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Hawaiian Islands Synoptic Discussion and Guidance

274
FXHW60 PHFO 130130
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 PM HST Wed Aug 12 2020

.SYNOPSIS...
A nearby upper low to the northwest of the state will bring a
slight chance of thunderstorms for areas around Kauai through
tonight. Otherwise, trade wind weather is expected into early next
week with a slight weakening of the wind speeds Thursday and
Friday. Clouds and passing showers will tend to favor windward and
mountain areas, but localized sea breezes in protected areas may
bring a few afternoon clouds as the background winds weaken.

&&

.DISCUSSION...
No significant changes to the forecast with the afternoon package.
Made some tweaks to the PoPs and related fields for the first 36
hours of the forecast package, to tap down the showers in several
locations based on the latest guidance and in light of today's
shower activity. Have left the slight chance of thunderstorm area
the same through tonight. There has been some thunderstorm
activity just outside the coastal waters today, but otherwise
things have remained pretty quiet.

The upper level low to the northwest was producing quite a bit of
thunderstorm activity this morning just northwest of the coastal
waters, but even the activity there has died down there since
midday. The afternoon sounding from Lihue shows 500 mb
temperatures colder than normal, with the models showing colder
temperatures over the coastal waters to the northwest and west of
Kauai. Precipitable water (PW) values remain near normal around,
with slightly higher values over the coastal waters. With little
change in the upper level low, have left the area for the slight
chance of thunderstorms the same near Kauai through tonight. Have
not extended the area into tomorrow, but will need to see how
things pan out tonight.

Otherwise, the high pressure system far north of the area will
dominate our weather maintaining a trade wind pattern with clouds
and shower focused over the windward slopes. The high is expected
to weaken over the next couple of days, with a new high building
northwest of the area this weekend. As the current high weakens
and new high settles in, we could see the winds weaken bit, which
may allow for some localized land breezes in the afternoon. This
could lead to some interior clouds and shower development.

&&

.AVIATION...
A weak upper level low northwest of Kauai, with associated cold
and unstable air aloft, will allow for increased shower and
isolated thunderstorm activity this evening and into tonight over
Kauai and surrounding waters. Elsewhere, persistent high pressure
north of the state will bring moderate trades to the islands. This
pattern will concentrate most isolated to scattered shower
activity along the windward coasts and slopes. Brief periods of
MVFR ceilings and visibility may occur with this activity tonight.

No AIRMETs are currently in effect.

&&

.MARINE...
A large and strong surface high far north of the area will slowly
weaken through Friday. A new high will build northwest of the
area over the weekend then move southeast through the middle of
next week. These highs will maintain moderate to fresh northeast
to east trade winds over Hawaiian waters through at least early
next week. Winds over the coastal waters are expected to remain
below Small Craft Advisory (SCA) speeds. A low aloft will bring a
slight chance of thunderstorms to waters around Kauai through
tonight, and some of the trade wind showers there may be heavier
than normal.

Surf will remain below advisory levels along all shores into next week.
A NE swell expected Thursday and Friday will boost surf along
east facing shores and may also reach some north shore locations.
A small long-period swell from Hurricane Elida is possible over
the weekend. Small surf along south facing shores will build
starting over the weekend and continuing into next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...R Ballard
AVIATION...Thomas
MARINE...Donaldson

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



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