Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 290645
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
845 PM HST WED SEP 28 2016

.SYNOPSIS...
Locally breezy trade winds will focus showers across windward and
mauka areas through Friday. Showers are expected to increase
during the weekend and into early next week, as a moist and
unstable airmass associated with the remnants of Ulika move over
the state.

&&

.DISCUSSION...
Currently at the surface, a 1038 mb high is centered well to the
north of the state, while Tropical Storm Ulika is located 1100
miles east-southeast of Hilo. Aloft, a broad upper level low is
located between the islands and Ulika, with upper ridging to the
north and west of the state. Infrared satellite imagery shows
partly to mostly cloudy skies in place, with some more extensive
cloud cover moving into windward locales. Meanwhile, radar imagery
shows scattered to numerous showers moving into windward and mauka
areas. Main short term concern for tonight revolves around rain
chances.

Tonight,
High pressure well north of the state will keep a moderate to
locally breezy trade wind flow in place. MIMIC total precipitable
water imagery shows values in excess of 1.5 inches within the band
of enhanced clouds and showers over and upstream of the state.
This band of moisture will move through the islands overnight,
resulting in a wet trade wind pattern for windward and mauka
areas. With inversion heights rising as the band of deeper
moisture moves through, some of the showers will make it into
leeward areas of the smaller islands as well.

Thursday through Friday,
Moderate to locally breezy trade winds are expected to remain in
place through the remainder of the work week as strong high
pressure persists to the north of the state. The enhanced band of
moisture should be through the eastern islands by daybreak, but
may linger into the morning hours across Oahu and Kauai. Behind
this band of enhanced moisture a drier than normal airmass will
move in, with precipitable water values dropping to between 1.0
and 1.2 inches, and inversion heights lowering to around 6 or 7
kft. As a result, morning showers favoring windward/mauka areas
of Oahu and Kauai, should give way to a drier trade wind pattern
across the entire state Thursday afternoon through Friday.

Friday night through next Wednesday,
High pressure will remain northeast of the state through the
period. Meanwhile, a trough of low pressure (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 troughing is then expected to further amplify
just to the west of the state Sunday through early next week,
before upper ridging builds back in from the east by the middle of
next week.

Moderate to breezy trade wind flow should continue through the
weekend as high pressure remains northeast of the state and the
remnants of Ulika approach from the east. As the remnant trough
of Ulika move over or just south of the islands early next week
and high pressure northeast of the state weakens, the trades will
diminish and shift around to a more east or east-southeast
direction.

As for forecast details, the 00z GFS and 00Z ECMWF are in good
agreement showing deep layer moisture increasing Friday night and
Saturday in response to rising inversion heights resulting from
the upper troughing 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. Deeper 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. This should result in a more active
showery pattern across the islands, with the potential for locally
heavy rainfall. The 00Z ECMWF shows the deeper moisture exiting to
the west on Tuesday, with the 00z GFS keeping the deeper moisture
in place through Wednesday. For now, will split the difference and
show improving conditions across the island chain by the middle of
next week.

&&

.AVIATION...
An area of slightly enhanced low level moisture embedded in the
trades will move across the state overnight into Thursday morning.
AIRMET SIERRA for mountain obscuration has been posted for
windward sections of the Big Island and Maui, and may be needed
for windward areas of other islands later tonight. VFR conditions
will prevail elsewhere. Expect improving conditions for windward
areas by Thursday afternoon as a drier airmass spreads in from the
east.

AIRMET TANGO remains posted for low level turbulence south
through west of the higher terrain of all islands due to the
locally breezy trade winds. Similar conditions are likely to
continue through Thursday.

&&

.MARINE...
High pressure far north of the state will maintain locally strong
trade winds across the area through the remainder of the week. A
small craft advisory remains in effect for the typically windier
areas near Maui and the Big Island, and will likely need to be
extended in time later tonight.

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 Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION/MARINE...Jelsema
AVIATION...Jacobson



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