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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 240628

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Sat Sep 23 2023

High pressure far northwest of the islands will keep moderate to
breezy trade winds in place through late next week, with only
minor fluctuations in strength. Fairly typical trade wind weather
will prevail, with showers favoring windward and mauka areas and a
stray shower reaching leeward communities at times.


Currently at the surface, a 1028 mb high centered around 1500
miles northwest of Honolulu continues to drive moderate to locally
breezy trade winds across the island chain this evening. Infrared
satellite imagery shows partly to mostly cloudy conditions across
windward and mauka areas as well as leeward Big Island, with
mostly clear to partly cloudy skies in leeward sections of the
smaller islands. Radar imagery shows scattered to numerous
showers moving into windward slopes and coasts, with some of these
showers making it into leeward communities as well. Main short
term focus revolves around trade wind trends and rain chances.

High pressure will remain anchored to the northwest of the islands
tonight and Sunday, while a cold front drops steadily southward
well to the north of the state. Little change in trade wind
strength is expected, with moderate to breezy trade winds
prevailing. The high northwest of the state will weaken and shift
eastward Sunday night and Monday, while the front edges southward
and closer to the island chain, eventually stalling out well north
of the islands Monday night. This should ease the trades ever so
slightly on Monday. The high will then re-strengthen north-
northeast of the state Tuesday through late next week, bringing a
return of breezy trade winds.

As for the remaining weather details, fairly typical trade wind
weather will prevail during the next 7 days, although there could
be a slight decrease in shower coverage Sunday afternoon through
Tuesday. Overall, showers will favor windward and mauka areas
through the period, particularly at night and during the early
morning hours, with a few showers reaching leeward areas at times.


A high pressure system far northwest of the islands will continue
to produce moderate to locally breezy trade winds. Satellite
imagery this evening shows increasing cloud and shower trends
just upstream of the island chain. Expect passing showers to last
through the early morning hours mainly over windward and mountain
sections of each island. Some of these stronger showers will
drift into leeward airfields. Expect decreasing shower trends
starting late Sunday morning. An AIRMET Sierra for tempo mountain
obscuration may be needed later tonight through early Sunday
morning as clouds and showers move into the windward mountain

AIRMET Tango for moderate low level turbulence remains in effect
downwind of the terrain of all islands. This AIRMET will likely
be needed through Sunday.


A surface high pressure system located far northwest of the state
will continue to produce locally strong trade winds into early
next week. As a result, a Small Craft Advisory (SCA) remains in
effect through Monday afternoon for the typically windier waters
adjacent to the islands of Maui County and the Big Island. The
latest forecast indicates the trade winds should remain locally
strong through most of next week. Therefore, the SCA will
eventually need to be extended beyond Monday for the same
typically windier waters in the vicinity of the eastern islands.
Note that SCA conditions may develop over additional waters by
the middle of next week as high pressure builds far north of the
state, and the trade winds strengthen a notch.

Forerunners associated with a new small, long-period southwest
(210 degrees) swell have arrived in the islands based on recent
observations at the nearshore buoys. This swell is expected to
continue to spread over the area through Sunday. This southwest
swell energy combined with a lingering southeast trade wind swell
will likely maintain small surf along south facing shores through
early next week.

The nearshore buoys are showing a small north-northwest (330
degrees) swell with a wave height of around 2 feet and a period
of 11 seconds, This swell will likely produce a small bump in
surf heights along exposed north and west facing shores through
Sunday. In addition, a small north (350-360 degrees) swell may
arrive around Tuesday, followed by a modest north-northeast (020
degrees) swell from Thursday into Friday.

Moderate, rough surf will likely persist along east facing shores
through early Sunday. The surf may lower slightly along east
facing shores from late Sunday into Tuesday due to a subtle
weakening of the trade winds. Moderate, rough surf may return
along east facing shores by the middle of next week due to
strengthening of the trade winds.


Small Craft Advisory until 6 PM HST Monday 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