Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 221957

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Fri Sep 22 2017

Breezy trade winds will continue today, then gradually trend down
over the weekend as the ridge of high pressure to the north weakens
and a weak surface trough approaches from the east. A typical
pattern of mainly windward showers will prevail, though interior
and leeward Big Island will experience extensive afternoon cloud
cover and a few showers each day.


A 1030 mb high far northeast of Hawaii while a weak trough of low
pressure positioned south of the islands. The pressure gradient
between these two synoptic features continues to support breezy
trade winds across the local area. Satellite-derived precipitable
water imagery and upper air soundings at Lihue and Hilo reflected
values around normal across much of the state for this time of
year that ranged from 1.2 to 1.4 inches. The exception was near
the southern half of the Big Island, where higher moisture values
are observed due to the aforementioned surface trough to the
south. Visible satellite shows a few windward and mauka clouds
with mostly sunny skies leeward. Radar shows very little
precipitation blowing in with the trade winds this morning.

The latest short-term model guidance remains in good agreement and
is lining up well with the current pattern and trends around the
state. A general consensus supports breezy trade winds persisting
today, then steadily trending down over the weekend as the high
to the north weakens and the trough south of the Big Island
continues westward. Mostly dry and stable trade wind weather will
continue each day with clouds and showers favoring windward
locations. Rainfall accumulations will be light, with precipitable
water values holding in the 1-1.3" range (near to below average)
through the weekend.

For the extended (Monday through midweek), light to moderate trades
are expected to persist across the islands due to a weakness within
the ridge north of the state. Clouds and showers will likely favor
windward locations. The GFS and ECMWF both depict a weak surface
trough approaching the islands from the east by midweek. This
combined with a slight increase in moisture, may be enough to bring
the rain chances back to normal over windward locations.


High pressure north-northeast of the State will keep a moderate
trade wind flow in place through tonight. Clouds and showers will
favor windward and mauka areas, with isolated spillover showers
trickling over into leeward areas from time to time. Brief MVFR
cigs/vsbys in showers, otherwise prevailing VFR conditions
expected all areas.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. This AIRMET will likely
remain in place through at least the evening hours.


Fresh to locally strong trades should gradually weaken later today,
decreasing wind speeds into the light to moderate range through
the weekend. Trade winds will continue to bring rough surf along
east facing shores today, easing this weekend.

Surf along south facing shores will remain small through early next
week, with mainly background southeast and southwest swell energy
expected. The next potential 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 Tuesday
through midweek.

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


Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.



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