FXHW60 PHFO 091919
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
919 AM HST Thu May 9 2024

.SYNOPSIS...
Trade winds will diminish through tonight as a broad surface
trough and upper disturbance move in. Added instability
associated with these features will lead to an increasing threat
for strong to severe thunderstorms and heavy rainfall late tonight
through Saturday. Although the thunderstorm potential will lower
Sunday into early next week, plenty of moisture lingering combined
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 trough developing just west of Kauai will decrease the trade
winds today with light to moderate east to southeast winds
expected. A plume of moisture moved in from the east last night
and is currently producing scattered showers mainly over windward
and mountain areas. Sea breezes developing later this morning will
lead to afternoon showers over interior and leeward areas. With
some enhanced moisture already in place, we could see some
downpours this afternoon. Also with the light winds, any showers
will be slow moving and could cause some minor flooding impacts
this afternoon through the evening hours.

An unusually strong upper level disturbance (for this time of
year) will begin to move into the area tonight and will bring the
threat of flash flooding 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.

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 during the afternoon.
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 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. Also sudden changes of the weather will be possible. It
could be sunny and fair weather one hour and you could be under a
severe thunderstorm the next hour. Although not widespread, due to
the potential for intense rainfall rates, a Flood Watch should be
issued later today for the State of Hawaii.

&&

.AVIATION...
Light to moderate trades will continue today and gradually weaken
tomorrow. Daytime sea breezes and nighttime land breezes are
expected over wind-protected leeward areas.

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. As
sea breeze activity resumes by this afternoon, SHRA and low cigs
will begin to develop across some leeward/interior locations. In
addition, a disturbance aloft 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.

AIRMET Sierra is in effect for mtn obsc across windward areas as
a band of moisture moves in from the east.

&&

.MARINE...
A strong disturbance aloft approaching from the north will bring
a threat for strong, possibly severe, thunderstorms late 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 tonight then linger through
the weekend. As a result, light east to southeast winds will
replace the usual trade wind flow at the surface on Friday and
Saturday. Moderate easterly trade winds will gradually rebuild
starting Sunday.

A long-lived south swell will affect the islands over the next
several days. Surf along south facing shores is expected to hold
just below the High Surf Advisory threshold of 10 feet today
through Friday, 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 represents the most sustained south swell of the year.

Otherwise, north shore surf will remain tiny today and tonight,
followed by a small reinforcing short-period north-northwest swell
late Friday 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...
None.

&&

$$

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