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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 010230 AAA
Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
430 PM HST Wed Sep 30 2020
Satellite loop suggests that increased clouds and showers may
reach the Big Island late tonight versus Thursday morning, so the
onset time for both was moved up 4 to 6 hours.
Light and variable winds will continue through Friday, allowing
for development of local land and sea breezes. A trough moving
over the area will 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 Friday. 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 and adjacent waters 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
beginning late tonight through Saturday night. This increased
moisture keeps a slight chance for thunderstorms in the forecast
Thursday through Saturday night, with the possible areas of
development drifting westward through this period alongside the
surface trough. The gridded database has been altered a bit to
depict a slight chance of thunderstorms over land only during the
day, leaving the slight chance of thunderstorms depiction over
water untouched. The database and forecast have also been altered
to bring increased clouds and showers to portions of the Big
Island beginning late tonight instead of Thursday morning. 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.
Variable winds will prevail with gentle onshore sea breezes
during the day and light offshore land breezes at night. The sea
breezes have produced broken to overcast clouds and spotty showers
over the slopes all around the Big Island, over the south slopes
of Maui, and over central Kauai. After dark, land breezes will
clear the clouds out of most areas, but the surface trough will
likely bring showers to the east side of the Big Island and the
waters to the southeast of that 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
could produce extensive enough mountain obscuration to require
AIRMET Sierra on the Big Island.
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 seasonal average Monday
through the middle of next week.
Current north-northwest swell is expected to peak tonight. CDIP
Waimea buoy was deployed this afternoon and is currently reading
wave heights around 7 feet 12 seconds, which is about a foot above
model wave guidance. This swell will decline slightly along the
island chain tomorrow before another larger north- northwest swell
arrives 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 rest of 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