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Hawaiian Islands Synoptic Discussion and Guidance

731
FXHW60 PHFO 182001
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST Thu Oct 18 2018

.SYNOPSIS...
Gentle to locally moderate trade winds will persist through the
next few days. Showers will remain focused across windward slopes,
while instability near the Big Island will likely trigger leeward
and interior showers and a few thunderstorms this afternoon. An
upper level trough slowly passing overhead from west to east on
Friday through Sunday will produce unstable conditions and spread
the risk of thunderstorms to much of the island chain. Trade winds
will rebuild on Sunday and early next week, though unstable
conditions may linger over portions of the state.

&&

.DISCUSSION...
A gentle to locally moderate trade wind flow remains in place,
along with slightly unstable conditions around the Big Island. A
stalled front sitting about 500 miles north of Kauai has weakened
the local pressure gradient and resulting trade winds. Winds
should remain strong enough to inhibit sea breezes across all but
the sheltered leeward areas of most islands, with widespread sea
breezes in store on leeward Big Island. A nearly stationary mid-
to upper-level trough remains overhead, sending high clouds over
most islands and producing some instability. While low level
ridging is strong enough to maintain modest stability over the
western half of the state, a stubborn pocket of moisture with
precipitable water values of 1.75 to 2 inches is contributing to
instability around the Big Island. This will fuel afternoon clouds
and a few thunderstorms on the Big Island. The rest of the state
will experience mainly windward showers, with afternoon clouds and
a few showers over leeward terrain.

On Friday and Friday night, trades will remain somewhat weak, and
unstable conditions will develop over portions of the state. A
shortwave aloft, currently located about 750 miles northwest of
Kauai, will drop into the mid- to upper-level trough already in
place over the islands. As the shortwave trough passes over the
western end of the state, isolated thunderstorms could develop
over all islands, especially over interior terrain during the
afternoon. A lack of deep moisture will likely limit chances for
widespread showers to windward slopes from Kauai to Maui, while
lingering moisture will maintain higher rainfall chances over the
Big Island and perhaps east Maui.

Gentle to moderate trades and unstable conditions will hold over
most the state on Saturday. The latest runs of the GFS and ECMWF
are showing some differences in the handling of the mid- to
upper-level trough. The GFS keeps the feature moving eastward,
while the ECMWF suggests that it will stall over the central
islands. Expect unstable conditions to persist. The trades will
be strong enough to produce showers over windward slopes.
Thunderstorms could pop up anywhere during this time, mainly over
interior terrain during the day and windward terrain at any time.
With 0-6 km wind shear climbing to around 30 kt and ample
instability, strong thunderstorms will be possible over central
and eastern portions of the island chain.

Building trade winds and decreased instability are expected on
Sunday. The GFS and ECMWF differ, so there is some uncertainty.
The GFS pushes the now cut- off mid- to upper-level low east of
the Big Island, while the ECMWF drifts the gradually weakening
feature just southwest of the state. The GFS solution would
suggest lingering instability over the Big Island and typical
trade wind weather elsewhere. The ECMWF solution points toward
decreasing chances of heavy showers and thunderstorms over most
islands. For now, the forecast leans heavily toward the GFS.

By Monday, trade winds will strengthen to locally breezy levels.
Uncertainty in the location of the low aloft remains, with the
ECMWF pointing toward a wetter and less stable pattern.

&&

.AVIATION...
Light trade winds will continue today, with clouds and showers
remaining focused over windward and mountain areas. While VFR
conditions will prevail over most areas, TEMPO MVFR conditions are
possible mainly over windward and mountain areas of Maui and the
Big Island. AIRMET SIERRA is in place for these areas for mountain
obscuration, and have extended that into this afternoon with
additional cloud development possible. Molokai and Maui were
recently dropped from the AIRMET due to improved conditions on
those islands.

Winds are expected to weaken further over the next 24 hours, so do
not expect any significant turbulence. Thunderstorms are possible
over the Big Island this afternoon, and again tomorrow afternoon,
which could bring brief IFR conditions.

&&

.MARINE...
Trade winds are forecast to remain weak through Friday night.
The trades may gradually strengthen from Saturday into early next
week. The current forecast does not have Small Craft Advisory
(SCA) conditions over any of the coastal waters or channels
through Sunday night. SCA conditions may develop across the
typically windy waters adjacent to the islands of Maui County and
the Big Island early next week.

Unstable atmospheric conditions are forecast to develop over
parts of the state beginning Friday night. This may cause locally
heavy downpours, and allow thunderstorms to form across some
areas this weekend. Mariners should monitor future weather
forecasts as this situation evolves.

The current north northwest swell will subside through this
weekend. The weak trades will cause surf to slowly decrease along
most east facing shores through Friday. However, some east facing
shores with a northerly exposure may continue to see small
breakers from the north northwest swell energy during the next
couple of days.

A new long period south swell is expected to Friday and peak this
weekend. Based on the latest guidance, surf will likely reach the
advisory criteria along south facing shores starting Saturday.
There is also a possibility surf may approach the High Surf
Warning threshold along south facing shores this weekend, before
the swell begins to gradually decline early next week.

See the latest Collaborative Nearshore Swell and Wind Forecast for
Oahu (SRDHFO), which was updated earlier Wednesday afternoon, for
more details on swells and surf.

&&

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

&&

$$

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

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



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