772
FXHW60 PHFO 100705
AFDHFO

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

.SYNOPSIS...
The threat for strong to severe thunderstorms and heavy rainfall
will continue across the state through Friday night as a strong
upper level disturbance settles southward into the area. 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...
The evening water vapor imagery showed broad upper troughing over
the region with a strong shortwave trough digging southeastward
into the area. The surface trough is responding to the upper
height falls, strengthening northwest of Kauai this evening. High-
resolution model guidance reflects this, with some depicting a
mesoscale low forming along the trough overnight into Friday.
MUCAPES of 1000 to 2000 J/Kg combined with effective deep layer
shear values of 35 to 45 knots will support a few strong to severe
thunderstorms and locally heavy rainfall that could lead to flash
flooding through Friday night.

Low-level convergent flow into this surface trough will be the
focus for heavy showers and thunderstorms overnight across the
western end of the state, initially impacting the nearshore
coastal waters of Oahu and Kauai. In addition to this activity
over the waters, expect showers to become active over the windward
areas of Oahu overnight, where 2 to 3 inches have already fallen
in the past 24-Hrs. A transition to more of a
widespread/statewide potential is possible Friday into Saturday
as the upper low and surface reflection drift eastward across the
area. As a result, a Flood Watch is in effect for the entire
state.

Guidance shows the upper low beginning to weaken as it lifts off
to the northeast later this weekend into early next week, which
will support a diminishing likelihood of thunderstorms. The
rainfall chances, however, will likely linger as the
aforementioned broad surface trough remains over or nearby to the
west Sunday through next week. Expect the light to moderate east
to southeast low-level flow to persist, which will allow the land
and sea breeze regime to hold for most areas. This combined with
a sufficient amount of moisture will keep the rainfall chances up
each day, especially through the afternoon and early evening
periods over interior/leeward areas with the sea breezes. The wet
pattern could persist through the second half of the week as
another upper low drops southward into the area drawing up deep
tropical moisture over the state.

&&

.AVIATION...
Light to moderate trade winds will continue to weaken as a broad
surface trough and upper disturbance approach from the northwest
and slowly move across the state over the next several days. This
will bring enhanced shower activity along with the threat of a few
thunderstorms. Thunderstorm activity will be greatest over western
islands tonight then slowly shift eastward Friday. Expect periods
of MVFR conditions as showers move through, with occasional IFR
possible under heavier showers.

AIRMET Sierra remains in effect for tempo mountain obscuration
across all islands with little change expected the next couple
of days.

AIRMET Tango may be necessary for moderate mid to upper level
turbulence as the upper disturbance moves across the state.

AMD NOT SKED for PMDY will continue for the next several days due
to equipment issues.

&&

.MARINE...
An upper disturbance approaching from the north will bring the
threat of strong, possibly severe, thunderstorms tonight through
Saturday. These thunderstorms may produce dangerous wind gusts,
hail, and frequent lightning. 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 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.

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...Gibbs
AVIATION...Ahue
MARINE...Powell