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Hawaiian Islands Synoptic Discussion and Guidance

299
FXHW60 PHFO 190639
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Sun Apr 18 2021

.SYNOPSIS...
Mostly dry weather will continue tonight as the trades ease. A
land and sea breeze pattern will become established Monday through
Wednesday with increasing showers over Oahu and Kauai on Tuesday.
There is also a slight chance of thunderstorms over Kauai late
Monday night through Tuesday. Moderate to breezy trades and more
typical trade wind weather will return Thursday and continue
through next weekend.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high is centered around 1600
miles northeast of Honolulu, while a cold front is located around
850 miles northwest of Kauai. Light to moderate trades are hanging
on across the islands this evening, strongest over the eastern end
of the state. Infrared satellite imagery shows mostly clear to
partly cloudy conditions in most areas, with a bit higher cloud
coverage over windward Big Island. Radar imagery shows scattered
showers affecting windward sections of Maui and the Big Island,
with nothing more than isolated showers affecting windward
sections of the other islands . Main short term concern revolves
around the potential for some unsettled weather Tuesday into
Wednesday.

A cold front approaching from the northwest will gradually ease
the trades and shift winds around to the east-southeast tonight
and Monday. The east-southeasterly flow will hold in place
through mid week, with terrain blockage bringing a land and sea
breeze set up to the smaller islands Monday through Wednesday. A
new surface ridge is forecast to develop north of the islands
Wednesday night, bringing a return of moderate trades Thursday
which are forecast to strengthen to breezy levels Friday through
next weekend.

As for the remaining weather details, a few windward showers can
be expected tonight as the low level boundary layer flow tips
east-southeasterly. A land breeze and sea breeze pattern will set
up Monday through Wednesday, with a few showers possible in
coastal areas favored in an east-southeasterly flow at night, and
over interior areas during the afternoon and early evening hours.
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 through Tuesday evening. A more typical but drier
than normal trade wind pattern appears to return Wednesday night
and linger through next weekend.

&&

.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...
Moderate to fresh easterly trade winds will shift out of the
southeast and diminish through the first half of the week as an
upper trough and front move into the region. This pattern shift
will translate to a land and sea breeze regime developing along
the coasts for most areas beginning later tonight into early
Monday morning. 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 Monday night through Tuesday night over the Kauai
and Oahu waters. A return of moderate to fresh easterly trades
will be possible for all waters late Wednesday through the second
half of the week.

Surf along north facing shores will hold steady through Monday
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.

Surf along south facing shores will remain up into Monday as the
Tasman swell that arrived Saturday moves through. A gradual
downward trend is anticipated Tuesday through midweek. A similar
south-southwest swell will be possible late 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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