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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 300655 CCA

Area Forecast Discussion
National Weather Service Honolulu HI
850 PM HST Fri Sep 29 2023

Moderate to locally breezy trade winds will deliver passing clouds
and showers through the weekend, mainly to windward and mauka areas.
Weak instability associated with a low aloft may bring an isolated
thunderstorm through Saturday. Light winds and mostly dry weather
are expected next week, with limited showers focused over interior
areas during the afternoon.


N Pacific surface analysis continues to feature a slow-moving trade-
wind-supporting 1030 mb high centered about 1200 miles NNE of the
islands, and a pair of strong extratropical lows to the distant N
and NW of the area. Afternoon sounding data indicates that a low
aloft several hundred miles NW of Kauai has destabilized the island
atmosphere, with a weak/absent subsidence inversion noted in the
Lihue and Hilo soundings respectively. Although prevailing moisture
depth is confined to the lowest 10-12 kft of the atmosphere, an area
of deeper moisture over windward waters about 30 miles E of Oahu did
manage to produce a few brief lightning strikes earlier this
evening. Infrared satellite imagery briefly indicated isolated cloud
tops near 40 kft in this area, while also depicting statewide
scattered to broken high clouds that added some color to this
evening's sunset. Radar shows a cluster of heavier showers
associated with the earlier lightning strikes over Oahu's windward
waters, with another cluster of heavier showers/towering cumulus
moving over windward Big Island. Elsewhere, isolated showers are

Little overall change to the current weather regime is anticipated
over the next couple of days, with bigger changes expected next week
as the trade wind flow breaks down. Until then, low-level moisture
will arrive on a moderate to locally breezy trade wind flow that
will be gustiest in the afternoons. An increase in PWAT tonight and
Saturday will ensure that windward areas receive some showers, while
also bringing a slight chance of thunderstorms (CB). The ongoing
forecast does indicate the potential for isolated CB over the Big
Island slopes Saturday afternoon, and over waters around Kauai and
Oahu, but some fine tuning of the forecast will likely be required
as the approaching moisture slug moves W over the chain through

A trend toward decreasing PWAT is expected on Sunday, although mid-
level instability will likely linger until Monday. From Monday
onward, trade wind speeds will diminish as the low to the distant NW
moves E and weakens the surface ridge N of the islands. The
associated high will move E, and as a weakening front associated
with the low draws closer to the islands, the surface ridge will
move over the islands for most of next week.

With a mid-level ridge building over the area from the W, a mostly
dry, light wind pattern is anticipated. Some interior and upslope
clouds may develop in response to daytime heating, but little in the
way of rainfall is expected, with mostly clear nights and mornings.
Depending on the evolution of the parent low N of the islands, the
weakening front may move over Kauai over the second half of next
week, with deterministic ECMWF guidance continuing this theme over
the past 24 hours. Thereafter, significant differences are noted in
the reliable guidance, with GFS favoring a weaker low and continued
dry with light winds, while ECMWF favors a stronger low driving weak
fronts across the area, with the potential for moderate NW to N
winds. Either way, the recent persistent trade wind flow will be


Surface high pressure well northeast of the islands will allow
moderate to occasionally breezy trades to persist through the
remainder of tonight. Thus, expect scattered showers to mainly
affect windward locations with isolated activity elsewhere. There
will be brief periods of MVFR ceilings and visibility associated
with any heavier showers.

AIRMET Tango remains in effect for tempo moderate turbulence below
9000 feet for areas downwind (south through west) of island terrain.
This AIRMET will likely remain in effect through most of the weekend
before conditions improve early next week.


Moderate to locally strong easterly trade winds will persist through
the weekend. The ongoing Small Craft Advisory for the windier waters
surrounding Maui County and Big Island will likely need an extension
in time through Sunday. The impetus to these recently robust trades,
surface high pressure centered near 40N150W creating a tight
pressure gradient down across the regional waters, will travel east
as a cold front moves into the central North Pacific early next
week. This will kill the upstream pressure gradient and considerably
weaken winds to just light and variable by Tuesday. The near and
offshore waters will experience these light to gentle breezes
through next weekend.

A pair of northern Pacific systems moving south of the Aleutian
Islands and into the Gulf of Alaska the next several days will
produce a couple of north to northwest swells that will reach the
islands Sunday and fill in Monday and then again Tuesday into
Wednesday. The will result in heightened north and west-facing shore
surf through next week. The first medium period near 4 foot
northwest (330 degree) swell is timed to fill in during the day on
Sunday. This swell should peak Sunday night and Monday surf to or
above head high and then slowly fall through Tuesday. A slightly
larger, medium period more northwest-turning-north swell (310
becoming 350 degree) swell is expected to reach the islands and fill
in from late Tuesday through Wednesday. This swell will reinforce
head to slightly higher surf heights through the middle of the week.

East-facing short period chop will significantly fall next week as
east trades transform to more light to gentle breezes. South-facing
shore surf will remain around waist high through early next week.
There may be a slight bump in southeast swell (mixed into the
background south swell) this weekend that is originating from a
small fetch region 1,000 miles southeast of the island chain. This
may lift surf from chest to near head high along more southeastern
exposures from Sunday into mid week.


Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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