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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 231931

Area Forecast Discussion
National Weather Service Honolulu HI
931 AM HST Sat Sep 23 2017

Trade winds will gradually ease during the next few days as a
surface ridge to the north weakens. Mainly windward clouds with
light passing showers will prevail through Tuesday, with a
possible boost in shower activity around the middle of the work
week as the surface trough moves through the islands. Gentle to
moderate trade winds will likely persist heading


A stable trade wind flow is producing a somewhat dry pattern of
mainly windward rainfall. A 1026 mb surface high centered near
42N138W and an associated ridge extending from the high to 600
miles north of Kauai are driving the moderate trade wind flow.
Precipitable water within this flow is running near to slightly
below September normal, and a mid level ridge to maintaining
rather stable conditions with an inversion between 6,000 to 7,000
ft. A shallow upper level trough cutting across the islands is
having no impact aside from a wisps of high clouds over the
eastern half of the island chain.

This weather pattern has produced modest rainfall of around a
tenth of an inch over windward slopes in the past 24 hours, with
standouts like Waialeale collecting about a third of an inch
during that time. Leeward areas of the smaller islands have been
dry, while leeward an interior slopes of the Big Island have been
seeing afternoon clouds and a few showers. Expect little change

Only subtle changes to the ongoing weather pattern are in store
during the next few days. The ridge will weaken and retreat slowly
southward, leading to a gradual decrease in the trades. This may
lead to more extensive leeward sea breezes by Monday, but showers
should remain confined to windward slopes.

An increase in shower activity is expected mid-week. A weakening
surface trough, currently sitting about 600 miles east of Hilo,
will dissipate as it passed over the islands on Wednesday, but its
associated moisture will fuel some increase in shower activity.
Gentle to moderate trade winds will persist.


Light to moderate trade winds will persist through 24 hours and
beyond. Ragged clouds and isolated showers embedded in the trade
flow will favor windward and mauka sections of all islands. Weak
sea breezes and convective showers will redevelop across leeward
Big Island this afternoon and linger through early evening. Brief
MVFR CIG/VIS possible in passing showers. Otherwise VFR conditions
will prevail all areas.

No AIRMETs currently in effect, and none anticipated at this time.


Trade wind speeds will remain below Small Craft Advisory
threshold through the forecast period.

Surf along south facing shores will remain small through early next
week, with mainly background southeast and southwest swell energy
expected. A long-period pulse out of the southwest from the
Tasman Sea is expected by Sunday and could lead to a slight
increase in surf. For the extended forecast, strong to gale-force
southwest winds associated with a low in the Tasman Sea may lead
to another small southwest swell for Tuesday through midweek.

A small northwest swell will continue today before easing.
Another small reinforcement from that direction is expected to
fill in around Sunday night, lasting through the middle of next





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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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