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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 292008

Area Forecast Discussion
National Weather Service Honolulu HI
1008 AM HST Wed May 29 2024

Surface high pressure far northeast of the state will maintain
breezy trade winds for the remainder of the week. Shower activity
will primarily be focused along windward slopes and coastlines,
becoming more prevalent during the nocturnal hours. Upper level
troughing will move in from the northeast and be over the islands
from Friday into early next week. This will likely introduce
frequent periods of increased trade wind showers in both coverage
and intensity.


Surface troughing approximately 700 miles west-northwest of Kauai,
along with a near 1030 mb surface high centered around 1,350 miles
northeast of Oahu, are both supporting a breezy trade wind regime
across the state. Winds will hold at Small Craft Advisory
thresholds through the eastern island bays and channels the next
several days. The regional atmosphere is fairly moist with morning
sounding pwats of around 1.5 inches pushing the upper percentile
climatological range for late May. This relatively higher moisture
within this breezy trade flow is producing mainly scattered to
broken mid level clouds, with occasional overcast windward mauka
conditions. Radar is spotty with the highest rainfall amounts of
just a few hundredths of an inch per hour confined to the higher
elevations. Better organized showers may drift over into the
smaller island leeward communities. The main short term forecast
theme will center around rain chances and trade wind trends.

High pressure will slightly strengthen northeast of the state and
settle more south-southeast as the trough of low pressure west of
the area lifts slowly northward the next couple of days. This
will provide a minor pick up of wind speeds as winds remain
breezy to locally windy into early June. The trough will wash out
through the weekend as high pressure northeast of the state inches
a bit southwestward and closer to the island chain early to mid
next week. This may provide a slight boost to trades the first few
days of June. A relatively drier air mass moves across the state
the next few days. This scenario will have light showers favoring
more windward slopes and coasts, particularly during the overnight
hours. Heights will lower over the state from Friday into the
weekend although extended guidance is a little out of agreement on
the extent of this troughing. Regardless of these minor model
differences, there is moderate confidence that next week's upper
troughing will increase areal shower coverage and intensity.


Moderate to breezy easterly trades will continue for the next
several days. Clouds and showers will favor wind ward slopes,
where periods of MVFR CIGs/VSBYs will move through especially in
the overnight and early morning hours. VFR conditions will prevail
over most leeward areas, with the exception of afternoon and
evening MVFR conditions over leeward Big Island.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of most islands this morning. Conditions are
improving and the AIRMET will likely be dropped by noon.


A trough northwest of the area and a high centered far northeast
of the area will maintain moderate to locally strong easterly
trade winds through today. As the trough weakens and drifts
further away, the trade winds will slightly strengthen Thursday
and back towards the east-northeast. Fresh to strong east-
northeast trade winds should persist through the weekend as a
ridge of high pressure remains anchored far north of the state. A
Small Craft Advisory (SCA) remains in effect for majority of the
waters around Maui County and the Big Island through Thursday and
additional zones around Oahu and Kauai waters may need to be
included for Thursday through the weekend.

Surf along exposed south-facing shores will remain below the
summer average (5 feet) through Friday. A series of south swells
will move in over the weekend into next week and will likely bring
a prolonged period of above average surf along south facing
shores. Forerunners from the first swell could arrive as early as
Friday, but should steadily fill in throughout the day Saturday
and peak late Saturday into Sunday. A reinforcing pulse should
arrive on Monday and will likely maintain surf heights near or
just below the High Surf Advisory threshold of 10 feet. Surf
heights should gradually trend down Tuesday through Wednesday as
the south-southwest (200 degree) swell declines. A gale low
currently developing just east of New Zealand, will likely bring
another south (190 degree) swell Thursday into Friday of next week.

Surf along exposed east-facing shores will continue to remain
rough and choppy due to the windswell generated by the breezy
trade winds. As the trades slightly strengthen Thursday into the
weekend, we should see a slight increase of rough and choppy surf
by this weekend.

Surf along north-facing shores will continue to remain nearly flat
over the next few days. A developing storm force low near
Kamchatka should produce an out of season northwest swell, which
should steadily fill in Sunday night into Monday. Latest guidance
is showing 3 to 4 feet of swell from 320 degrees Monday into
Tuesday. A typhoon tracking off the coast of Japan will likely
bring a small reinforcing swell from the west-northwest to
northwest Tuesday into the middle of next week.


Small Craft Advisory until 6 PM HST Thursday for Kaiwi Channel-
Maui County Windward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.




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