628
FXHW60 PHFO 100203
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
403 PM HST Thu May 9 2024

.SYNOPSIS...
A strong upper level disturbance will bring unstable weather
conditions through Saturday. Added instability will lead to an
increasing threat for strong to severe thunderstorms and heavy
rainfall tonight through Saturday. Although the thunderstorm
potential will lower early next week, plenty of moisture
lingering combined with afternoon sea breezes and a broad surface
trough nearby will support afternoon showers over interior and
leeward areas each day. The wet pattern could continue through the
latter half of next week as another upper disturbance approaches.

&&

.DISCUSSION...
A Flood Watch is now in effect through early Saturday morning for
the entire state of Hawaii. An unusually strong upper level
disturbance (for this time of year) is starting to approach the
state and will bring the threat of intense rainfall rates and
isolated severe thunderstorms across the state. Initially the main
threat will start over the smaller islands of Kauai, Oahu and
Maui County and expand eastward to the Big Island during the day
Friday.

For tonight, the upper level trough will dig southward towards
Kauai and will enhance instability over the smaller islands. With
the low level winds primarily out of the southeast direction
tonight, some of the favored areas for rainfall will across
windward and mountain areas, especially along the Koolau
mountains on Oahu. Any showers will be slow moving and could
become anchored over the terrain, which could bring the threat of
flash flooding. Over on the Big Island, any showers this evening
should decrease tonight with mainly scattered showers expected.

For Friday, the upper level trough centered near Kauai in the
morning will begin to move eastward and will bring an unstable
environment across the state by Friday afternoon. Latest guidance
is showing 500 mb temps ranging from -12C to -15C across the state
with lapse rates greater than 7C/km in the low to mid levels of
the atmosphere. CAPE values greater than 1000 J/kg with bulk shear
values of around 40 knots should be enough to support isolated
severe thunderstorms capable of producing large hail and gusty
winds of greater than 50 mph. At this time, the best combination
of moisture and instability and shear looks to be from Oahu
towards the Big Island. Guidance has been fairly consistent with a
line of showers and thunderstorms developing south of the islands
Friday morning and spreading northeastward towards Oahu, Maui
County and the Big Island during the late morning through the
afternoon and evening. If this line of showers and thunderstorms
were to develop, this very well could lead to the main impactful
period on Friday.

The main takeaway for Friday is that we are not looking at a
widespread rain event. Rather, we are looking at a scattered
heavy shower event with isolated strong to possibly severe
thunderstorms. We do want to emphasize that flash flooding can
occur well downstream from the mountains, where the weather could
be fair. Due to the scattered nature of the event, sudden changes
of the weather should be expected. It could be sunny and fair
weather one hour and you could be under a severe thunderstorm the
next hour. So please keep an eye on the sky.

Over on the summits of the Big Island, the strong upper level
disturbance will bring very strong winds and periods of heavy
snow. The combination of very strong winds and heavy snow could
produce blizzard-like conditions Friday afternoon through the
evening. Due to the hazardous conditions, a Winter Storm Warning
and a High Wind Warning has been issued for the Big Island
summits.

Over the weekend, although conditions should somewhat stabilize,
the upper level low will be slow to move away from the state.
Light to moderate east to southeast flow will likely lead to
afternoon sea breezes, which will allow for showers to develop
over interior and mountain areas each afternoon. With the
instability still lingering somewhat, conditions will likely be
more wet than normal especially during the afternoons.

&&

.AVIATION...
Light to moderate trades will continue today and gradually weaken
through tomorrow. SHRA will continue to favor windward and mauka
areas with the greatest coverage and intensity during the late
night and early morning time period. MVFR conds are expected with
these SHRA. With sea breeze activity by this afternoon, SHRA and
low cigs will begin to develop across some leeward/interior
locations. In addition, a disturbance aloft and a surface front
will approach the area later today, helping to enhance shower
activity across the state and bringing instability that may help
to spark a few TS near Kauai sometime tonight into tomorrow
morning and continue eastward through the island chain.

AIRMET Sierra is in effect for mtn obsc across the whole chain.

AIRMET Tango may be needed for turb later this evening.

AMD NOT SKED for MDY will continue for the next several days due
to equipment being down.

&&

.MARINE...
A strong disturbance aloft approaching from the north will bring
the threat of strong, possibly severe, thunderstorms tonight
through Saturday. These thunderstorms will have the potential to
produce dangerous wind gusts, hail, and frequent lightning. At the
surface, a trough to the northwest of Kauai will strengthen and
expand southward over the island chain by morning then linger
through the weekend. As a result, light and variable winds will
replace the usual trade wind flow at the surface on Friday and
Saturday. Light to moderate easterly trade winds will return
starting Sunday.

A long-lived south swell will affect the islands over the next
several days. Surf along south facing shores is expected to
remain at the High Surf Advisory threshold of 10 feet today
through tomorrow, then gradually decline Saturday through Tuesday.
A series of overlapping long- period south swells will give
another sizable boost to south shore surf Wednesday through late
next week. This event represents the most sustained south swell of
the year.

Otherwise, north shore surf will remain tiny through tomorrow
morning. A small reinforcing short-period north- northwest swell
will build in tomorrow afternoon and continue through the weekend.
A small long-period northwest swell will build Sunday night,
giving north shore surf a boost early next week. Surf along east
facing shores will be well below normal levels through the middle
of next week.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late Friday night 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.

High Surf Advisory until 6 PM HST Friday for Niihau-Kauai
Leeward-Waianae Coast-Kahoolawe-Maui Leeward West-Kona-Kauai
South-East Honolulu-Honolulu Metro-Ewa Plain-Molokai Leeward
South-Lanai Leeward-Lanai South-Maui Central Valley South-South
Maui/Upcountry-South Haleakala-Big Island South-Big Island
Southeast.

High Wind Warning from 6 AM Friday to 6 AM HST Saturday for Big
Island Summits.

Winter Storm Warning from noon Friday to 6 AM HST Saturday for
Big Island Summits.


&&

$$


DISCUSSION...Kino
AVIATION...Walsh
MARINE...Bedal/Wroe