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Hawaiian Islands Synoptic Discussion and Guidance

428
FXHW60 PHFO 241954
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
954 AM HST SAT SEP 24 2016

.SYNOPSIS...
Moderate trade winds will prevail over the next 24 hours, followed
by an increase expected on Sunday and Monday as high pressure
strengthens far north of the state. A typical trade wind weather
pattern will dominate through at least Friday, focusing showers
along windward slopes of all islands along with afternoon clouds
and showers over the Kona slopes of the Big Island. A brief
increase in mainly windward shower activity is expected on Monday.

&&

.DISCUSSION...
Deeper moisture that affected portions of the Big Island and Maui
has largely cleared the state to the southwest, setting the stage
for a typical trade wind weather regime over most areas today.
Precipitable water (PW) values over and just east of the islands
are still running about 10 to 25 percent above normal according to
the overnight soundings and satellite data. However, mainly
scattered cumulus clouds are observed upwind of the state, and
recent aircraft data and the soundings suggest that there is an
inversion developing around 6000 ft and that a portion of this PW
is elevated above the inversion. As a result, expect moderate
trade winds to focus clouds and modest showers over windward
slopes today. While leeward portions of the smaller islands should
be rather dry, lingering moisture will provide elevated chances
for clouds and afternoon showers over leeward and interior
sections of the Big Island this afternoon.

Trade winds will be gradually increasing on Sunday and Monday,
with an expected increase in showers due on Monday. The pressure
gradient over the islands is somewhat weak due to the presence of
a surface trough several hundred miles northwest of Kauai and a
cold front 700 miles to the northwest. The front will stall and
begin to slowly dissipate on Sunday, allowing strong high pressure
passing to the north near 40N to exert a greater influence. As a
result, trade winds will be on the rise on Sunday and will become
locally breezy on Monday. The air mass will continue to dry out
over the islands on Sunday as high pressure builds aloft, leading
to modest windward rainfall and rather dry conditions leeward. A
weak mid level trough passing overhead is expected to produce an
increase in shower activity on Monday.

Locally breezy trade winds and a typical shower pattern focusing
clouds and showers over windward areas will prevail. from Tuesday
through at least Friday.

&&

.AVIATION...
A fairly typical trade wind regime, albeit with trades in the
moderate to slightly weaker than moderate speeds, will continue
into Sunday. Thus windward sections of the islands may see some
MVFR ceilings at times. Current satellite imagery shows most of
the incoming low clouds headed mostly into the Big Island today
with some around Kauai this morning.

Soundings indicate quite a bit of moisture around the islands.
Most of it is below 20000 feet on the Hilo side and below 12000
feet on the Lihue side. Could see some afternoon convective
cloudiness over some interior sections today. Otherwise, VFR
conditions will be prevalent.

&&

.MARINE...
Moderate to fresh trade winds will prevail through tonight
followed by an increase in winds Sunday and Monday as high
pressure builds to the north of the state. A Small Craft Advisory
(SCA) will likely be posted for the typically windy waters around
the Big Island and Maui on Sunday, and the SCA will likely remain
in place through much of the week.

Surf will generally be on the small side through the week. A
series of mainly small pulses of south-southwest swells will
continue. A very small northwest to north-northwest swell will
peak tonight and Sunday, then fade early next week. Easterly
trade winds will maintain moderate short period surf along east
facing shores.

&&

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

&&

$$

Public and Marine...Wroe
Aviation...Fujii

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



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