Gale Watch 4, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
951 AM 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 1040 mb high is centered around
2000 miles north-northwest of Kauai. The pressure gradient remains
tight across the island chain this morning, and is driving breezy
to windy trades across the State. Infrared satellite imagery shows
partly cloudy skies in most areas, with some more extensive cloud
cover in some windward 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.

Today 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 expanded to include
southeast sections of the Big Island, primarily locations to the
south of Volcano where sustained winds are nearing 35 mph with
gusts to 55 mph. The current Wind Advisory continues for Lanai and
portions of the Big Island through tonight, and this will likely
need to be extended through the remainder of the Holiday weekend.
In fact, winds are forecast to strengthen further Saturday night
and Sunday, so additional areas may need to be included in the
Advisory later this weekend.

The airmass over the islands will remain quite dry initially,
with precipitable water values between 0.8 and 1.0 inches today.
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 Thursday,
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
supports 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 get 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 high pressure north of the state will continue to drive
strong trade winds over the area for the next several days.
AIRMET TANGO for tempo moderate low level turbulence is posted
over and immediately south through west of the mountains across
the island chain. Likewise, AIRMET TANGO for strong surface winds
is also posted for Lanai, and the Kohala, Hamakua, Hilo, Puna, and
Kau districts on the Big Island. Big Island terminals, especially
PHKO are experiencing stronger NE winds than normal.

Clouds and showers will favor the windward and mountain areas,
however, strong winds will transport some showers to leeward
areas. Most showers should remain brief and fast moving, with
predominately VFR conditions.

&&

.MARINE...
The very large north swell continues to run higher than the models
had indicated, so the High Surf Warning continues today for north
and east facing shores. The strong winds are also producing high
wind waves, contributing to hazardous conditions. The trend is
down, and currently expecting surf to fall below warning levels by
this evening, and then remain at advisory levels overnight. Will
continue to monitor the buoys and surf reports today.

The strong northeast trade winds will also bring increasing rough
surf to east facing shores which will persist into the new week.

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.

This current swell will slowly subside through tomorrow, before a
new north swell arrives Sunday, which is expected to peak near
advisory levels on Monday, and then subside through Tuesday. A
northwest swell is expected next Wednesday.

A Small Craft Advisory (SCA) is in effect through tomorrow morning
for Big Island Windward waters and through Sunday evening for strong
to near-gale force northeast trade winds and large seas due to a
combination of large wind waves and the large north swell. Trade
winds are expected to remain strong through the first half of
next week, and a new north swell will keep the seas high, so the
SCA will likely be extended further.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Big Island North and East.

Wind Advisory until 6 AM HST Saturday for Lanai-South Big Island-
Big Island North and East-Kohala.

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

Small Craft Advisory until 6 AM HST Saturday for Big Island
Windward Waters.

&&

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