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Hawaiian Islands Synoptic Discussion and Guidance

811
FXHW60 PHFO 230604
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
804 PM HST Tue Aug 22 2017

.SYNOPSIS...
A drier and more stable airmass has built into the island chain
this evening and will remain in place through the remainder of the
work week and into the upcoming weekend. Tropical moisture and
muggy conditions may return Sunday through early next week. Clouds
and showers will favor windward and mauka areas through the
period, with a stray shower spilling into leeward areas from time
to time. Moderate to breezy trade winds will continue through the
remainder of the work week, with the trades then easing over the
weekend into early next week as high pressure weakens to the north
of the State.

&&

.DISCUSSION...
Currently at the surface, a 1026 mb high is centered around 1200
miles northeast of Honolulu, with another 1030 mb high around 1300
miles north of Kauai. Meanwhile aloft, the upper level low which
resulted in the unstable conditions over the last few days, is now
around 400 miles west of Kauai and is shifting westward and
further away from the state. A drier and more stable airmass is
building into the island chain from the east, and this has
resulted in a notable decrease in shower activity across the
islands. Radar imagery shows a few showers affecting windward
areas, with coverage highest over Kauai where moisture remains a
bit deeper and the airmass less stable. Meanwhile, infrared
satellite imagery shows partly to mostly cloudy skies in place
across the State. Main short term concerns revolve around rain
chances and the increasing trades over the next few days.

Tonight through Saturday,
The two areas of high pressure will merge and settle southward
closer to the islands over the next couple of days, increasing the
trade winds across the state. Moderate to breezy trade winds are
expected through the remainder of the work week due to the tighter
pressure gradient across the State, with the peak in the trades
expected on Thursday. We should then see the trades ease to
moderate levels on Saturday, as high pressure weakens north of
the island chain.

As for sensible weather details, a drier airmass will continue to
build over the islands tonight, with these drier than normal
conditions then expected to remain in place through Saturday.
Clouds and showers will favor windward and mauka areas through the
period, with a stray shower reaching leeward areas from time to
time due to the strength of the trades. Showers are expected to be
most prevalent during the overnight and early morning hours.

Saturday night through next Tuesday,
The high pressure ridge north of the islands as a weakening front
passes by well to the north of the State. This will weaken the
trades into the light to locally moderate range Sunday through
early next week. Both the GFS and ECMWF show some deeper tropical
moisture trying to lift northward into the area Sunday through
early next week, and this may result in an increase in trade winds
showers across the island chain. In addition to the increase in
showers, muggy conditions will likely make a return as well, with
both models showing dewpoints climbing back into the lower 70s.

&&

.AVIATION...
A strong high pressure center well north of Hawaii will produce
moderate to locally breezy easterly trade winds over the islands.
Scattered shower activity will continue into the morning hours
with prevailing VFR conditions. Brief periods of MVFR conditions
in showers are possible tonight and Wednesday morning along
northeastern slopes of all islands potentially affecting Hilo and
Lihue terminals. Drier air will filter into the islands from the
northeast on Wednesday with a reduction in shower activity
statewide through the rest of the week.

AIRMET Tango for low level mechanical turbulence remains in
effect.

&&

.MARINE...
A Small Craft Advisory (SCA) remains posted this evening for the
windier zones around Maui and waters south of the Big Island. This
SCA is good through Thursday afternoon, but there is a strong
possibility that it will be extended into the weekend.

The warm muggy air mass continues to exit the islands, and the
upper level low near Kauai that helped set off a few
thunderstorms across the region, continues to drift further away
from the islands. There is a slight chance of a thunderstorm over
the western offshore waters west of Kauai tonight, and this threat
should be over with by Wednesday afternoon.

With the trades on the strengthening trend, the surf on the east
facing side will see a slight bump on Wednesday. A longer period
east swell may also arrive during this second half of the week as
a result of tropical cyclone activity in the eastern Pacific.
A swell from the Tasman Sea, generated by a gale about a week
ago, is due to arrive Friday afternoon. This swell and a southeast
swell will maintain some level of small surf along the south
facing shores into the weekend and beyond.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Lau

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

Current Conditions
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RH		39 %
Wind		NE 19 mph
Road		Open (4x4)