Flash Flood Warning 1, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
941 AM HST Mon Jan 8 2024

.SYNOPSIS...
An approaching front will cause winds to strengthen and veer to
southwesterlies today, pulling deep tropical moisture across the
islands. Unstable showers will spread eastward across the state
through Tuesday. Expect periods of heavy showers and isolated
thunderstorms. Gusty winds and a potential for flooding remains a
threat for all islands, with the strongest impacts expected along
the front as it passes by. Southern slopes of the Big Island will
also see enhanced rainfall into Wednesday. A second front is
forecast to deliver another round of wet weather from Thursday
through Friday.

&&

.DISCUSSION...
Satellite loop shows a front approaching Kauai from the
northwest. Prefrontal southwesterlies will strengthen today across
the state. Shower bands are already developing in the warm
unstable air ahead of this front and will increase rainfall across
portions of Kauai and Oahu this morning before spreading to Maui
County this afternoon. Stronger shower bands are currently moving
into Kauai and Oahu this morning along with isolated
thunderstorms.

Models show wet and unstable weather spreading eastward across
the state through Tuesday as the front moves through. This front
will slowly weaken as it travels eastward down the island chain.
Strong prefrontal southwesterlies may enhance downslope wind
gusts along north and east slopes of island mountains. We will
monitor conditions to see if they warrant issuance of a wind
advisory. Showers may become heavy at times and thunderstorms are
expected, especially along the front as it passes through. A
Flood Watch remains in effect through Tuesday afternoon for all
Hawaiian Islands.

The Big Island will likely see some enhanced rainfall along the
southern slopes as winds begin to shift there today. Showers
could become heavy there at times, with a potential for
thunderstorms lasting through Tuesday night. Strong winds and
snowfall are possible on the summits of Mauna Kea and Mauna Loa.
We will monitor conditions across the summits to determine the
need for appropriate advisories there later tonight through
Tuesday. Weather will briefly improve across each island, in
turn, after frontal passage from late Tuesday near Kauai to
Wednesday for Oahu and Maui County. The Big Island will see
lingering showers on Wednesday with improvement forecast from
Wednesday night into Thursday. Another front is forecast to move
into the islands from the northwest, with showers spreading down
the island chain from Thursday through Friday. Improving weather
is forecast statewide from Friday night into next weekend.

&&

.AVIATION...
Moderate to breezy south to southwesterly winds will continue in
advance of a cold front that will be making its way down the
island chain through the day and into tonight. Numerous showers
with embedded heavy downpours ahead of the front and near the
frontal boundary can be seen on radar across Kauai and Oahu this
morning. This activity is bringing widespread MVFR conditions with
isolated IFR conditions to the western end of the state. These
conditions will slowly progress down the island chain as the front
moves through and will impact portions of Maui County later this
morning into the afternoon and the Big Island late tonight.
Isolated thunderstorms with lightning and gusty winds will be a
concern with this activity, with the best chances near the
frontal boundary. In addition, the moist southerly flow in advance
of the front will bring showers to the southern slopes of the Big
Island through the day.

An upper level trough moving over the state is expected to bring
moderate upper level turbulence today. AIRMET Tango for moderate
turbulence between FL220 and FL420 is in effect. AIRMET Tango for
low level turbulence below 090 is also in effect downwind of the
terrain. AIRMET Sierra for tempo mountain obscuration is in effect
for Kauai, Oahu, Molokai and Maui and will likely be expanded to
include the Big Island sometime tonight. Light icing is also
possible in layer 130 to FL220. Aviation interests are encouraged
to monitor for forecast updates throughout the next several days
as conditions evolve.

&&

.MARINE...
Fresh to strong south southwest winds are occurring far ahead of
a cold front that is associated with a deepening surface low
entering our far northwest waters. Periods of prefrontal rain and
isolated thunderstorms over the Kauai and Oahu surrounding waters,
locally strong at times, will spread east into the Maui and Big
Island area waters through Tuesday night as rainfall gradually
diminishes over Kauai and Oahu Tuesday. The deepening (potentially
storm force) low will pass just north of the offshore waters
Tuesday and maintain a near all-water Small Craft Advisory (SCA).
Winds will be particularly gusty with severely limited
visibilities in and around areas of heavy showers and occasional
thunderstorms. The main cold front will travel through the islands
Wednesday with gentle to moderate breezes veering northwest in its
wake. Any remaining prefrontal areas of stratiform rain and
embedded isolated thunderstorms will be east of the state by early
Thursday. Forecast guidance indicates the development of another
surface low in the vicinity of the far north offshore waters late
Thursday into Friday that will pull in a front across the west
islands Friday. This system will briefly tighten the pressure
gradient across the north northwest waters enough to produce gusty
near gale force winds.

A small north northwest (310-320 degree) swell will continue to
slowly decline today. A relatively small albeit longer period
northwest swell will gradually build in through the day. The far
northwest offshore buoys have detected and been observing this
swell's steadily increasing energy within the higher spectral
frequency bands bands this morning. This swell energy will likely
result in lifting surf heights along better exposed northern Kauai
and Oahu shores today. A slow-moving system north of the offshore
waters produce a gale force northwest fetch along the backside of
the low that will be aimed at the western islands later today.
The relative close proximity of this source will lead to what is
expected to be a short-lived large, medium period north northwest
(340-350 degree) swell that will build in tonight and Tuesday.
Surf will likely exceed High Surf Advisory levels along Kauai and
Oahu's north facing shores. Seas north and west of the islands
will become elevated and meet SCA criteria Tuesday. The location
and intensity of another surface low developing close to the
islands Friday may potentially produce a large, short to medium
period north swell. A fetch of persistent trade winds far east of
the islands will send shorter period, wind wave swell into our
coastal waters through Tuesday. Eastern exposures will experience
chop from this swell as well as periods of larger surf from north
wrap. South facing shore conditions will remain unusually choppy
from enhanced south southwest winds until winds veer more west
late Tuesday into Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Tuesday afternoon for Niihau-Kauai Leeward-
Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central
Oahu-Waianae Mountains-Lanai Mauka-Kahoolawe-Maui Windward West-
Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island
Interior-Kauai North-Kauai East-Kauai South-East Honolulu-
Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward-Molokai-
Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley
North-Maui Central Valley South-Windward Haleakala-Kipahulu-
South Maui/Upcountry-South Haleakala-Big Island South-Big Island
Southeast-Big Island East-Big Island North.

Small Craft Advisory until 6 PM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters-Big Island
Leeward Waters.

&&

DISCUSSION...Powell
AVIATION...Farris
MARINE...Blood