Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 211954

Area Forecast Discussion
National Weather Service Honolulu HI
954 AM HST Thu Sep 21 2017

Locally breezy trades will gradually weaken during the next couple
of days as surface high pressure far northeast of the state
erodes. 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 through at least
the weekend.


A locally breezy and generally stable trade wind flow persists.
The trades are being driven by a somewhat strong 1032 mb high
sitting 1600 miles northeast of the islands and its associated
ridge stretching about 650 miles north of Kauai. A trough within
the ITCZ remains north of 10N, and this is helping to enhance the
winds as the pressure gradient over the state is somewhat tight.
Mid level ridging is producing an inversion around 5,000 to 7,000
ft according to the overnight soundings and recent aircraft data,
and precipitable water within the trade wind flow ranges from near
September normal of almost 1.5 inches to as low as 1.1 inches in
dry slots. This is resulting in typical pockets of showers that
are producing modest rainfall along windward slopes. The far
southwestern edge of an upper level trough extends over the
islands. This feature is having little effect other than pulling
up some elevated moisture near the Big Island.

Expect little change this afternoon. Breezy trade winds will hold,
and windward shower activity will diminish. Afternoon clouds
along leeward and interior terrain on the Big Island will be more
extensive than normal, thanks to elevated moisture being pulled
northward by the upper level trough overhead.

A gradual decrease in trade winds is due on Friday and Saturday as
the ridge currently 650 miles north of the state is pushed
southward. Moderate trade winds are forecast to persist through
the upcoming week. No big changes in the moisture content and
stability of the trade wind flow are noted in the near future. As
a result, showers will continue to be largely confined to windward
slopes. The upper level trough will meander in place through the
weekend, which will likely lead to continued afternoon cloud cover
and spotty showers over leeward and interior Big Island.


VFR conditions prevail at all TAF sites and these conditions are
expected to continue through the day today and on into the evening
hours. There will likely be some brief MVFR conditions along
windward slopes at times as moisture riding in on the trades
interacts with island terrain.

AIRMET Tango remains in effect for low level turbulence over and
immediately south and west mountain ranges of all islands. The
AIRMET will likely continue through the day today.


Fresh to locally strong trades associated with 1032 mb high
pressure north-northeast of the state across the eastern Pacific
will hold through Friday. Trades should weaken to the light to
moderate range by the end of the weekend as high pressure shifts
eastward and weakens in response to a frontal boundary passing far
north of the state. This pattern should hold through the first
half of the upcoming week.

Trade winds will generate rough surf along east facing shores into
Friday. Surf along east facing shores will trend down over the
upcoming weekend as the trades weaken locally and upstream.

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 will be possible by Sunday, which should lead to
a slight increase in surf. For the extended, strong- to gale-force
southwest winds associated with a low that has been impacting the
Tasman Sea over the past 24 hrs may lead to another small southwest
(210-220 deg) swell Tuesday through midweek.

A small northwest (310 deg) swell associated with broad and complex
low pressure that has been centered over the central Pacific near
the Date Line over the past couple of days will fill in today and
hold into the weekend before easing. The latest surface analysis
showed 990 mb low centered near the Aleutians around the Date
Line with a decent pocket of strong to near gale-force winds
focused within the 320 to 330 deg band relative to the islands.
This may end up reinforcing what is left of this weekend`s 310 deg
energy by Monday, before completely fading Tuesday through
Wednesday. As a result, a slight increase in surf is expected
along north and west facing shores today, which should hold into
early next week before dropping off.


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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