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Hawaiian Islands Synoptic Discussion and Guidance

854
FXHW60 PHFO 211350
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Tue Nov 21 2017

.SYNOPSIS...
Surface low pressure 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. A diffuse band of low
clouds and showers will move down across the islands from this
afternoon through Wednesday. A new surface high building far
northwest of the area on Wednesday will move southeast into early
next week. Gusty trade winds are expected Thanksgiving Day and
Friday, with trade showers over windward facing slopes and
coastlines, occasionally spreading leeward on the smaller islands.
The trades are forecast to weaken slightly Saturday and Sunday.

&&

.DISCUSSION...
The pressure gradient between a 1034 mb surface high far
northwest of the islands and areas of low pressure 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. Strong low-level temperature inversions of around 5500
thousand feet on Kauai, and around 8000 feet over the eastern Big
Island were observed in the early morning balloon soundings.
Precipitable water values were below 1 inch, which was indicative
of the dry conditions across the area.

The northerly winds continue to transport isolated showers over
some north and northeast facing slopes and coasts early this
morning according to loops of satellite imagery and radar
reflectivity data. However, rainfall totals remain relatively
light over most areas. Note that the leading edge of a diffuse
band of broken low clouds and scattered showers is evident about
250 miles north-northwest of Lihue. This leading edge appears to
be moving toward the south-southeast at about 15 mph. The forecast
guidance shows this band reaching Kauai County this afternoon.
This feature is expected to move over Oahu this evening and then
across Maui County later tonight, before pushing down over the Big
Island by late Wednesday. This band will bring an increase in low
clouds and showers as it passes down the island chain, especially
over windward sections.

The trough aloft continues to produce strong west winds over the
summits of Mauna Kea and Mauna Loa on the Big Island of Hawaii
early this morning. The winds have decreased slightly during the
past few hours, so we have cancelled the High Wind Warning for
the Big Island summits, and replaced it with a Wind Advisory.
This Wind Advisory is currently in effect through this morning,
but may need to be extended into this afternoon if the winds
remain stronger than forecast.

Through tomorrow, we anticipate the surface low pressure far
northeast of the area will continue departing toward the
northeast, while the surface high moves southeastward and weakens.
Winds over the islands will gradually shift out of the northeast
later today and tonight. Starting Wednesday night, a new strong
surface high far northwest of the islands will start moving
southeast. The trade winds will strengthen and become breezy
starting Thanksgiving Day. Low clouds and showers carried by the
trades will favor windward facing slopes and coastlines. However,
the strong trades will occasionally transport brief showers
leeward on some of the smaller islands. The leeward Big Island is
expected to have increasing clouds and a chance of showers each
afternoon, followed by clearing late at night. Trade wind speeds
will decrease slightly starting Saturday, but the trades are
forecast to persist through early next week.

&&

.AVIATION...
Cool northerly winds will continue push bands of clouds and
showers over the islands today. 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.
Overnight buoy observations, both nearshore and to the north of
the state, show little change during the past several hours, and
actual wave heights remain above Wave Watch III guidance. The
effect of the low bias in the guidance is that surf height along
north-facing shores will remain above High Surf Advisory (HSA)
criteria through this afternoon, and we have extended the HSA
through 6 PM.

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 lingering north swell causing the combined seas to be
above the 10-foot threshold. With wave heights running above
guidance, the SCA has also been extended through this afternoon.

Moderate to fresh northeast winds over the coastal waters today
and tonight are expected to increase starting on Wednesday, as
high pressure builds northwest and north of the state. Winds will
likely reach SCA levels by Wednesday afternoon or evening in the
channels, with widespread SCA winds and possible gales in the
channels by Thursday, continuing into Friday. Fresh to strong
trade winds are expected to continue into the weekend.

In addition to the increasing trades, a new north swell is
forecast to arrive Wednesday, then peak Wednesday night into
Thursday. The swell is currently projected to be a few feet
larger than the current swell with a slightly longer period. This
will translate to likely 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 advisory level surf is a possibility for these areas
as well.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST his evening for north facing
shores of Niihau, Kauai, Oahu, Molokai, Maui, and the Big Island.

Wind Advisory until noon HST today for the Big Island Summits.

Small Craft Advisory until 6 PM HST this evening 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.

&&

$$

Discussion...Houston
Aviation...Bohlin
Marine...Jacobson

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



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