435
FXHW60 PHFO 151829
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
829 AM HST Mon Dec 15 2025

.UPDATE...
The flood watch now ends at 6 PM HST for all areas. Latest model
guidance shows a notable decrease in QPF for the overnight period.
Kauai and Niihau may need to be extended beyond 6 PM HST, and we
will be closely monitoring the evolution of convection on
satellite and radar this afternoon to make that determination.
Otherwise, another system will move in Wednesday with the
potential to prompt a new flood watch Wed-Thu. The most likely
location for any new watch later this week will be Kauai and
Niihau.

&&

.SYNOPSIS...
The remnants of a cold front have transitioned to a trough that
is draped over the western third of the state. While the trough
will be weakening through the day, not providing as much focus as
on Sunday, the trough will continue to pull southern moisture-
rich air up across the islands through tonight. The next front is
approaching Kauai and is expected to absorb this trough around
Wednesday. The bonding of these two boundaries will keep chances
high for returning showers and possible isolated storms through
early Tuesday, especially over Kauai and Oahu. Rain chances will
lower going through the middle of the week for Oahu but will
remain moderately high for Niihau and Kauai due to the close
proximity of the western coastal water stationary boundary.

&&

.PREV DISCUSSION.../ISSUED 425 AM HST Mon Dec 15 2025/

A fairly active night rain-wise over Kauai and Oahu where many
locations have picked up another 1 to 2 inches since late Sunday
evening. The front, or the channel for southern latitude moisture
to travel north across the western isles and therefore the impetus
to a very wet Sunday, has stalled in the vicinity of the Kauai
Channel and is troughing out this morning. While forcing is
becoming more benign, a greater than 2 inch precipitable water
air mass is still being channeled northward along the boundary
from the southern latitudes. This alone, with the weak lift
provided by the lower level trough, will keep high chances of
mainly light showers in place through the day. The Flood Watch
remains in effect through early Tuesday; mainly to account for
less rain becoming runoff as it falls on saturated grounds.
Moderate to locally heavy rainfall could move up within light
southerlies and train over already soggy grounds hence
exacerbating minor flooding or ponding of water in lower lying
areas.

The next cold front is making its way to Kauai and is packing
quite the punch as it crosses 165W as evidenced by the cooling
cloud tops and frequent lightning. While this front is expected
to progressively reach the far offshore waters by Tuesday morning,
it is then forecast to slow up and merge with what is left of the
tough laying between Kauai and Oahu this morning. Reasoning is
that eastern ridging anchored off of the western Mexico coast will
budge very little and the associated upper low will ride up along
the ridge`s northwest flank. This northeast-exiting low will
stretch the frontal boundary, troughing the front out as it merges
with this current resident trough. High lower to middle level
moisture will still stream up along southwesterly steering flow
through a good part of the week. Confidence is highest that the
boundary will meander just west of Kauai and hang in there for
several days. Placement of this trough is crucial for Kauai
weather this week. There will likely be several days of mainly
overcast skies and return rain regardless of where the trough
decides to camp, but the threat for return flooding all hangs on
the eventually settling of the trough over the western nearshore
waters.

The majority of the model ensemble members place the boundary
along or just west of Niihau and Kauai with just a few members
introducing periodic rainfall for Oahu. Thus, it appears the
flooding threat will significantly diminish over Oahu by early
Tuesday but the threat will still remain over Kauai and Niihau
through the middle of the week. The main threat will be flooding
with locally heavy 1 to 2 inch per hour rain rates inducing return
flooding of both roads and streams. As the upper low advances well
northeast of the islands by Friday, western state rain chances
will remain moderate high but gradually taper off Thursday and
Friday. The Flood Watch may need to be extended into Tuesday or
Wednesday for Kauai and Niihau. Light to occasionally gusty
southerly winds will prevail through Wednesday as the pesky trough
hangs in place over the western third of the island chain. What
is left of the trough will lift out by the weekend and, after an
extended hiatus, trades will return for the weekend. Trades will
usher in a drier air mass and assist in clearing out skies while
providing a relatively drier weekend/early next week; typical
trade shower regime going into the holiday week.

AVIATION...
A trough in the vicinity of the western islands of Kauai and Oahu
will meander through the day. Moisture riding up from the
southwest along the trough will create periodic IFR/MVFR cigs and
vsbys within heavier showers. Lingering instability will keep a
mention of heightened chances for isolated thunderstorms. Winds
along or in the wake of the trough are variable. Light winds will
tend to be more southerly downstream of the boundary. Expect
statewide southerlies tonight into Tuesday morning as the trough
completely washes.

AIRMET Sierra in effect for mtn obsc for Kauai and Oahu and for IFR
conditions.

MARINE...
A surface trough continues to lift northwestward through Kauai
Coastal Waters today. Expect periods of showers, locally heavy at
times with isolated thunderstorms possible along the trough axis.
Elsewhere, expect decreasing shower activity in a gentle
southeasterly to southerly wind pattern. Another cold front will
approach Kauai from the northwest on Wednesday and Thursday,
possibly increasing shower activity and thunderstorms for the
northwest waters. A Small Craft Advisory remains in effect through
this afternoon for most coastal waters around Kauai and Oahu due
to high seas from the passing north-northwest swell.

The current large medium period north-northwest (320-340 degree)
swell will peak today and then slowly decline into Tuesday. Swell
heights around 9 feet at 13 second periods are being reported at
both the offshore and several nearshore buoy locations this
morning. This means we will continue to see surf heights at High
Surf Advisory (HSA) levels for north and west facing shores
exposed to this large north-northwest swell. Surf heights will
fall just below HSA levels by tonight for Kauai and Oahu. The HSA
was extended in time for north and west facing shores of Molokai
and north facing shores of Maui. Predicted surf heights for west
facing shores of the Big Island are lower then previously forecast,
and will only reach HSA levels, therefore the High Surf Warning
was lowered to an advisory starting later tonight. The HSA will
likely expire for all islands by early Tuesday morning.

A combination of moderate northwest and north swells will keep
surf elevated through the week, possibly reaching HSA levels along
north and west facing shores from Wednesday into Thursday.

East facing shores remain tiny to small given the lack of trade
winds but shorelines exposed to north swell energy may remain
slightly elevated through the week. Surf along south facing
shores increases due to short period southerly wind wave chop.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 6 PM HST this evening for Niihau-Kauai Leeward-
Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central
Oahu-Waianae Mountains-Kauai North-Kauai East-Kauai South-East
Honolulu-Honolulu Metro-Ewa Plain-Koolau Windward-Koolau
Leeward.

High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Leeward-Waianae Coast-Oahu North Shore-Kauai North.

High Surf Advisory until 6 AM HST Tuesday for Maui Windward West-
Molokai Windward-Molokai North-Molokai West-Maui Central Valley
North-Windward Haleakala.

High Surf Advisory from 6 PM this evening to 6 AM HST Tuesday
for Kona-Kohala.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters.


&&

$$


UPDATE...Parker
PREV DISCUSSION...Blood
AVIATION...Blood
MARINE...Bohlin
FIRE WEATHER...Blood