369
FXHW60 PHFO 161351
AFDHFO

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

.SYNOPSIS...
Southerly kona winds will continue pulling up unstable tropical
moisture from the deep tropics over the next few days. A kona low
remains in place roughly 600 miles north to northwest of the
Hawaiian Islands as a weakening low level trough begins to move
into Kauai. Bands of showers producing moderate to heavy rain and
thunderstorms will develop in this tropical air mass and affect
most islands through Friday. The kona low will begin to drift away
from the state this weekend with improving trends over the
western islands into the upcoming weekend. By Monday all islands
will see a dramatic improvement in weather conditions as easterly
trade winds return.

&&

.DISCUSSION...
In the larger scale, satellite imagery this morning shows a kona
low system roughly 600 miles north to northwest of the Hawaiian
Islands with a weakening trough moving into the islands of Kauai
and Niihau from the northwest. Southerly winds ahead of this
system continues to pull up deep unstable tropical moisture over
the islands. A combination of local radar and satellite imagery
show convective shower bands moving into most of the islands from
the south. These shower bands will produce periods of moderate to heavy
rain across much of the state through Friday. Soil moisture levels
remains saturated for most areas, which means any additional heavy
rainfall could swiftly turn into runoff and a threat for flash
flooding. For these reasons a Flood Watch remains in effect for
most of the state. The flood threat for the Big Island remains at
a lower threat level, therefore the Big Island is not in the
watch at this time.

Kauai County and Oahu weather impacts...The low level trough
moving into Kauai County this morning will combine forces with the
unstable tropical air mass over the next few days. Increasing low
level surface convergence with upper lift from a 500 mb trough
passing over the islands will help to lift the unstable moisture
over the mountain slopes of Kauai and Oahu, increasing the threat
for flooding due to moderate to heavy showers and thunderstorms
potentially training over the island. The medium range forecast
guidance show drier and more stable east to southeasterly winds as
the ridge builds in from the east and the kona low drifts
northward from Friday night into Saturday morning. Any delay in
the ridge building back in will slow down these improving weather
trends this weekend.

Maui County weather impacts...Deep tropical moisture in southerly
kona winds periods of moderate to heavy showers over the islands
of Maui County over the next few days. The main threats for Maui
County will be if several storms line up over any one location
potentially producing 1 to 2 inches per hour rainfall for several
hours. These small scale convective shower bands are notoriously
hard to predict well in advance. These areas affected by this
flood threat may often see fair conditions that rapidly
deteriorate into heavy rain with rapid runoff and flooding in a
very short time period.

Big Island weather impacts...Tropical moisture will affect the Big
Island into Friday. The southeastern slopes of the Big Island
would be in the highest threat level for moderate to heavy rain
during this event. Most of the deep convective showers should
remain west of the Big Island this week with just a slight risk of
flooding over Hawaii County.

Saturday through Monday...We gradually transition back to a drier
weather pattern as high pressure builds in from the east
producing a drier and more stable southeasterly wind flow pattern
across the Hawaii region as the kona low drift farther northward
away from the state. Easterly trade winds should return by Monday.

&&

.AVIATION...
A strong kona low about 650 nm northwest of Kauai is pulling
tropical moisture out of the south southeast across the state.
This will keep the Hawaiian Islands in an active weather pattern
through the remainder of the week. Moderate to heavy showers and
possible thunderstorms are expected over the central islands
today and tomorrow. On the Big Island, enhanced shower activity
is expected in the afternoon and early evening hours.

Latest radar imagery reveals a broad band of intensifying showers
and thunderstorms streaming across Oahu and the Kaiwi Channel.
This band is expected to shift eastward today, spreading across
the whole of Maui county by late this evening.

AIRMET Sierra is in effect for Kauai and Oahu for mountain
obscuration. Conditions will likely spread across Molokai and
Lanai by midmorning. Occasional IFR conditions could develop over
Kauai after sunrise.

AIRMET Tango is in effect for mechanical turbulence below 120 kft
N thru E of the Big Island. Conditions could spread to East Maui
later today as the gradient aloft tightens.

&&

.MARINE...
Fresh to locally 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
direction is expected over the weekend through early next week as
it lifts northward and away from the area. A Small Craft Advisory
has been issued for the Windward and Leeward Big Island waters
(Hamakua Coast and waters near South Point). In addition to the
winds, expect locally rough seas in heavy showers or storms that
develop over the next few days, with the best chances being over
the waters west of Maui County. Seas should remain just below the
advisory level over the 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 overlapping, long-period south-southwest swells
expected. The first will rise through the day today, then peak
tonight through Friday, likely reaching the advisory level.
Offshore buoys 51003 and 51004 (51002 remains down) reflect this
with the peak energy coming in within the 19 and 21+ second
bands. As a result, a High Surf Advisory has been issued through
Friday. Expect rough conditions for most southerly 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.

Surf along north and west facing shores has trended up overnight
on Kauai, with observations coming in larger than expected. Given
the energy being spread over the short-period bands, heights
should remain below the advisory thresholds for north and west
facing shores. Expect a similar trend over exposed waters down
the island chain today as it continues to build. This 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...Bohlin
AVIATION...Bedal
MARINE...Gibbs