunder also remains in the forecast over the slopes of the Big
Island where lapse rates will locally steepen once heating of the
higher elevations commences.
Deeper moisture gradually departs westward late tonight through
Saturday even as gently sloped isentropic ascent persists around
Kauai. Given the close proximity of the mid-level low during this
time, suspect transient pockets of showers, a few locally heavy,
will continue through the night over Kauai. However, poor
organization and decreasing coverage precludes any extension of the
Flood Watch. Low-level ascent shuts off altogether once trades
return during Saturday. Even so, the light gradient on Saturday may
lend itself to a hybrid sea breeze pattern with potential for
afternoon showers over the western slopes of Kauai and Oahu.
Strengthening 590+dm mid-level ridge builds in from the east
Saturday through Monday signaling the return of deep layer stability
and a typical moderate trade wind pattern by Sunday.
&&
.AVIATION...
A surface trough in the Kauai Channel between Kauai and Oahu will
slowly drift westward today. Enhanced clouds and showers will
linger over Kauai, Niihau and Oahu this morning. Periods of MVFR
conditions will develop in any shower bands. Expect decreasing
shower trends over Oahu by this afternoon, with continued enhanced
clouds and showers over the western islands of Kauai and Niihau
into the overnight hours. The southeastern slopes of the Big
Island may also see some low clouds and enhanced shower activity
today as deeper unstable tropical moisture moves up from the
south. Isolated thunderstorms are also possible for Kauai, Niihau
and the Big Island today, thunderstorm chances are diminishing by
this evening.
AIRMET Sierra remains in effect for Mountain Obscuration due to
low clouds and showers over Kauai and Oahu this morning. These
low cloud ceilings will likely improve over Oahu later this
afternoon.
&&
.MARINE...
Surface low in the far northern offshore waters associated with a
trough west of Kauai will drift west and weaken through the
weekend. The chance for isolated to scattered thunderstorms will
remain over northwest and far southern offshore waters through the
weekend. Gentle to locally moderate southeast to south winds are
expected over the eastern coastal waters, with variable winds over
western coastal waters. Due to the lighter wind speeds, localized
land and sea breezes expected through tonight. High pressure far
north of the area will slowly build north of the region and move
southeast through early next week. Moderate to locally fresh
easterly trade winds are expected to build back next week through
the forecast period.
The current small to moderate north-northeast swell will slowly
decline through the weekend. A small medium to long period
northwest swell is expected to start filling in through the
afternoon and hold through Saturday before declining Sunday. Surf
along east facing shores exposed to the north swell will remain
elevated today before declining Saturday. Another small medium
period northwest swell is expected to fill in Monday and decline
slowly through next week. Tiny to small background north and
northwest pules will keep north shores from going completely flat
through the middle of next week.
Surf along south facing shores will gradually build as a small
medium to long period south swell fills in today and holds through
the weekend. Trade wind chop should increase by Sunday as trade
winds return.
&&
.FIRE WEATHER...
Conditions remain below the critical fire weather threshold
through the weekend as light winds persist through Saturday and
high RH associated with resident low pressure persists. Moderate
trades and drier conditions expected by next week.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this afternoon for Niihau-Kauai-
&&
$$
DISCUSSION...JVC
AVIATION...Shigesato
MARINE...Bohlin
FIRE WEATHER...JVC

NWS HFO Office Area Forecast Discussion