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Hawaiian Islands Synoptic Discussion and Guidance

265
FXHW60 PHFO 031354
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
354 AM HST Mon Aug 3 2020

.SYNOPSIS...
Surface high pressure far north of the state will produce
moderate to locally breezy trade winds through next weekend. A
fairly dry trade wind weather pattern is expected early this
week. An upper-level disturbance passing near the islands
combined with increasing moisture around mid-week may cause
an uptick in showers over most windward sections from Wednesday
through Friday. Expect a more typical dry trade wind weather to
return by next weekend.

&&

.DISCUSSION...
An east to west oriented surface ridge is almost 1100 miles north
of Honolulu. In addition, there is an east-northeast to west-
southwest oriented surface tough about 500 miles south of South
Point on the Big Island. Since the islands are sandwiched between
these two features, the pressure gradient remains relatively
tight across the region. This is maintaining moderate to locally
breezy trade winds across the main Hawaiian Islands early this
morning.

Loops of water vapor imagery depict an upper-level trough
northwest of the islands. Southerly flow aloft along the eastern
flank of this feature is transporting a band of cirrus clouds,
which consist of small ice crystals, above the western end of the
island chain this morning. Satellite imagery and radar
reflectivities also show scattered to broken low clouds with
embedded showers being transported by the low-level trade wind
flow toward the windward sides of most of the islands.

The early morning balloon soundings from Lihue and Hilo indicated
stable atmospheric conditions persist with a strong trade wind
inversion of 6 to 7 thousand feet. In addition, the precipitable
water (PW) values were 1.1 to 1.2 inches. This is below the
typical August PW values of around 1.35 inches, so the atmosphere
remains somewhat drier than normal in the vicinity of the state.

The forecast models indicate surface high pressure will remain
far north of the state for the next several days. This will likely
produce moderate to locally breezy trade winds through next
weekend. The models also suggest the current relatively dry trade
wind weather pattern will persist for the next couple of days.
However, pockets of slightly greater moisture being carried by the
trades will continue to produce some low clouds and brief showers
over windward sections through Tuesday, especially at night and
during the early morning hours.

By mid-week, the forecast guidance continues to depict a weak
mid-level trough passing from east to west in the vicinity of the
islands. This might cause a slight destabilization of the
atmosphere. In addition, areas of slightly enhanced moisture may
move in from the east, which would likely increase the PW values,
as well as the dew point temperatures. The result may be a
noticeable uptick in showers, especially over windward facing
slopes from Wednesday into Friday. Upslope sections of the
leeward Big Island could also see some rainfall each afternoon.
Finally, the higher dew point temperatures may make the air feel
warmer than normal, even with the locally breezy trades. By next
weekend, the forecast models show a return to a more typical dry
trade wind pattern with mainly brief windward showers next
Saturday and Sunday.

&&

.AVIATION...
A stable, moderate to breezy trade wind flow will persist as high
pressure remains anchored far north of the state. Brief MVFR
conditions will develop along windward slopes as pockets of
moisture ride along the trades. VFR conditions will dominate over
leeward areas, though the Kona slopes of the Big Island will
experience MVFR ceilings and reduced visibility in SHRA each
afternoon.

AIRMET TANGO for turbulence remains in effect below 8,000 feet
over and immediately south through west of mountains.

&&

.MARINE...
A surface ridge will remain far N of the islands throughout the
forecast period, supporting locally strong trade winds. A Small
Craft Advisory is currently posted for the typically windy waters
around Maui County and the Big Island through Tuesday, and will
likely need to be extended.

There will be small long-period S swells arriving through the
week, with surf heights remaining below advisory levels. There
will also be a short-period SE swell that will linger through the
week, keeping surf slightly elevated along exposed shorelines.
Easterly trade winds will continue to produce short-period choppy
surf along E facing shores. A small NNE swell will linger today
before diminishing, with an even smaller NNW swell possible around
Thursday/Friday.

&&

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

&&

$$

DISCUSSION...Houston
AVIATION...TS
MARINE...Birchard

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



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