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Hawaiian Islands Synoptic Discussion and Guidance

874
FXHW60 PHFO 191339
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 AM HST Mon Apr 19 2021

.SYNOPSIS...
An approaching front will disrupt the trade winds across much of
the state today through Wednesday, resulting in a land and sea
breeze pattern across the island chain. This light wind regime
will focus showers over interior locations during the day and
locations near the coast at night. Shower activity will remain
limited today, with an increase in showers expected tonight and
Tuesday over Oahu and Kauai as an upper level disturbance moves
through. A thunderstorm or two is also not out of the question
over the Garden Isle. Moderate to breezy trades and more typical
trade wind weather will return Thursday and continue through next
weekend, with showers favoring windward and mauka locales.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high is centered around 1650
miles northeast of Honolulu, while a cold front is located around
750 miles northwest of Kauai. Light winds have developed over the
western islands, with land breezes present in most areas, while
light to moderate east-southeasterly boundary layer flow now
established over the eastern end of the state. Infrared satellite
imagery shows mostly clear to partly cloudy conditions in most
areas. Radar imagery shows a few showers moving into east and
southeast facing slopes and coasts of most islands, although most
of the shower activity remains offshore. Main short term concern
revolves around the potential for some unsettled weather Monday
night through Wednesday.

A cold front approaching from the northwest and a building high
to the far northeast, will keep southeasterly boundary layer flow
in place through mid week. The Big Island terrain will block the
stronger winds from reaching the smaller islands, allowing for
widespread land and sea breeze development today through
Wednesday. A new surface ridge is forecast to build to the north
of the islands during the second half of the work week, bringing a
return of moderate trade winds by Thursday, with the trades then
ramping up to breezy levels Friday through the weekend.

As for the remaining weather details, the land breeze and sea
breeze pattern expected through mid week, will favor showers
developing over interior locations during the day and areas near
the coast at night. Models continue to advertise an increase in
deep layer moisture lifting up from the south and over the western
islands late Monday night through Wednesday. This will likely
increase shower coverage over Kauai and Oahu during this time,
with some locally heavy rainfall and even a thunderstorm or two
possible over Kauai late Monday night and Tuesday as an upper
level disturbance moves overhead. A more typical trade wind
pattern will return Wednesday night and linger through the
weekend, with showers favoring windward and mauka areas.

&&

.AVIATION...
A cold front approaching the islands from the northwest will
weaken the ridge north of the islands, causing trade winds to
transition to a light and variable wind pattern for most areas.
These light large scale winds will allow local scale land and sea
breezes to develop over each island. Converging sea breeze winds
during the daylight hours will build clouds over island mountain
and interior sections. A few showers may develop over these areas
in the late afternoon to early evening time periods.

Mostly VFR conditions are forecast with the possibility for Tempo
BKN025 along the east to southeast slopes of the Big Island, and
over island interior sections in the afternoon to early evening.

No AIRMETs in effect and none are expected.



&&

.MARINE...
Winds have shifted out of the southeast overnight due to an
approaching front an upper trough from the northwest. This shift
will translate to a land and sea breeze regime along the coasts
for most areas through the first half of the week. The exception
will be over the windward waters of the Big Island along the
Hamakua coast and around South Point where moderate to fresh
southeast winds will continue. In addition to the winds, a few
heavy showers and even a thunderstorm or two will be possible
tonight through Tuesday night over the Kauai and Oahu waters as
the front nears Kauai. A return of moderate to fresh easterly
trades will be possible for all waters through the second half of
the week.

Surf along north facing shores will hold steady today due to a
persistent short-to-medium period mix of north-northwest to
north-northeast energy moving through. As this energy eases into
Tuesday, a medium-period, moderate northwest swell from a nearby
source should arrive and begin to build down the island chain
Tuesday night through Wednesday. This will be a long-lived event
that should hold into next weekend as it shifts out of a more
north-northwest direction.

Surf along south facing shores will remain up today, then
trend down Tuesday through midweek as the south-southwest swell
eases. A similar south-southwest swell will be possible later in
the week.

Surf along east facing shores will remain small through the
week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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