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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 301938
Area Forecast Discussion
National Weather Service Honolulu HI
938 AM HST Wed Sep 30 2020
Light and variable winds will continue through the rest of the
work week, allowing for development of local land and sea breezes.
A trough moving over the area could introduce a slight chance of
thunderstorms near or over the islands Thursday through Saturday.
Trade winds may return this weekend.
A weakened ridge just north of the islands will keep winds light
across local waters through the rest of the work week. Trade
winds may return this weekend as the ridge rebounds northward and
the local pressure gradient steepens. Until then, expect
development of land and sea breezes, especially across leeward
areas. There may be enough instability to trigger a brief
thunderstorm or two across portions of the Big Island Thursday and
Friday during afternoons and evenings.
A surface trough to the east of the state is forecast to pull an
area of significantly increased moisture across the islands
Thursday night through Saturday night. This increased moisture
keeps a slight chance for thunderstorms in the forecast through
Saturday night, with the possible areas of development drifting
westward through this period with the surface trough. Drier and
more stable conditions are expected Sunday and afterwards as the
trough passes west of the islands.
Expect trade winds to return to at least the eastern portion of
the state beginning Saturday, then spread westward and last
through the rest of the forecast period. Models disagree a bit on
wind strength, but a trade wind weather pattern should return as
well, with clouds and showers favoring windward and mauka areas.
A surface ridge will remain near Kauai through tomorrow as a
surface trough moves over the Big Island of Hawaii from the east.
VFR conditions will prevail through tomorrow morning. Variable
winds will prevail with gentle onshore sea breezes during the day
and light offshore land breezes at night. The sea breezes will
produce clouds and spotty showers over interior areas this
afternoon with the best chance for showers over the slopes of the
Big Island. Land breezes will bring clearing skies over most areas
tonight, but the surface trough will likely bring showers to
waters around the Big Island. The atmosphere will become more
unstable tomorrow as a trough aloft digs down from the north. The
instability from the trough aloft and moisture from the surface
trough will likely produce more active afternoon showers over
interior areas tomorrow. Thunderstorms will be possible over the
Big Island tomorrow afternoon. MVFR ceilings and visibilities are
possible and could produce extensive enough mountain obscuration
to require AIRMET Sierra.
Light background trades will persist across the marine area
through Thursday, with locations near the coast seeing localized
land and sea breezes. High pressure will gradually build north of
the islands Friday through the weekend, allowing moderate trade
winds to build from east to west across the island chain Friday
and Saturday. The trades will become moderate to fresh in strength
by Sunday, and hold at these levels through early next week.
Locally strong trades will be possible in some of the windier
waters around Maui and the Big Island on Sunday.
Surf along south facing shores will trend lower through the rest
of the week. A late season south swell is expected early next
week, likely boosting surf above the summertime average Monday
through the middle of next week.
Current north-northwest swell is expected to continue to rise and
peak tonight. Another larger swell is anticipated to arrive
Thursday night and Friday, which could bring advisory level surf
along north facing shores. Surf will then gradually lower Friday
night through early next week.
Surf along east facing shores will be driven primarily from wrap
associated with the series of north-northwest swells moving
through the islands today through the weekend. East shore surf may
increase substantially Sunday night through early next week, as a
long-period east swell associated with east Pacific tropical
cyclone Maria moves through. This could result in surf reaching
advisory levels along most east facing shores Monday through the
middle of next week.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office