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Hawaiian Islands Synoptic Discussion and Guidance

393
FXHW60 PHFO 251410
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
410 AM HST Mon Mar 25 2019

.SYNOPSIS...
A high pressure center far to the north of the Hawaiian Islands
will keep moderate to breezy trade winds in the forecast today
with increasing shower trends. The high pressure ridge north of
the islands begins to weaken tomorrow with trade winds decreasing
in strength and becoming light and variable on Wednesday ahead of
an approaching cold front. Clouds and showers along the frontal
band will reach the northwestern islands from late Wednesday night
into Thursday morning, showers will spread southeastward to all
islands from Thursday afternoon to Friday morning. Cool northerly
winds will develop after frontal passage.

&&

.DISCUSSION...
The morning satellite imagery shows the upper level cirrus clouds
moving out as expected. An extensive band of unstable low clouds
just northeast of the island chain will move in with the trade
winds increasing shower trends today over the Big Island and Maui
County this morning. Clouds and unstable showers will then
spread westward to Oahu and Kauai this afternoon.

In the big picture, a weakening 1028 mb high pressure center
roughly 1000 miles north of the Hawaiian Islands will continue to
produce moderate to breezy trade winds across the state today.
This high pressure system will continue to weaken and drift
southeastward through Tuesday as another cold front approaches the
islands from the northwest. Meanwhile an approaching cold frontal
trough will weaken the ridge north of the state decreasing the
strength of the trade winds on Tuesday. By Wednesday, model
solutions show the cold front roughly 400 miles northwest of
Lihue, wind speeds will become light and variable across the
state, with onshore sea breezes each day, and offshore land
breezes in the overnight hours. Models are also showing an upper
level trough supporting a band of low level convergence ahead of
the front producing elevated clouds and showers over Kauai on
Wednesday night. Expect enhanced showers spreading southeastward
to Oahu by early Thursday morning with additional showers drifting
over Maui and Hawaii Counties Thursday night into Friday morning.

The precipitation chances for Thursday were adjusted a bit this
morning to slow down arrival timing of the frontal cloud band by
about six hours. Our confidence in the arrival timing on these
frontal showers remains fair at this time due convergence bands
developing ahead of the front, the relative weakness of the
frontal boundary, and the four day forecast time period. Cool
northerly winds will move in behind the front with temperature
dropping statewide by a few degrees on Thursday and Friday.

The medium range solutions for both the American (GFS) and the
European (ECMWF) models show the forward motion of the front
stalling out over the central islands. We continue to forecast
light to moderate trade winds as another weak high pressure system
moves through the Central Pacific. Unsettled shower activity
will linger over windward and mountain slopes of Maui and Hawaii
Counties from Friday into the upcoming weekend. In the extended
forecast another cold front will move into the region with light
and variable winds forecast across the state through the first
half of next week.

&&

.AVIATION...
High pressure centered far north of the Hawaiian Islands will
drive moderate to fresh trade winds through Monday night. Bands of
stratocumulus moving along the trade wind flow will bring periods
of showers to the islands. Brief MVFR cloud ceilings are to be
expected, especially over the windward mountain slopes of each
island. Mountain obscuration could become an issue over Kauai
later this afternoon. Otherwise, VFR conditions will prevail.

&&

.MARINE...
High pressure north of the state will keep fresh to strong trades
across the coastal waters through tonight. The trades will ease
Tuesday and Tuesday night as a front approaches from the
northwest, with winds becoming light and variable by Wednesday.
The front is expected to move into the islands late Wednesday
night and Thursday, then slowly dissipate across the central or
eastern islands through the end of the week. Moderate to fresh
northerly winds are expected to fill in behind the front, with the
winds returning to a more typical trade wind direction at light
to moderate speeds over the weekend as weak high pressure builds
north of the state.

A Small Craft Advisory (SCA) remains in effect for the typically
windy waters around Maui and the Big Island through 6 AM HST
Tuesday. In addition, the Maui County windward waters, Big Island
Windward waters and Kaiwi Channel were added to the SCA through 6
PM this evening due to winds, seas or a combination of the two.

The current north-northwest swell will gradually decline today
and tonight. A small reinforcement out of the north is expected
tonight through Wednesday, followed by a small northwest swell
Tuesday night and Wednesday. Surf will remain below advisory
levels during this period. A new moderate sized northwest swell
will build Wednesday and Wednesday night, and could approach low
end advisory levels Thursday through Friday along north and west
facing shores. Surf will remain elevated Friday night through the
weekend as new overlapping moderate sized north-northwest and
northwest swells move through the island chain.

Surf along east facing shores will peak today due to a slight
uptick in trade wind speeds. A downward trend is expected tonight
through Wednesday with small surf expected through the rest of
the week.

Surf along south facing shores will remain small but several long
period south swells will produce background surf throughout the
week. A slight bump is expected Wednesday night through Friday,
followed by more background south swell over the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 PM HST this evening for Kaiwi
Channel-Maui County Windward Waters-Big Island Windward Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Jelsema

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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