480
FXHW60 PHFO 221926
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
926 AM HST Wed Jan 22 2025

.SYNOPSIS...

A cold front will bring a brief round of moderate to locally heavy
showers with slight chance of thunderstorms today into Thursday.
This will be followed by a reinforcing shot of cool weather.
Trades return briefly Friday into Saturday before they are
disrupted again by another front approaching from the northwest.

&&

.DISCUSSION...

Mid-level shortwave energy is driving a progressive cold front
through the forecast area at press time. Presently shifting east
of Oahu, a line of moderate to locally heavy showers accompanies
the front and is anchored along the its convergent wind shift
boundary. The initial band of showers will push through any given
location quickly. This will then be followed by a transition to
moderate northwesterlies, falling dewpoints, and a gradually
diminishing coverage of showers during the following hours. A few
wind gusts to around 35 mph within heavier showers can`t be ruled
out as taller showers will have access to stronger winds above the
boundary layer. Generally speaking, however, winds remain on the
lighter side even as strong dry advection forces boundary layer
winds to the surface. Modeled soundings suggest it will be a
struggle to generate winds of even 20 mph outside of convection. A
wave of scattered post-frontal showers is also noted on radar
over and west of Kauai. These showers are developing as the
initial wave of cold advection above the surface destabilizes the
boundary layer and will extend the chance of showers behind the
front by a couple additional hours before they diminish in
response to falling dewpoints. Expect the front to advance through
Oahu during the peak heating period today which may put a dent in
high temperatures which have been lowered to 78 at HNL. Worth
noting that the most recent MAV guidance tops HNL out at a rather
cool 75F. Dewpoints fall into the low to mid 50s behind the front
allowing interior and sheltered locales to fall into the 50s
tonight over Kauai, Oahu, and Molokai. The front will reach Maui
tonight and dissipate in the vicinity of the Big Island on
Thursday as upper-level support is lost and showers diminish
considerably in coverage.

Light northwesterlies will give way to sea breezes on Thursday,
though interior clouds may be hard to come by given the extremely
dry state of the boundary layer. The exception will be the Big
Island where deeper frontal moisture will linger and mix upslope
during daytime heating yielding typical extensive cloud cover and a
few interior showers. Dewpoints then rebound and the chance for
windward and mauka showers increases as frontal moisture shifts
westward into central and western portions of the island in response
to the return of moderate trades.

The next round of upper-level energy then potentially pushes the
frontal zone eastward back through the forecast area. The final
position of this remnant frontal zone may ultimately be critical as
it potentially serves as a focus for rainfall late next week given
model consensus on another round of upper energy digging southward
toward the islands.

&&

.AVIATION...

A fragmented band of low clouds and showers associated with a cold
front will track SE across the islands today, then stall and
dissipate near the Big Island late tonight and Thursday. Periods
of MVFR VIS/CIG can be expected in SHRA, with ISOL IFR VIS in
+SHRA as the boundary passes, with cloud tops potentially as high
as 20 kft. VFR conditions will prevail over the Big Island today,
with the potential for MVFR CIG/VIS possible over leeward Big
Island by tonight.

AIRMET Sierra for mountain obscuration is posted for Kauai and
Oahu, and will be modified to include Maui County as the front
moves SE. Improving conditions can be expected on Kauai and Oahu
as early as this afternoon/evening. AIRMET Tango for low level
turbulence over and SE of the terrain is posted for Kauai and
Oahu, as low-level NW winds briefly surge as the front passes.
Additionally, moderate upper level turbulence is expected to
develop this afternoon and tonight across much of the state.
&&

.MARINE...

Conditions will quickly change today as a cold front currently
near Oahu continues to move southeast towards Maui County later
today. Gentle to moderate southwest to west winds are expected
ahead of the front with fresh to strong northwest wind behind the
front. The front will likely be weakening and slowing in forward
speed after crossing Oahu, dissipating near the Big Island
Thursday. The front will bring a relatively brief period of
rainfall, near gale force winds, and the potential for
thunderstorms and heavy showers, mainly over Kauai and Oahu
waters this morning.

A Small Craft Advisory (SCA) is in effect and has been expanded to
included Maui Windward waters due to quickly building seas
associated with the building northwest swell and the quickly
rising short-period wind swell from the cold front. Winds will
diminish as the front dissipates, with a surface ridge moving over
the islands from the west by Thursday night. Although winds will
decrease, seas will remain elevated due to a new north to north-
northwest swell filling in Thursday into Friday. A surface high
will pass north of the islands Friday into Saturday, which should
bring the return of moderate northeast trade wind flow by Friday
evening. Long range guidance indicates the potential for another
frontal passage late in the weekend into early next week.

A new moderate long period northwest swell will build through the
day today. Nearshore buoys at Hanalei and Waimea continue to show
a steady increase this morning and we should see surf continue to
rise into Thursday. A developing storm-force low currently located
roughly 700 miles north of Oahu is expected to send a large north
to north-northwest swell Thursday into Friday. Due to the close
proximity of the low and the strength of it, there is potential
for the swell to come in much larger than guidance. Surf should
rise above High Surf Advisory (HSA) thresholds along north facing
shores by Thursday and likely hold at advisory thresholds through
Friday. A series of small to moderate northwest to north-northwest
swells are expected early next week.


&&

.FIRE WEATHER...

Low dewpoints will cause afternoon RH values to fall well into the
40s on Thursday and Friday but winds are forecast to remain light
and firmly below the critical fire behavior threshold.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel.


&&

$$


DISCUSSION...JVC
AVIATION...Birchard
MARINE...Kino
FIRE WEATHER...JVC