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Hawaiian Islands Synoptic Discussion and Guidance

972
FXHW60 PHFO 201950
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
950 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...
An initial band of showers is moving across the islands from north
to south. This band has been driven southward from an upper level
shortwave that developed into an upper level trough overnight.
The back end of the high clouds across the area are moving a
little faster to the south than originally thought, however the
upper level trough is expected to set up over the islands later
today. This could halt the progress of the back end of high
clouds.

No changes to the grids at this time, with the forecast generally
on track.

The trough is then expected to start a westward movement Saturday
with a mid to upper level low developing along the trough by
midday Saturday. The coldest 500 mb temperatures reach down to the
islands on Saturday, with the highest precipitable water (PW)
values reaching the islands on Sunday. The forecast has the chance
for some thunderstorms over the Big Island on Saturday afternoon
and evening, but this could be extended/expanded depending on how
the situation evolves.

The increase of PW expected to move northward over the islands on
Sunday will increase rain activity, and with the upper low
over/near the islands, the instability could lead to some heavy
showers. The low is expected to move west of the islands on
Sunday and into early next week, which will allow for a slightly
more stable environment over the islands, but the increased
moisture will remain.

A surface reflection of the upper level low will likely develop
over the islands this weekend, resulting in lighter east to
southeast winds over the islands. Lighter winds could lead to some
sea breezes that would contribute to afternoon and evening shower
activity over leeward and interior areas.

The upper level low is expected to lift northward as we head into
the middle of the week. This will be drawn into a new upper level
low approaching from the far northwest. This could keep a surface
trough in the area, maintaining a wet pattern over the islands. The
forecast grids continue to lean toward a light to moderate trade
wind weather regime from Wednesday onward, in line with recent ECMWF
guidance, but will be re-examining this situation with the latest
model runs later today.

&&

.AVIATION...
A surface high far NE of the area will maintain locally strong
east low level winds over the area through Saturday. The low level
winds will push some clouds and showers over windward (east)
facing sections of the islands and produce ISOL MVFR ceilings and
visibilities. VFR conditions will prevail over most leeward areas,
but afternoon sea breezes may produce ISOL MVFR ceilings over the
southwest (leeward) slopes of the Big Island of Hawaii and Maui.
There will be a slight chance of thunderstorms and heavy showers
tomorrow on the Big Island.

A northeast to southeast trough aloft lies just northwest of the
area. There is a low within the trough about 350 nm northeast of
Maui. A band of layered middle and high clouds associated with the
trough blankets the area. The trough is producing moderate
turbulence over the area and light icing is present in the layered
cloud.

AIRMET TANGO is in effect for the moderate turbulence. Light
icing (below AIRMET intensity) is noted in the AIRMET bulletin.

&&

.MARINE...
High pressure far north-northeast of the state will maintain fresh
to locally strong northeast trades through Saturday. Starting
Saturday night, a trough passing just south of the state will
bring increasing showers, and a decrease in wind speeds. Winds are
expected to decrease to light to moderate speeds by Sunday and
veer to the east. Sea and land breezes will be possible on Sunday
especially along the leeward coast. Winds should veer to the
east-southeast on Monday as the trough begins to move west of the
state.

The Small Craft Advisory for most of the typical windy areas
around Maui and the Big Island is in-effect through Saturday and
is expected to drop below advisory levels by Saturday night.

A series of small swells from the southwest, west-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.

Looking out into next week, a strong storm tracked west to east
south of New Zealand centered around 55S over the last several
days. This storm is expected to track north with a captured fetch
along the great circle route to Hawaii today through Saturday.
This should bring a prolonged south swell starting mid next week
with surf potentially reaching advisory levels towards the end of
next week.

&&

.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...M Ballard
AVIATION...Donaldson
MARINE...Kino

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



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