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Forecast Discussion for Honolulu, HI

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415
FXHW60 PHFO 200634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Thu Jul 19 2018

.SYNOPSIS...
A ridge of high pressure located far north of the islands will
maintain trade winds through the weekend and into the first half
of next week. The locally breezy trades will ease off to moderate
speed by Friday night, and continue through the weekend. A
relatively dry and stable air mass will limit the trade showers
until late Sunday when some tropical moisture roll in from the
east.

&&

.DISCUSSION...
A relatively dry trade pattern is expected for the next couple of
days. The pressure gradient will continue to relax over the
islands through Friday as a weak low pressure system located 500
miles SSW of Oahu continues to move west, further away from the
islands. Expect the current locally windy trades to settle to
moderate speed by Friday evening.

The air mass over the islands is stable, with a strong inversion at
around 5k feet as noted by both the Lihue and Hilo soundings from
this afternoon. So, any showers will be light in intensity. Radar
is picking up a few showers moving into the active volcano area
of the Puna district, as well as possibly virga from the dense
layered clouds that have developed rapidly over Maui County in the
past couple of hours. Otherwise, it has been a dry evening thus far.
We may pick up a few more trade showers as the evening progresses.

An upper low is centered 600 miles NNW of Kauai this evening. The
SW winds aloft to the SE of the low is drawing up some moisture,
by ways of cirrus clouds, from the deep tropics and bringing them
to the eastern half of the island chain, and apparently, some of
this cirrus is pretty dense. The upper low is expected to move
SW in the coming days, keeping the islands under a SW wind flow
for most of the upcoming weekend. Its only effect on our island
weather will be the cirrus clouds that may be dense enough to
block out some of the sunshine at times. The upper low will affect
the offshore waters W and N of Kauai, with a slight chance of a
thunderstorm.

By late Sunday afternoon, the leading edge to a batch of tropical
moisture, should be at the Big Island door steps. This moisture
will be working gradually westward, clearing Kauai by Tuesday night
of next week. What the islands will be getting will be a boost of
enhanced trade showers. There could be a surge in the trade winds
as well when this moisture area moves through, from a disturbance
passing south of the islands. This disturbance and moisture area are
two different sources that appears to come together as they approach
the islands from the east.

&&

.AVIATION...
Moderate to strong high pressure northeast of Hawaii will keep
the breezy/windy conditions in placed through Friday. AIRMET
Tango continues for mechanical turbulence south through west of
the mountains of all islands as a result.

Expect some clouds and showers over the typical windward and
mountain areas with passing MVFR conditions. Otherwise, expect VFR
conditions to prevail. Additional cirrus and some mid level clouds
are moving over the Big Island from thunderstorms south-southwest
of the Big Island.


&&

.MARINE...
Moderate to locally breezy trade winds continue. A disturbance
located about 500 miles southwest of Hawaii will continue to track
west and away from the islands tonight. The Small Craft Advisory
(SCA) has been extended for the typically windier waters around
the Big Island and Maui County through Friday. The passing of
this disturbance has helped to tighten the local pressure gradient
over the state, but this will begin to weaken as the disturbance
tracks farther from the state. Winds are then expected to decline
a notch by Friday night, possibly below SCA thresholds. Another
weak disturbance is then forecast to pass south the of state early
next week, once again tightening the pressure gradient over the
state, leading to an increase in winds.

A series of reinforcing south-southwest swells will keep surf
along the south facing shores slightly above the summer average
through Friday. A larger and longer period south-southwest swell
is expected to begin filling in Saturday and peak on Sunday. This
swell may cause surf to approach the High Surf Advisory (8 ft)
threshold along south facing shores from Saturday into Monday.
Surf along east facing shores will continue its downward trend
Friday into Saturday as the winds ease.

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.



&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay, Pailolo
and Alenuihaha channels, and waters south of the Big Island.


&&

$$


Lau/Chevalier/Eaton

Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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