Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 201354
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 AM HST Fri Sep 20 2019

.SYNOPSIS...
Island weather will trend wetter through the weekend, with
diminishing trade winds and increased moisture also making it feel
quite muggy by Sunday. The chances for a few locally heavy showers
will increase as well, as a low pressure system in the upper
atmosphere passes near the islands. Light winds may continue for
most of next week.

&&

.DISCUSSION...
A sharp trough in the mid- to upper-atmosphere is digging toward the
islands from the north, bringing with it broken to overcast layered
high clouds that are currently blanketing skies over all islands.
The high clouds are efficiently obscuring a satellite view of the
incoming low cloud field, while radar shows a few showers moving
quickly westward within a moderate to breezy trade wind flow. 12Z
soundings show a capping subsidence inversion remains in place at
around 8000 feet, and shower coverage has remained limited
overnight.

As the trough continues to dig south today, it will push most of the
high clouds south of the smaller islands, while also helping to
carve out a cut-off closed low aloft tonight into Saturday. With
moderate to locally breezy trade winds in the low-levels supplied by
a surface high to the distant northeast, the most likely area for
showers will be windward areas through Saturday. While the trades
will also keep showers from sitting over any one area for too long,
they will also push a few sprinkles leeward on the smaller islands.
The low aloft is expected to develop just to the northeast of the
islands, bringing moderate instability as it drifts west and
northwest over the islands through Saturday. In response, 500 mb
temperatures are forecast to drop ~4 degrees C into Saturday,
bottoming out near -9C. As the low moves west of the islands Sunday
into early next week, instability will diminish somewhat as mid-
level temps warm, but moisture is expected to increase.

The low aloft is expected to induce the development of a low-level
trough over the area that will bring light east to southeast winds.
Layered high clouds south and east of the low`s center will spread
from the Big Island to the other islands by Sunday and Monday, and
periods of considerable cloudiness are possible. However, the
greatest instability may not phase together with the deepest
moisture, thus reducing thunderstorm chances, except over leeward
Big Island, where daytime heating may spark an isolated thunderstorm
on Saturday. As the low moves west of the area, a plume of deep
moisture will be pulled northward over the islands, and this appears
to be conducive for the development of locally heavy showers, and
these have been added to the forecast for Sunday and Monday.
Difficult to pinpoint exactly where and when these would occur, but
the diminished winds may allow sea breezes to drive heavy shower
development over leeward and interior areas during the afternoons
and evenings, while the light east to southeast winds deliver
windward showers that would favor nights and mornings.

The low aloft will move west of the islands Tuesday and then get
drawn northward into the circulation of a new low aloft that is
forecast to approach from the distant northwest by the middle of
next week. This is forecast to allow a mid-level ridge to build over
the islands from the northeast, but latest GFS guidance indicates
that this new low may keep the surface trough from weakening or
leaving the area. Lingering low-level convergence could potentially
support continued shower development, and winds would remain on the
lighter side, if this were to verify. The forecast grids lean toward
a light to moderate trade wind weather regime from Wednesday onward,
supported by latest ECMWF guidance.

&&

.AVIATION...
Trade wind flow continues at the surface, but a swath of mid and
high clouds is moving from northwest to southeast across the island
chain. There are some trade wind showers moving across windward
areas that will bring brief periods of MVFR conditions, but overall,
VFR should prevail.

The potential for light icing exists, but does not seem widespread
enough to warrant issuance of AIRMET zulu at this time.

&&

.MARINE...
Fresh to locally strong, easterly trade winds will continue into the
weekend. The Small Craft Advisory for most of the typical windy
areas around Maui and the Big Island has been extended through
Saturday. Winds will gradually weaken through the weekend as a
surface trough approaches from the east. The trough will increase
showers starting Saturday, as well as shift winds out of the
southeast by Sunday, allowing sea and land breezes to develop over
leeward areas.

A series of small swells from the southwest and south are expected
through the rest of this week. A small northwest swell is possible
late in the weekend into early next week. A small east swell from
Tropical Cyclone Kiko will persist into the weekend.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...Dye
MARINE...TS


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