Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 240137
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
337 PM HST Sun Apr 23 2017

.SYNOPSIS...
High pressure to the northeast of the islands, with a ridge to the
north will maintain a trade wind flow into the middle of the week.
Limited shower activity is expected into Tuesday. From Wednesday
and into the weekend, increasing showers are expected, with the
potential for unsettle weather at the end of the week.

&&

.DISCUSSION...
Showers continued to fade over the islands of Maui County and Oahu
this afternoon. Have knocked PoPs and associated fields down for
those areas through tonight. The east side of the Big Island also
ended up staying drier, so dropped the PoPs there as well. Showers
have been more persistent on Kauai with radar suggesting a few
briefly heavier showers. Expect conditions to improve some over
Kauai overnight, but the possibility for briefly heavy showers
remains in the forecast for this evening.

A mid to upper level trough remains on the west side of Kauai this
afternoon. There was a break in the high clouds over the islands
for most of the day, however another batch of high clouds is
moving over the southern end of the state this afternoon. The
trough is expected to move to the east, which should help to push
the high clouds to the east of the islands on Monday.

The afternoon sounding at Lihue had a 500 MB temperature of -8.5
which is near normal for this time of year, while Hilo was -5.7
which is on the warm side of normal. As the upper level trough
moves to the east, we could see some slightly colder temperatures
at 500 MB, but with limited moisture riding in on the trades, we
don`t expect the slightly colder temperatures to impact shower
activity.

A front approaching from the northwest will impact the western
end of the surface ridge north of the islands tonight and Monday.
This will help to veer the winds to the east or east southeast.
Increasing trade showers are possible Wednesday night and
Thursday, with a trend toward an unsettled weather pattern toward
week`s end.

A significant surface trough or front is forecast to be near or
over the islands at the end of the week, supported by an
amplifying trough aloft. A colder upper level airmass could help
to significantly destabilize the island atmosphere. Models are
currently honing in on the Thursday night through Saturday time
frame for the greatest instability, with the ECMWF now more
aggressive in destabilizing the atmosphere. The forecast continues
to reflect wet conditions across most of the islands at the end of
the week, but it is still too early to refine the forecast.
Forecast confidence remains low due to poor run- to- run and
model- to- model consistency.

&&

.AVIATION...
Expect VFR conditions to dominate tonight and Monday. A diffuse
band of moisture that affected the western end of the state today
is dissipating, allowing a drier trade wind flow to fill in from
the east. AIRMET Sierra remains in effect over windward terrain of
Kauai, but it will likely be dropped before sun down. An upper
level trough centered about 250 nm west of Kauai will generate a
few high clouds, but as a low level ridge builds in tonight, the
entire state will be stable, with isolated -SHRA confined to
windward slopes.

A stable easterly trade wind flow prevails from the Big Island to
Oahu and will spread to Kauai later today. The afternoon Hilo
sounding showed an inversion near 6000 ft, and since east winds at
850 mb (roughly 5,000 ft) will increase tonight, some low level
turbulence will occur to the west of terrain on the Big Island and
Maui County. However, we do not anticipate needing AIRMET Tango
tonight, and conditions will be border line on Monday.

&&

.MARINE...
The Small Craft Advisory (SCA) has been extended through Monday
night for coastal water zones surrounding the Big Island and some
areas around Maui County. See the advisory list below for the
affected areas. The SCA extension was primarily due to strong east
southeast winds interacting with the island terrain and is
adjusted slightly from the typical windy areas due to a slight
veering in the normal trade wind direction. Wind speeds will
maintain through at least Monday night with possible extension for
the same or similar zones into Tuesday.

A number of mainly small swells will affect the state this week
and High Surf Advisories are not expected for at least the first
half of the week. Wind waves, along with a little bit of a trade
wind fetch upstream, will bring small but choppy waters across the
east facing shores through most of the week. Small north and
northwest swells will fade in and out over the next week as well.
Small, long period south swells will also fade in and out this
week.

A stationary upper trough with an axis approximately 250 nm west
of Kauai will trigger scattered thunderstorms in the northwest
offshore waters today and tonight. These thunderstorms are not
expected to reach the western coastal waters zones.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for Maui County
Windward Waters-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...Wroe
MARINE...Foster



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