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000
FXHW60 PHFO 091332
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
332 AM HST Tue Apr 9 2024

.SYNOPSIS...

Trade winds will ease today and turn toward the southeast late
tonight and Wednesday, fewer showers are expected of the smaller
islands. Developing low pressure northwest of the islands will
keep winds light and out of the southeast after midweek. The low
could also bring the potential for increased showers, some heavy,
mainly to Kauai toward the end of the week into the weekend.

&&

.DISCUSSION...

Latest satellite and radar imagery show scattered light to
moderate showers primary focused along windward and mauka regions
with some of these showers spilling over to leeward areas at
times. An area of enhanced clouds and moisture embedded in the
trade wind flow is moving on shores most windward and mauka areas
and may reach windward Kauai later today. The high far northeast
of the state will maintain breezy trades through today.

The high to the distant northeast will weaken Wednesday as a
potent cutoff low develops over distant portions of the
Papahanaumokuakea Marine National Monument. The surface reflection
of this slow- moving, deep-layer low will have high end gale
force winds peaking west of Laysan Thursday. Over the main
Hawaiian Islands, trades will veer to the southeast late Tuesday
and Wednesday. This flow regime typically places the smaller
islands in the wind "shadow" to the lee of the Big Island,
resulting in light winds and limited rainfall, especially as a
weak ridge builds over the islands the next couple of days.
Guidance is also showing strengthening southwesterly winds over
the Big Island summits as the sub- tropical jet moves overhead
which could produce advisory level winds as early as Tuesday night
and peak near high end advisory level Wednesday night into
Thursday.

Limited showers will favor windward areas during nights and
mornings, and leeward areas may see isolated afternoon showers as
localized sea breezes develop, with typical trade wind weather
over the Big Island. Having said that, the forecast vertical wind
profile has produced terrain- anchored heavy rainfall over Oahu`s
Koolau range in the past (sometimes referred to as "WOOSEE"
rainfall here at the office - "Windward Oahu Orographic SouthEast
Effect"), although anticipating its development with much lead
time can be problematic. Greatest potential at this time appears
to be Wednesday into Thursday. Deep moisture pooling near the Big
Island and near freezing temperatures could also produce mixed
snow and freezing rain rear the summits Thursday into Friday.

The surface low will gradually weaken while loitering to the
distant northwest through Friday, then move slowly northeast into
next week, with an associated convergence band lurking close to
Kauai from Wednesday night through at least Sunday. This slow-
moving convergence band will bring the potential for heavy showers
and thunderstorms that may linger into next week, with the flow
becoming southerly near Kauai. Initially, Kauai appears to be the
most likely to get heavy rainfall, but the potential for localized
heavy rain may spread eastward to other areas of the state in
about a week as the band drifts east and winds remain light. Given
the lead time, confidence in the details of the forecast are low.


&&

.AVIATION...
Strong high pressure north of the state will slowly weaken as it
shifts eastward, causing trade winds to decrease. AIRMET Tango
remains in effect for moderate low level turbulence over and
downwind of terrain of all islands. Conditions are expected to
gradually improve from the west today.

MVFR ceilings may occur across windward slopes within showers
brought in by the trades. However, VFR conditions will prevail at
the TAF sites. AIRMET Sierra for mountain obscuration above 2500
feet remains in effect for north and east sections of the Big
Island. This may be cancelled later today.

High clouds are moving eastward away from the islands this
morning. This greatly reduces the likelihood for light rime icing
and the need for any issuance of AIRMET Zulu within the forecast
area.


&&

.MARINE...
A weakening cold frontal trough approaching Hawaii from the
northwest direction will weaken wind speeds over the next few
days. A cutoff surface low deepens along the trough northwest of
Kauai by Wednesday as a strong high pressure system farther to the
north, builds eastward across the Central Pacific basin. This
cutoff low will cause trade winds to veer from a more southeasterly
direction from Wednesday to Saturday. Wind speeds will likely
remain near low end Small Craft Advisory (SCA) levels for the
windward waters and channels surrounding Maui and the Big Island
through much of this week. The current SCA continues through this
afternoon.

Surf along east facing shores will continue to slowly trend lower
over the next two days as wind waves ease. Surf along north
facing shores will trend higher today as a small, medium- to long-
period north-northwest swell fills in across the islands. This
swell will peak today and hold through Wednesday. Guidance for the
second half of the week shows a low developing north of the state
that could aim a moderate medium period north-northwest swell
towards the islands. This swell would arrive late Friday or
Saturday, but confidence remains low at this time.

For south facing shores, the current small long period southwest
swell will decline today. The next long period south swell will
arrive in the islands late Thursday, peak on Friday, and then
slowly decline through the weekend.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.


&&

$$


DISCUSSION...Almanza
AVIATION...Powell
MARINE...Bohlin