Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 210628
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Mon Nov 20 2017

.SYNOPSIS...
A surface low far northeast of the islands will continue to move
away from the region, while an upper level trough will remain
over the area through this weekend. Low clouds and showers will
remain focused along north facing slopes and coasts tonight. A
strong new surface high will build far northwest of the area on
Wednesday, then move southeast into early next week. Gusty trade
winds are expected Thanksgiving and Friday, before easing slightly
this weekend. Expect windward trade showers, occasionally
spreading leeward.

&&

.DISCUSSION...
The pressure gradient between a surface high far northwest of the
islands and a surface low far northeast of the region is
producing light to locally moderate northerly winds. This low-
level flow continues to advect cool and relatively dry air into
the islands. Aloft, loops of water-vapor satellite imagery
indicate middle- and upper-tropospheric troughing extends
northeast from the islands. This upper-level troughing is having
little significant affect on atmospheric stability across the
state. A significant low-level temperature inversion of around 5
thousand feet was observed on Kauai, and increased to near 7
thousand feet over the eastern Big Island in the early afternoon
balloon soundings.

The northerly winds continue to transport isolated showers over
north and northeast facing slopes and coasts early this evening
according to loops of satellite imagery and radar reflectivity
data. However, rainfall will continue to be relatively light over
most areas. Note that there is a chance the north winds, and some
slightly higher inversion heights could boost rainfall along
portions of the north and northeast slopes of the Big Island later
tonight and early Tuesday morning.

The trough aloft continues to produce strong west winds over the
summits of Mauna Kea and Mauna Loa on the Big Island of Hawaii. As
a result, a High Wind Warning has been issued for the summits
through early Tuesday morning. The winds are expected to gradually
subside as the trough weakens later tonight and Tuesday. We
expect to issue a Wind Advisory for the summits on Tuesday once
the High Wind Warning is lowered.

Through Wednesday, we anticipate the surface low far northeast of
the area will move northeastward and weaken, while the surface
high moves southeastward and weakens. Winds over the islands will
gradually shift out of the northeast Tuesday. The forecast
guidance shows a low-cloud band moving down from the northwest and
reaching Kauai County by Tuesday afternoon. This feature is
expected to then move over Oahu and Maui County Tuesday night,
before pushing down over the Big Island by Wednesday evening. This
cloud band will bring an increase in low clouds and showers,
especially over windward areas.

Starting Wednesday night, a new strong surface high far northwest
of the islands will start moving southeast. The northeast trade
winds over the islands will strengthen and become breezy. Wind
speeds will likely decrease again starting Saturday, but trade
winds will likely persist through early next week.

&&

.AVIATION...
Cool northerly winds will continue to push bands of clouds and
showers over the islands tonight. The highest coverage for shower
activity will occur over mountain areas and the northern slopes of
all islands. Relatively drier air will lead to higher than normal
cloud bases. VFR conditions prevail with only isolated MVFR in
moderate showers.

No AIRMETs are in effect and none are expected.

&&

.MARINE...
Surf along the north-facing shores of the state remains elevated.
Evening buoy observations show little change during the past
several hours, and heights remain above Wave Watch III guidance.
The effect of the low bias in the guidance is that surf height
will remain above High Surf Advisory (HSA) criteria through
Tuesday morning. The north swell is expected to diminish Tuesday
afternoon.

A Small Craft Advisory (SCA) remains in effect for all Hawaiian
waters except Maalaea Bay and waters south of the Big Island due
to the large north swell causing the combined seas to be above the
10-foot threshold. With wave heights running above guidance, the
SCA has been extended through Tuesday morning.

Winds over the coastal and offshore waters remain generally from
the north due to a low pressure system northeast of the state.
The low is in the process of weakening and moving northeastward
away from the island chain. As this occurs, a high pressure ridge
will slowly build north of the forecast area through the week.
Winds will slowly veer toward a trade wind direction and increase,
reaching SCA levels Wednesday with near gale-force winds possible
in some of the channels by Thursday.

In addition to the increasing trades, a new north-northeasterly
swell is forecast to arrive later this week. The swell is
currently projected to be a few feet larger than the current swell
with a slightly longer period. This will translate to possible
warning level surf for north facing shores, building late
Wednesday and peaking on Thanksgiving Day. The swell direction
will result in some wrap into the east facing shores so warning
level surf is a possibility for these areas as well.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until noon HST Tuesday for north facing shores
of Niihau, Kauai, Oahu, Molokai, Maui, and the Big Island.

High Wind Warning until 6 AM HST Tuesday for the Big Island
Summits.

Small Craft Advisory until noon HST Tuesday 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-Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters.

&&

$$


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