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Hawaiian Islands Synoptic Discussion and Guidance

148
FXHW60 PHFO 230142
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
342 PM HST Fri Jan 22 2021

.SYNOPSIS...
Strong and gusty trade winds will become especially strong over the
weekend, delivering passing clouds and showers, some briefly heavy,
mainly dampening windward areas. More widespread rainfall may
develop early next week as winds remain strong and shift to the
southeast.

&&

.DISCUSSION...
Winds did eventually nudge upwards today, so the adjustments to
the winds for today were in line. Have made some tweaks to the
winds for tonight through tomorrow night to better align some
areas with the Wind Advisory that has now been posted for several
areas. Winds are likely to remain elevated for several of these
areas into the start of the new week, so expect the Wind Advisory
to be extended, and possibly expanded as the event unfolds. While
the Wind Advisory has been issued for portions of most of the
islands, advisory levels winds are most likely over and
immediately downwind of the mountains. Otherwise, no changes in
the grids or the forecast philosophy with the afternoon page.

The morning surface analysis continued to depict a 1035 mb high
to the northeast of the islands, along with a trough and a dying
stationary front to the northwest. The ridge associated with the
high is expected to nudge closer to the islands tonight,
tightening the pressure gradient over the island, which should
cause an uptick in winds. As the trough and front to the
northwest move to the east over the weekend, and a low potentially
develops along these boundaries, we can expect the wind direction
to become a little more southeasterly by early next week. At the
same time, the high to the northeast will be reinforced over the
weekend and strength, causing the winds to increase further.

A broad mid to upper level low to the south of the state continues
to drift away from the islands. Some mid, upper and layered clouds
associated with the system are found just outside the coastal
waters to the south of the Big Island.

Cooler than normal mid-level temperatures associated with the low
are supporting a somewhat unstable island atmosphere. Radar wind
profile data continues to show low level moisture riding in on the
trades extending upwards to around 10 thousand feet. Combined
with the stronger trade winds mentioned above, orographic forcing
is helping to generate showers over and immediately downstream of
the terrain. Meanwhile, many leeward communities remain mostly
dry.

As the winds become more southeasterly, a moisture convergence
band extending southeast from the low creeps towards the islands
from the south. The latest guidance indicates this band will move
over portions of the islands from Monday into Wednesday, bringing
precipitable water values close to 2", layered cloudiness, and
widespread rainfall. Deep moisture and modest instability are
expected, so the chance of thunderstorms appears too low to
mention thus far. A drying trend is loosely expected thereafter as
the band shifts west of the area. High pressure to the distant
northeast will continue to support locally strong trade winds.

&&

.AVIATION...
A strong surface high will remain far northeast of the area
through the next few days, so locally strong easterly trade winds
will continue. Sustained speeds of 30kts or greater tonight and
tomorrow in the Alenuihaha Channel and south of the Big Island
could warrant an AIRMET Tango. Low-level wind shear will also need
to be closely monitored throughout this period.

Showers will continue to favor eastern sections of the islands
during the night and early morning hours. VFR conditions will
prevail across the main Hawaiian Islands throughout the period,
with the exception of brief MVFR conditions in any heavier showers
that do occur.

AIRMET Tango remains in effect for moderate low-level turbulence
in areas southwest to west of the mountains of all islands. The
persistence of breezy to strong easterly trade winds across the
islands will likely allow for this low-level turbulence to
continue well into the weekend. AIRMET Tango also remains in
effect moderate turbulence between FL250 and FL300, but Ellrod
turbulence index values over the islands should become less
favorable over the next few hours.

&&

.MARINE...
A surface high far northeast of the area is generating moderate
to locally strong trade winds. The high will strengthen Saturday,
increasing trade winds across the entire area through the first
part of next week. A Small Craft Advisory is in effect for all
coastal waters for strong to near gale force trade winds as well
as building seas from the trade wind swell and a large west-
northwest swell.

Offshore NDBC buoy 51101 just registered at swell height of 7
feet at 21 second period which means the long period forerunners
of the new northwest swell should arrive tonight for Kauai,
overnight to early morning hours for Oahu, and in the morning for
Maui. A High Surf Advisory (HSA) has been issued for exposed north
and west facing shores of the smaller islands starting tomorrow
morning. This swell is expected to peak Saturday with an 18 second
period and high- end advisory or low- end warning level. The more
westerly direction of this swell will allow west facing shores of
the Big Island to experience advisory level surf, likely later in
the day Saturday. Swell and surf will lower gradually Sunday and
Monday, then become rather small for this time of year on Tuesday
and Wednesday.

Trades will strengthen this weekend. Rough, short- period surf
will gradually rise and a HSA has been issued for exposed east
facing shores starting tomorrow. The HSA for east facing shores
may need to be extended as strong trades persists for several
days.

Small, mainly background, southerly swells are expected over the
next several days. A small bump in south-southwest surf is
expected to gradually fill in Monday, peak Monday night and
Tuesday, then lower Wednesday through Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 AM HST Sunday for Kauai Leeward-Kauai
Mountains-Oahu South Shore-Oahu Koolau-Olomana-Waianae Mountains-
Molokai Windward-Lanai Makai-Kahoolawe-Maui Windward West-Maui
Leeward West-Windward Haleakala-Leeward Haleakala-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

High Surf Advisory from 6 AM Saturday to 6 AM HST Sunday for east
facing shores of Kauai-Oahu-Molokai-Maui-Big Island.

High Surf Advisory from 6 AM Saturday to 6 AM HST Sunday for north
and west facing shores of Niihau-Oahu-Molokai-Big Island.

High Surf Advisory from 6 AM Saturday to 6 AM HST Sunday for north
facing shores of Maui.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters-

&&

$$


DISCUSSION...M Ballard
AVIATION...Donaldson/Vaughan
MARINE...Almanza

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



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