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Forecast Discussion for Honolulu, HI

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136
FXHW60 PHFO 210207
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
407 PM HST Sat Jul 20 2019

.SYNOPSIS...
A broad area of high pressure far north of the islands will
continue to feed the area with locally breezy trade winds through
at least the middle of next week. There could be a boost in
enhanced trade wind showers between Monday and Tuesday, in
association with a low level disturbance passing south of the
islands and its interaction with an upper level low northwest of
the islands.

&&

.DISCUSSION...
A well anchored surface high north of the islands will be the
continuing source for trade winds through at least the middle of
next week. These trade winds will be mainly be in the moderate to
locally strong range.

The GFS continues to suggest a deep low level trough, currently
400 miles south of the Big Island, to head northwest during the
next 72 hours. The trough will be accompanied by tropical moisture
that will sweep across the island from east to west. Influencing
this low level feature is an upper low at 500 mb, located 350 miles
north- northwest of Kauai this afternoon. It is forecast to deepen
slightly in the next 24 hours while remaining in that general
vicinity through Monday, before drifting westward, away from the
islands. By Tuesday afternoon the upper low is expected to be 700
miles west- northwest of Kauai. Both the GFS, ECMWF, and NAM, are
in pretty good agreement with this scenario, although the EC is a
few hours behind the GFS with the initial onset of the weather on
the Big Island.

Current grids have thunderstorms slated for the Big Island Monday
morning, as per GFS solution. We have kept the influence of this
tropical moisture to the Big Island, and Maui at the fringe, for
the time being, and that is due to the unknown amount of low level
moisture spreading up the island chain. The prevailing low level
winds through Tuesday will be trades, and not southeast, although
there is a slight tinge of south over Kauai on Tuesday. But should
the winds stay trades, it may limit the amount of low level
moisture for the islands. Instability from the upper low is likely
to be sufficient to enhance the trade showers, however, but just
leading to some spotty heavier showers. That is what we are
banking on at this time. The weather should be regular trades by
Wednesday, given the upper low will be further away and weaker.

Comparing the latest satellite imagery with the models, there is
a presence of a deep northeast to southwest oriented low- level
trough and a possible low 400 miles south of South Point, Big
Island, fitting the models solution. Scattered deep convection is
noted near its center.

Satellite imagery also show a broken band of low clouds over Maui,
extending upwind for 200 miles. With night time coming, expect the
band to increase in clouds and showers this evening. More sunshine
is in store for Maui on Sunday. Should the band shifts a little
to the north or south, it will mean more showers to Oahu or
windward Big Island, respectively. A patch of shower- bearing
clouds blew through Oahu a little while ago, and is now clearing
up. Oahu may get a couple of showers towards sunset. Kauai is
partly to mostly sunny, and not much clouds moving in. So the
early evening looks good. As for the Big Island, it has partly
cloudy and mainly dry east of Laupahoehoe, down across the Kau
coast. It is cloudy along the rest of the Hamakua coast to Upolu
Point with a few showers. The lee side has cloud up with scattered
showers in progress.


&&

.AVIATION...
A high will remain nearly stationary far north of the main Hawaiian
islands and maintain locally strong east winds over the area.
AIRMET TANGO is in effect for moderate turbulence below about 7,000ft
and this AIRMET will likely continue through Sunday.

Clouds and showers that developed over the southwest slopes of
the Big Island and Maui will dissipate after sunset. The low level
winds will carry scattered to broken low clouds over east sections
of the islands. Isolated MVFR ceilings are possible over these
areas tonight, but we don`t expect the ceilings to be extensive
enough to require AIRMET SIERRA for MTN OBSC.

&&

.MARINE...
Fresh to locally strong easterly trades are forecast to continue
through next week as high pressure remains positioned north-northeast
of the area. The stronger winds will remain across the typically
windier locations from Maui County to the Big Island due to
terrain accelerations. The current Small Craft Advisory has been
extended through Sunday night, but will likely need to be extended
further into next week.

Rough surf will continue along east facing shores each day due to
strong onshore winds. Latest local buoy observations are showing a
gradually decrease in heights and we should continue to see a slow
downward trend through Tuesday due to the upstream trades
trending down. A slight uptick is expected once again Wednesday
into Thursday of next week as upstream trades increase.

Surf along south facing shores will remain small through the
weekend, then trend up through the first half of the upcoming week
due to a recent system passing through/near the Tasman Sea.
Heights should remain below advisory levels as this southwest
swell moves through and peaks Monday through Tuesday. In addition
to this long-period source, a combination of small south to
southeast swells will be enough to keep the surf from going flat
along southern exposures through midweek.

A south-southeast (150-160 deg) long-period swell associated with
a compact gale that has developed southeast of the Tuamotus will
be possible late next week with a peak around Thursday night into
Friday. Surf will rise along exposed shores but should remain
below advisory levels.

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

&&


.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Sunday night for Maalaea Bay,
Pailolo and Alenuihaha Channels, and waters south of the Big
Island.


&&

$$


H Lau/Donaldson/Kino

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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