Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 250155
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Fri Nov 24 2017

.SYNOPSIS...
High pressure north of the State will keep breezy to windy trades
over the islands through the Holiday weekend and into early next
week. Passing showers will favor windward and mauka areas, with
a few stray showers reaching leeward areas of the smaller islands
under the robust trades. A wet trade wind weather pattern is
expected to develop on Sunday and continue through much of next
week, as deep tropical moisture spreads into the area from the
southeast. The wettest conditions are expected over the eastern
end of the island chain.

&&

.DISCUSSION...
Currently at the surface, a strong 1038 mb high is centered
around 1650 miles north-northwest of Kauai. The pressure gradient
remains tight across the island chain this afternoon, and is
driving breezy to locally windy trades across the State. Visible
satellite imagery shows partly cloudy skies in most areas. Radar
imagery shows scattered light showers rolling into windward areas
on the robust trades, with a few of these showers making it into
leeward areas from time to time. Main short term concerns focus on
the rain chances and the strong trades over the next few days,
with longer term concerns revolving around the potential for heavy
rainfall for portions of the State much of next week.

Tonight through Sunday,
The strong high to the north-northwest of the State will drop
southward through Saturday, then shift eastward along 34/35N
Saturday night and Sunday. This will keep breezy to windy trades
blowing across the island chain through the remainder of the
Holiday weekend. The Wind Advisory has been cancelled for the
North Kona District and Big Island south and east where winds
have dropped below Advisory levels. The Advisory remains in effect
for Lanai and over and downwind of the Kohala mountains on the
Big Island however, and has been extended in time through the day
on Sunday. The strongest winds are expected to move over the
island chain Saturday night and Sunday, and the Wind Advisory may
need to be expanded to include other areas later this weekend.

The airmass over the islands will remain quite dry initially,
with precipitable water values running between 0.8 and 1.0 inches
based on the 00Z soundings from PHTO and PHLI. Under the
persistent northeasterly trades, the airmass will slowly and
steadily moisten up tonight through Sunday, and deep tropical
moisture with a connection to the intertropical convergence zone
is expected to lift northwestward and into the eastern end of the
State during the day on Sunday. Clouds and showers will continue
to favor windward and mauka areas through the period, with a few
showers reaching leeward areas from time to time due to the strong
trades. Shower coverage should increase each day through the
period, with some heavy rainfall possible over the Big Island by
late in the day Sunday. In fact some snow could begin to fall at
the Big Island Summits as early as Sunday afternoon, and a winter
storm watch will likely be required tonight or Saturday.

Sunday night through next Friday,
There remains some differences between the GFS and ECMWF with
regard to moisture transport shifting northwestward into the State
initially, but both models trend toward a wet trade wind pattern
by mid to late next week. The GFS ensembles more closely support
the ECMWF solution, so will lean the forecast in its favor.
Overall, deep tropical moisture is expected to remain over the
eastern end of the State Sunday night through late next week, so
it looks very wet here, and a Flash Flood Watch may eventually be
needed. Across the smaller islands an old frontal band will be
shifting southward and enhancing showers Sunday night through
early next week, so it looks like a wetter than normal trade wind
pattern here as well, but rainfall amounts are not expected to be
as high. By the middle to latter part of next week, it appears
that the entire State will be overspread by deep tropical
moisture, along with the threat for heavy rainfall. Moderate to
breezy trade winds are expected to prevail through the period, and
may be locally windy at times.

Additionally, with deep moisture in place over the Big Island
through the entire period, some hefty snow amounts may occur over
the summits, with accumulations likely measured in feet.

&&

.AVIATION...
Strong trade winds are expected to continue through the weekend,
with a slight increase tomorrow. AIRMET TANGO for low level
turbulence will remain posted over and immediately south through
west of the mountains, with mention of tempo moderate to isolated
severe cases likely. AIRMET TANGO for strong surface winds is also
posted for Lanai and portions of the Big Island. The strong north
winds seen at PHKO today will ease this evening, and a more
typical land/sea breeze wind pattern should be expected tomorrow
as the prevailing trade winds veer a little more east.

Otherwise, clouds and showers will continue to stream over the
islands, focusing along the windward and mountain areas. The
stronger winds will help push some showers to leeward areas.
Diurnal cooling should help lead to an increase in showers
tonight, and AIRMET SIERRA for mountain obscuration may become
necessary if the showers become more widespread.

&&

.MARINE...
The current north swell has been on the decline today, with
periods growing shorter. Surf along the north and east facing
shores of the affected smaller islands has fallen to advisory
levels. A High Surf Advisory is in effect for those areas through
10 am Saturday. The strong trade winds however are bringing rough
conditions to the east facing shores, and an increase in winds is
expected Saturday and Saturday night, persisting into Sunday. The
advisory may need to be extended for east facing shores as a
result.

The combination of the north swell and the strong trade winds has
produced rough and varied surf along the north and east facing
shores of the Big Island. Impacts like water on roadways and in
nearby parks have been felt on the Big Island in places near
Honokaa, Richardson and in downtown Hilo along Bayfront Dr. The
High Surf Warning continues for the Big Island through 10 am
Saturday. The strong trade winds mentioned above will continue to
impact the Big Island as well, and could necessitate an advisory
Saturday and beyond.

The strong trade winds will peak Saturday night through Sunday,
and could reach gale force in the Pailolo and Alenuihaha Channels
as well as Maalaea Bay. A Gale Watch has been posted for those
waters starting Saturday night, with the Small Craft Advisory in
effect until then. Other waters remain under the Small Craft
Advisory for a combination of winds and seas through the weekend.

A new north swell is expected to arrive on Sunday, peak Monday
near advisory levels, and subside through Tuesday. This swell is
expected to be smaller than this current event, but the strong
trade winds will be a factor in the condition of the surf along
north and east facing shores.

The swell and strong winds will continue to bring large waves
breaking in north facing harbor entrances and possibly surges in
those harbors, so a Marine Weather Statement remains in effect.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 10 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala.

Wind Advisory until 6 PM HST Sunday for Lanai-Big Island North
and East-Kohala.

High Surf Warning until 10 AM HST Saturday for Big Island North
and East.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

Gale Watch from Saturday evening through Sunday afternoon for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel.

Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

$$

DISCUSSION...Jelsema
AVIATION...Eaton
MARINE...M Ballard



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