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Hawaiian Islands Synoptic Discussion and Guidance

063
FXHW60 PHFO 300226 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
426 PM HST THU SEP 29 2016

.SYNOPSIS...
High pressure north of the state will drive fresh trade winds
through the weekend, with showers primarily across windward and
mauka areas. Showers are may increase in coverage and intensity
over the weekend and into early next week, as a moist airmass
associated with the remnants of Ulika moves over the state.

&&

.DISCUSSION...
A 1038 mb surface high is centered far the north of the state
this afternoon driving fresh trade winds across the state.
Meanwhile, Tropical Storm Ulika is located around 1000 miles east of
Hilo and is moving northwest at 8 mph as of 11 am HST. The
Central Pacific Hurricane Center forecast has Ulika dissipated
well before reaching Hawaii.

Aloft, water vapor imagery shows a broad upper level trough located
between the islands and Ulika, with upper ridging to the north and
west of the state. Visible satellite imagery shows partly to
mostly cloudy skies this afternoon, with the more extensive cloud
cover over Kauai, Oahu, as well as the typical cloud build ups
across the Kona slopes of the Big Island. Radar shows scattered
showers carrying in with the trades concentrated across the
windward sides of Kauai and Oahu.

Fresh trade winds are expected to remain in place through the
weekend as the surface high slowly digs south and weakens,
essentially keeping the same pressure gradient over Hawaii. The
showers over the western end of the state will push west of the
area tonight while a relatively drier airmass is ushered in from
the east by the trade winds. Afternoon soundings already show
precipitable water (PW) value of 1.09 inches at Hilo where the
drier airmass has pushed in. Lihue is a bit wetter at around 1.39
inches this afternoon but is expected to dry out as the airmass
progresses westward. The drier trade wind pattern will continue
through Friday.

A surface trough (the remnants of Ulika) will approach from the
east over the weekend and become ill-defined as it passes over or
just to the south of the state early next week. Aloft, upper level
troughing will develop over the islands Friday night and linger
over the state through Saturday night. The upper trough is then
expected to cut off to two distinct lows to the northeast and
northwest of the state before an upper ridge builds back in from
the east by the middle of next week. Meanwhile, the trades will
diminish and shift around to the east or east-southeast as the
trough associated with the remnant of Ulika passes to the west.

As for forecast details, recent model runs have been in good
agreement showing deeper moisture arriving Friday night and
Saturday as the upper trough developing over the islands. PW
values increase to between 1.3 and 1.7 inches, highest over the
eastern islands, and this should promote wetter than normal trade
wind weather across the state. Showers will continue to favor
windward and mauka areas, but with the elevated inversion, some
showers will reach leeward locales as well. Most of the models
have been showing deep tropical moisture with PW values around or
in excess of 2 inches (associated with the remnants of Ulika) will
overspread the state from east to west Saturday night and Sunday,
and linger over the island chain through early next week. However,
this is not set in stone yet, as the models have been showing some
run-to-run variability. For example, the 12Z ECMWF run is not as
aggressive with the moisture and dissipates the remnants of Ulika
far to the southeast of the islands, and keeps the upper trough a
little farther west. Even if ex-Ulika does not get involved, the
upper trough will be enough to lead to a more showery pattern.
There is a greater potential for locally heavy rainfall if ex-
Ulika gets closer as some of the other models are showing.


&&

.AVIATION...
AMDAR soundings into Lihue/Honolulu/Kahului show 25 knots of
winds at low-levels. The inversion is more pronounced over the
eastern half of the state and weak/elevated over the western half.
We have not received any pilot reports with more than light
turbulence. However, wave clouds over Oahu are becoming
increasingly well defined extending downstream from the Koolau
Range. With these signals, will hold onto the existing AIRMET for
potential moderate turbulence in the lee of the mountains.

Otherwise conditions will be primarily VFR. Occasional MVFR
ceilings are expected in showers, mainly across windward
sections but also briefly passing leeward as well. Showers and
associated MVFR ceilings and visibilities will increase tonight,
but at this time an AIRMET for mountain obscuration is not
anticipated.


&&

.MARINE...
High pressure far to the north of the state will keep locally
strong trade winds in place through the weekend. A Small Craft
Advisory is in effect for the typically windier areas near Maui
and the Big Island through Friday, and will likely be extended
through the weekend.

Short period easterly swell resulting from the trade winds will
continue through the remainder of the week. A bit of a bump in
surf heights is expected for east facing shores over the weekend
as the remnants of Ulika approach from the east. Elsewhere, surf
heights will remain small through early next week.


&&

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


&&

$$

Discussion/Marine...Foster
Aviation...Bravender

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