274
FXHW60 PHFO 161931
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
931 AM HST Thu May 16 2024

.SYNOPSIS...
A kona low north of the state will continue drawing unstable
tropical moisture from the deep tropics northward over the islands
over the next few days. Bands of showers producing moderate to
heavy rain and thunderstorms will develop in this tropical air
mass and affect most islands through at least Friday. The kona
low will begin to drift away from the state this weekend with
improving trends over the eastern islands, while enhanced rainfall
will linger over the western end of the state into Sunday. All
islands will see an improvement in weather conditions next week as
trade winds gradually return.

&&

.DISCUSSION...
A kona low currently centered roughly 600 miles north-northwest
of the main Hawaiian Islands will bring widespread impacts to the
state over the next few days.

Latest surface analysis this morning shows a stationary front
extending from the kona low into the Kauai Channel, with a surface
trough roughly 200 miles northwest of Kauai. Surface observations
ahead of the frontal boundary show southerly kona winds that are
bringing unstable, deep tropical moisture from the tropics over
the islands. CIMMS MIMIC Layer precipitable water shows PW values
ranging from roughly 1.50 inches to 2.30 inches across the state,
with the axis of highest PW located very near Oahu just ahead of
the front. Morning radar imagery shows numerous showers moving
into the western and central islands from the southwest. Radar-
derived instantaneous rainfall rates of 1 to 2 inches per hour
suggest that these showers are bringing moderate to heavy
rainfall, particularly over Oahu and Molokai, where surface
convergence is maximized. Fortunately, these heavy showers are
moving rather quickly, which has somewhat limited flooding
impacts thus far this morning. However, soils have become
saturated with 24 hour rainfall totals over Oahu in the 1-2 inch
range for many locations. As showers continue to train over these
saturated areas throughout the day, expect flash flooding concerns
to amplify and streams to become more responsive to downpours. A
Flood Watch remains in effect for most of the state through Friday
afternoon, but this will likely need to be adjusted as the event
unfolds.

For the rest of the day through Friday, surface convergence and
forcing for ascent will be maximized over the smaller islands,
with Oahu, Molokai, and Lanai being the islands with the greatest
flash flooding threat today due to moderate to heavy showers and
thunderstorms and potential training. This focus area is expected
to shift westward towards Kauai on Friday, and then linger near
the western end of the state throughout the weekend. Hi-res
guidance generally agrees with this prognosis, with the FV3, ARW,
and HRRR all depicting similar scenarios. HREF heavy rainfall
probabilities and GEFS/ENS model ensembles increase confidence
these islands are most likely to see the heaviest rainfall rates
over the next couple of days. While flood impacts are also
possible for the island of Maui, guidance suggests that the
potential for heaviest rain will shift westward towards Kauai by
early Friday, lessening the threat for flash flooding for Maui by
that time.

The flood threat for the Big Island remains lower than the other
islands, therefore, the Big Island is not in the watch at this
time. The southeastern slopes could still see moderate to heavy
rain during this event, but most of the deep convective showers
should remain west of the Big Island.

The medium range forecast guidance shows drier and more stable
east to southeasterly winds as the ridge builds in from the east
and the kona low drifts northward this weekend. Any delay in the
ridge building back in will slow down these improving weather
trends this weekend.

Next week, there will be a gradual transition back to a drier
weather pattern as high pressure builds in from the east. This
will produce a drier and more stable southeasterly wind flow
pattern across the islands as the kona low drift farther northward
away from the state. Easterly trade winds will begin to return by
the middle of next week.

&&

.AVIATION...
A strong kona low about 600 nm NW of Kauai is pulling tropical
moisture out of the south across the state. This will keep the the
islands in an active weather pattern through the remainder of the
week into the weekend. Moderate to heavy SHRA and possible TS are
expected over the western side of the state today and tomorrow.
On the Big Island, SHRA is expected this afternoon into the
evening hours. LIFR to MVFR conds expected across the smaller
islands especially in heavier SHRA/TS.

AIRMET Sierra is in effect for for mtn obsc for Kauai, Oahu,
Molokai, Lanai, Maui and windward Big Island.

AIRMET Sierra for IFR conds over Kauai, Oahu, Molokai and Lanai.

AIRMET Tango is in effect for turb blw 120 N thru E of the Big
Island. Conds could spread to Maui later today as gradient
tightens.

AIRMET Zulu is in effect for light rime icing from Kauai to Maui
in layer 140-FL250.

&&

.MARINE...
Strong south to southeast winds associated with a kona low north-
northwest of the state will persist through Friday for most
waters. A shift out of the east to southeast is expected over the
weekend through early next week as the low lifts northward and
away from the area. A Small Craft Advisory remains in effect for
windward and leeward Big Island waters (Hamakua Coast and waters
near South Point). Expect rough seas in heavy showers or
thunderstorms that develop over the next few days, especially over
waters west of Maui County. Seas should remain just below
advisory level over exposed Kauai waters as a short-period
northwest swell moves through (peaking today through tonight).

Surf along south facing shores will remain up through early next
week due to expected overlapping, long-period south-southwest
swells. The first swell will rise through the day today, then peak
tonight through Friday, likely reaching advisory level. The High
Surf Advisory remains in effect for south facing shores through
Friday. Expect rough conditions for most south exposures through
this period due to a combination of the onshore winds, swell, and
periodic heavy showers moving through. As this swell slowly eases
over the weekend, a fresh, long-period south-southwest swell will
begin to fill in through the day Sunday. This will peak early next
week before easing through midweek. Ongoing short-period
northwest swell will keep surf along north and west facing shores
below advisory thresholds. This swell will linger into Friday,
then ease through the weekend. Surf along east facing shores will
remain well below average through the weekend. An upward trend is
possible next week as the trades return locally and far upstream
over the eastern Pacific.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Friday evening for Kauai, Niihau, Oahu,
Molokai, Lanai and Maui.

High Surf Advisory until 6 PM HST Friday for south facing shores
of all islands.

Small Craft Advisory until 6 PM HST Friday for Big Island
Windward Waters-Big Island Leeward Waters.

&&

$$


DISCUSSION...Vaughan
AVIATION...Walsh
MARINE...Powell