Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 260631
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Sun Jun 25 2017

.SYNOPSIS...
A mid-level trough will shift west of the state tonight, allowing
high pressure to have a greater influence on the local weather.
The trade winds will strengthen tonight and Monday, reaching
breezy and locally windy speeds by Tuesday. Drier and more stable
conditions are expected Monday through Thursday, but a more
showery trade wind pattern will return Friday through the upcoming
weekend as an upper level low passes by just to the north of the
island chain.

&&

.DISCUSSION...
Currently at the surface, a 1032 mb high centered around 1600
miles north-northeast of Honolulu, is driving light to moderate
easterly trade winds across the island chain this evening.
Meanwhile aloft, a mid-level 700 mb trough, is located over Kauai
and is enhancing some of the shower activity there. Infrared
satellite imagery shows partly to mostly cloudy skies in place
across the State, with radar imagery showing a few light showers
moving into windward areas. There remains a few lingering showers
over leeward areas as well, primarily over Kauai and Niihau. Main
short term concern revolves around rain chances.

Tonight,
The mid-level trough will exit to the west of the State, allowing
high pressure to exert more of an influence on the island chain.
This will result in strengthening trade winds over the eastern end
of the State, but the trades are expected to remain light over
Oahu and Kauai. A convergent band of clouds and showers associated
with the returning/strengthening trades, will move from east to
west across the state overnight. This should result in a bit of
and increase in mainly windward showers as the feature moves
through. Across leeward areas, the lingering convective showers
should end this evening, with isolated showers remaining possible
after midnight in association with the convergent band moving
through.

Monday through Thursday,
High pressure will build northeast of the State, increasing the
pressure gradient across the islands. This will result in
strengthening trade winds Monday and Monday night, with the
trades reaching breezy to locally windy levels Tuesday through
Thursday. In addition to the strengthening trades, a drier more
stable airmass will move into the area. Precipitable water values
will remain in the 1.0 to 1.2 inch range through the period,
below normal for this time of year, and this will result in a
drier than normal trade wind shower pattern. Showers will continue
to favor windward and mauka areas, with a stray shower reaching
leeward areas from time to time due to the strengthening trades.
Rainfall amounts are expected to remain light.

Thursday night through next Sunday,
Both the GFS and ECMWF are in good agreement showing an upper
level low tracking eastward toward the islands Thursday night and
Friday, but then differ on the progressiveness of this feature
tracking westward to the north of the State over the upcoming
weekend. High pressure will continue to hold in place north of
the islands through the period, with breezy trade winds expected
to continue through at least Thursday night. The trades are then
expected to weaken Friday through next Sunday as the gradient
slackens due to the upper level low passing by to the north of the
islands. As for sensible weather details, both models show deep
layer moisture increasing beginning Thursday night, with this
enhanced moisture remaining over the islands through next
Sunday. As a result, we should see a more showery/wet trade wind
pattern through the period, with showers continuing to favor
windward and mauka locales.

&&

.AVIATION...
The high pressure ridge north of the Hawaiian Islands will
strengthen and bring moderate strength northeasterly trade winds
on Monday. Cloud ceilings and TEMPO showers, mainly during
morning and evening hours, will trend back to north through east
sections of all islands with isolated showers elsewhere.

No AIRMETs are in effect. AIRMET Tango may be needed after 22Z
Monday for low level moderate turbulence across Maui and the Big
Island.

&&

.MARINE...
Trade winds will gradually increase later tonight and Monday as
a surface ridge strengthens far north of the state. This will
produce locally strong trade winds over the typically windy
waters adjacent to the islands of Maui County and the Big Island.
Therefore, a Small Craft Advisory (SCA) has been issued starting
Monday morning, and continuing through Thursday afternoon for
these waters. The current forecast indicates trade wind speeds
may decrease slightly on Friday.

As the trade winds strengthen, expect a slight increase in short-
period wind waves along east facing shores this week. In
addition, a northeast swell with dominant wave periods of 12 to
13 seconds, which was produced by a fetch of gales west of Oregon
and California, will spread across the Hawaiian waters through
Monday. The energy from this swell is being observed at all of the
windward near shore buoys early this evening. There is a low
probability that this northeast swell combined with the trade wind
waves may produce surf approaching the High Surf Advisory
criteria along east facing shores Monday or Tuesday. Some of this
northeast swell energy will also likely wrap into exposed north
facing shores across the state early this week.

Pulses of swell energy from the south-southeast through south-
southwest will produce near to just below average summer surf
heights along south facing shores this week.

Extreme tides that have been observed during the past several
days are gradually trending back toward normal heights early this
week. As a result, flooding impacts along the coast will continue
to slowly diminish during the next couple of days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from 6 AM Monday to 6 PM HST Thursday for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston



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