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000 
FXHW60 PHFO 290634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Wed Jun 28 2017

.SYNOPSIS...
Breezy trade winds associated with high pressure north of the state 
will continue through Friday, before trending down slightly over 
the weekend. Dry and stable conditions will limit shower activity 
over the state through Thursday. Showery weather will become more 
likely beginning Thursday night and continuing through the 
weekend. Windward and mountain areas will remain the focus for 
most of the shower activity, with showers reaching leeward areas 
more regularly Thursday night through the weekend. The trade 
winds will strengthen on Monday and reach breezy levels over the 
4th of July. A drier airmass will begin to move into the islands 
from the east on Monday, decreasing the coverage and intensity of 
trade wind showers through the middle of next week.

&&

.DISCUSSION...
Currently at the surface, an east to west oriented ridge of high
pressure is located around 550 miles north of Kauai, and is
driving breezy trade winds across the island chain this evening.
Infrared satellite imagery shows partly cloudy skies in place
across the State, with some more showery clouds developing
upstream (to the northeast) of the islands. Radar imagery shows
scattered showers moving into windward sections of Maui and the
Big Island, with isolated showers over windward sections of Kauai,
Oahu and Molokai, as well as leeward sections of the Big Island. 
Elsewhere, rain free conditions prevail. Main short term concern
revolves around rain chance.

Tonight and Thursday,
The ridge of high pressure will remain in place to the north of
the State, keeping a breezy and gusty trade wind flow in place
across the island chain. The airmass is initially quite dry, with
precipitable water (PW) values from the afternoon soundings
indicating 0.94 and 1.00 inches at PHLI and PHTO respectively.
Additionally, the low level inversions from the soundings were
between 5 and 7 kft, indicating quite stable conditions remain in
place. Model solutions show moistening trend over the next 24
hours, with PW values climbing into the 1.2 to 1.4 inch range by
Thursday evening. As a result, we expected to see an uptick in
trade wind shower activity beginning after midnight tonight and
continuing through the morning hours on Thursday. The shower
coverage should decrease Thursday afternoon as the sun erodes 
some of the incoming showery clouds moving in with the trades.
Rainfall amounts are expected to remain light through the period.

Thursday night through Sunday night,
Breezy and gusty trade winds are expected to continue through the
Friday night as the ridge of high pressure remains in place to the
north of the State. The trades are then expected to trend down
slightly over the weekend as an inverted trough pushes westward 
to the north of the islands, acting to disrupt the trade wind 
flow a bit. Both the GFS and ECMWF are in good agreement showing
an increase in moisture over the State beginning Thursday night
and continuing through the weekend. PW values climb to between 1.4
and 1.7 inches, and inversion heights will rise in response to
lowering upper level heights as an upper low tracks by to the 
north of the island chain. As a result, we expect a showery and
wetter than normal trade wind pattern beginning Thursday night and
continuing through the weekend. Showers will continue to favor
windward and mauka areas, but will reach leeward areas more
regularly due to the elevated inversion heights and deeper 
moisture over the region.

Monday through next Wednesday,
The GFS and ECMWF exhibit some minor timing differences early 
next week, but overall they both agree on a return of a drier 
trade wind weather and strengthening trades. For now will utilize 
a blended approach for forecast details. As a result, we expected 
the trade winds to strengthen on Monday, becoming breezy and gusty
for the 4th of July and into the middle of next week. Showery 
trades will likely continue into Monday morning, with a drier 
airmass overspreading the state Monday afternoon and Monday night 
and continuing through mid week.

&&

.AVIATION...
Expect a VFR evening as a relatively dry and stable trade wind 
airmass remains in place across the main Hawaiian Islands. The 
latest infrared satellite imagery shows a patch of scattered 
mainly light showers affecting Hana area of Maui, stretching 
southeast to the waters off the NE facing coast of the Big Island.
Radar is also picking up an isolated shower over the Koolau Mtns 
of Oahu. Otherwise, relative dry condition prevails elsewhere.

Satellite is showing a bit more low clouds with embedded showers
approaching the islands from the NE. These trade showers will be
favoring the windward and mountain areas of the larger islands. 
Brief MVFR cigs can be expected with these showers. They will be 
mainly light in intensity with restriction to vis limited to 
no less than 5 miles. A late night popup shower may occur along 
the Kona coast as well.

Strength of the trades remains strong enough to keep AIRMET Tango
in place through at least 1600 UTC. No other AIRMETs are expected.

&&

.MARINE...
A Small Craft Advisory (SCA) is in effect and includes the Kaiwi 
Channel, Maui County windward waters, Oahu leeward waters, and the
typically windy zones around Maui and the Big Island, due to 
strong trade winds. The SCA is in effect through Thursday, and may
continue for some zones into Friday, followed by slightly weaker 
trade winds this weekend. Trades will then strengthen again early
next week. The locally strong trades will produce choppy surf 
along east facing shores.

Long-period swells out of the southwest and southeast will 
generate moderate surf along south facing shores through the 
weekend. Surf will taper down around Sunday night or Monday.

A small, short-period northwest swell is forecast from Friday 
into the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Oahu Leeward 
Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big 
Island Southeast Waters.

&&

$$

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