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Hawaiian Islands Synoptic Discussion and Guidance

768
FXHW60 PHFO 221905
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
905 AM HST Mon Jul 22 2019

.SYNOPSIS...
A trough of low pressure will pass a few hundred miles to the south
of the islands through Tuesday, spreading clouds and showers from
southeast to northwest across the island chain. Drier weather and
decreasing cloud cover is expected by Wednesday. Moderate to locally
breezy trades and typical windward and mauka showers will then
prevail into the weekend.

&&

.DISCUSSION...
Currently at the surface, a trough of low pressure is located
around 400 miles southwest of Honolulu, while a 1027 mb is
centered around 950 miles north-northeast of the island chain. The
resulting gradient is producing moderate to locally breezy trade
winds across the state this morning. Visible satellite imagery
shows mostly cloudy skies in most areas, with a considerable
amount of convective debris cloud spreading northeastward towards
the islands. Radar imagery shows some light showers moving into
windward areas, with rain gauge observations indicating the most
shower activity is affecting the windward slopes of the Big
Island. A considerable amount of heavy shower and embedded
thunderstorm activity is affecting the waters to the south of the
islands.

Today through Tuesday,
Both the GFS and ECWMF are in relatively good agreement showing
the trough of low pressure tracking steadily off to the west-
northwest and nearly parallel to the island chain over the next 24
to 36 hours. Moderate to locally breezy trade winds are expected
through the period, although there will likely be a brief easing
of the trades late this afternoon through Tuesday morning as the
pressure gradient relaxes.

As for weather details, the deepest tropical moisture associated
with the surface trough is expected to remain to the south of the
islands. This should keep rainfall amounts in check, and no
significant accumulations are expected. Shower activity is
expected to be most concentrated over the Big Island today. There
remains the potential for an isolated thunderstorm during the
afternoon and early evening here, but this will be highly
dependent on the amount of convective debris cloud thinning which
occurs. As the trough moves off to the west-northwest, the
chances for showers will increase over Oahu and Kauai late this
afternoon and tonight, with shower activity winding down quickly
across the Big Island this evening as drier air begins to work in
from the east. Maui County is expected to remain the driest
through the period as the deepest moisture slides by to the south
and west.

Most areas will have dried out by daybreak Tuesday, with only a
few showers affecting windward and mauka showers. Some lingering
moisture may keep rain chances a bit higher across Oahu and Kauai
in the morning, and Kauai during the afternoon, before the drier
airmass overspreads the entire island chain by late in the day.

A considerable amount of high cloud cover is expected across the
entire state today and tonight, with the high clouds then thinning
from east to west on Tuesday.


Tuesday night through Sunday,
Model solutions are in good agreement showing high pressure
strengthening northeast of the islands through the weekend. At the
same time, upper level ridging will build over the islands through
the end of the work week, with an upper level trough then begin to
approach from the east during the weekend, lowering upper heights
across the island chain. Overall, moderate to locally breezy trade
winds are expected through the period. A fairly typical windward
and mauka focused shower pattern is expected to prevail, with a
stray shower reaching leeward areas from time to time. Stable
conditions will keep conditions fairly dry through the end of the
work week, with shower coverage picking up a bit over the weekend
as the airmass becomes less stable due to the approaching upper
level trough. No significant rainfall is expected.

&&

.AVIATION...
The pressure gradient between a high north of Hawaii and a trough
passing southwest of the area will maintain locally strong east
winds over the islands. AIRMET TANGO is in effect for moderate
turbulence below 8,000ft and will likely continue through tonight.

The low-level winds will carry scattered to broken low clouds and
showers over east sections of the smaller islands. Isolated MVFR
ceilings are possible over these areas, but are not expected to
be extensive enough to require AIRMET SIERRA for MTN OBSC.

Moisture from the trough passing southwest of the area may fuel
heavy showers and isolated thunderstorms over the Big Island
later today. AIRMET SIERRA for MTN OBSC is currently in effect
for southeast portions of the Big Island and will likely continue
through this afternoon, possibly expanding to include additional
areas.

&&

.MARINE...
Fresh to locally strong easterly trades will continue through the
week as high pressure remains north of the area and a weak low
pressure passes to the south. Stronger winds will remain across
the typically windier locations from Maui County to the Big Island
due to terrain accelerations. The current Small Craft Advisory
has been extended through Wednesday.

Heavy showers and a few thunderstorms will remain a possibility
across the leeward waters of all islands through Tuesday night as
the aforementioned area of low pressure to the south sends clouds
and moisture northward.

Rough surf will continue along east facing shores due to strong
onshore trades. A slight downward trend is expected for a brief
period through Tuesday as the upstream trades relax, but winds
will increase again Wednesday.

Surf along south facing shores will remain small today, then trend
up tonight through midweek due to a storm system that passed near
the Tasman Sea. Heights should remain below advisory levels as
this swell moves through and peaks Tuesday. A long-period south-
southeast swell associated with a compact gale southeast of the
Tuamotus will be possible Thursday night through Friday. Surf will
rise along exposed shores but should remain below advisory
levels.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...TS
MARINE...Morrison

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



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