Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 192000

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Sat Aug 19 2017

Light to moderate trade winds will prevail through early next
week as a ridge of high pressure north of the State weakens. The
lighter winds will allow local sea breezes to be more prominent
across leeward areas, with an increase in clouds and showers each
afternoon and evening. Meanwhile, low clouds carried by the trades
will still continue to affect windward and mauka areas at times.
Tropical moisture from the east will bring an increase in shower
coverage and muggy conditions beginning tonight and continuing
through Tuesday. Expect drier and windier conditions to return
Wednesday through late next week as high pressure strengthens
north of the island chain.


Currently at the surface, a 1030 mb high is centered around 1850
miles northeast of Honolulu, with a ridge axis extending
southwest from the high to a location around 800 miles north of
Kauai. The resulting gradient is driving light to moderate trade
winds across the island chain this morning. Visible satellite
imagery shows clear to partly cloudy conditions in place across
much of the state, however a band of low clouds and showers is
bringing mostly cloudy conditions to windward sections of Maui
County and the Big Island. Radar imagery shows trade wind showers
moving into windward areas, with a few showers occasionally
spilling over into leeward areas. Shower coverage is greatest
over windward sections of Maui County and the Big Island. Main
short term concern revolves around rain chances over the next few

Rest of today,
The high pressure ridge to the north of the islands will keep
light to moderate trade winds in place across the State, with
localized sea breezes developing in the more sheltered leeward
areas. Clouds and showers will favor windward and mauka areas,
with a few showers drifting into leeward areas, primarily this
morning. Showers are expected to be most prevalent over windward
sections of Maui County and the Big Island this morning, with
daytime heating then decreasing shower coverage during the
afternoon hours.

Tonight through Tuesday night,
Light to moderate trade winds will continue through early next
week, with some slight strengthening of the trades expected late
Tuesday and Tuesday night as high pressure strengthens to the
north of the State. Aloft, an upper level low will drift
southwestward and closer to the islands tonight through Sunday
night, then pivot off to the west and further away from the state
early next week. This will act to de-stabilize the airmass over
the islands a bit, particularly over the western end of the State.
At the same time, some deeper moisture will begin to work its way
into the area from the east tonight, with precipitable water
values climbing into the 1.5 to 2.0 inch range Sunday through
Tuesday night.

As for sensible weather details, we should begin to see an
increase in trade wind showers beginning tonight, with periods of
showery weather expected Sunday through early next week,
particularly in windward areas. In leeward areas, we should also
see an increase in showers, with some localized downpours possible
each afternoon into the early evening in association with sea
breeze enhancement. Additionally, due to the more unstable airmass
over the islands, isolated thunderstorms will be possible over
leeward and interior sections of the Big Island each afternoon
into the evening. Isolated thunderstorms may also affect the
waters north of Oahu and around and including the island of Kauai
Sunday night through Monday evening as the upper level low makes
its closest approach. Conditions will also become quite humid
Sunday through early next week, as the trades lighten up and
dewpoints climb into the lower and middle 70s across the area.

Wednesday through Friday,
High pressure will strengthen north of the State, bringing
moderate to breezy trade winds back to the island chain through
the end of the work week. Drier more stable conditions are also
expected, with more comfortable trade wind weather expected.
Showers will continue to favor windward and mauka areas, with a
stray shower reaching leeward areas from time to time due to the
strength of the trades.


VFR conditions prevail at most TAF sites with occasional MVFR
conditions being reported at PHTO and PHLI. Occasional MVFR
conditions are also occurring over windward slopes for most of
the state as moisture riding in on the trades interact with island
terrain. AIRMET Sierra is currently in effect for N through E
sections of Maui and The Big Island as satellite imagery shows an
area of enhanced moisture moving into those areas. Conditions may
improve somewhat later today.


Winds and seas are expected to remain below Small Craft Advisory
levels through early next week as a trough aloft helps weaken the
pressure gradient. The trough may make showers more active, and
could even produce some thunderstorms Sunday night through Monday
night. Wind speeds will increase again later next week.

Surf along all shores is expected to remain well below the High
Surf Advisory threshold through the middle of next week.

The peak high tides for the month will occur over the next few days.
Water levels are running as much as a foot above the predicted
levels due to an eddy moving westward through Hawaiian waters.
Coastal flooding is possible around the times of the high tides
through early next week. See the Special Weather Statement for
more details.





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