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Hawaiian Islands Synoptic Discussion and Guidance

198
FXHW60 PHFO 200649
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Mon Nov 19 2018

.SYNOPSIS...
Strong high pressure passing north of the islands will bring
increasing trade winds on Tuesday that will remain locally strong on
Wednesday. Winds will gradually weaken and shift to the southeast on
Thursday before becoming light on Friday, with winds remaining light
over the weekend. Brief showers will favor windward areas through
Wednesday, with a few showers spreading leeward with the strong
trade winds. An increase in moisture is expected Wednesday night and
Thursday, and this moisture may linger into the weekend, fueling
increased showers.

&&

.DISCUSSION...
A 1033 mb surface high centered about 1100 miles N of the islands is
supplying local breezy trade winds this evening. Latest satellite
imagery shows broken low clouds approaching the islands from the
ENE, and radar shows abundant small showers over windward waters,
with showers over leeward waters primarily confined to the low cloud
plumes shedding SW from most islands. Mostly cloudy skies prevail
over windward areas, with partly cloudy to mostly clear conditions
leeward.

The main weather highlight over the next 24 to 48 hours will be
increasing trade winds as the high to the N moves SE. Winds are
expected to increase to the point that a Wind Advisory will likely
be required for portions (if not all) of the islands, with winds at
their peak Tuesday night/Wednesday.

Winds are not expected to remain strong for an extended period
however as the high will be moving quickly E while it gradually
weakens later in the week. A front approaching from the NW will
weaken the ridge on Thursday, causing winds to veer to the SE while
gradually weakening. As the front passes N of the islands on Friday,
winds are expected to become light and variable. Thereafter, some
divergence in model solutions, with GFS indicating light winds will
persist into next week while ECMWF guidance depicts a return of
trade winds. The existing forecast indicates a blend of these
solutions, with winds on the lighter side.

The trade winds will deliver randomly spaced showery and stable low
clouds to the islands through Wednesday, with showers most active
during nights and mornings. While showers will focus over windward
areas, the strong trades will send a few leeward on the smaller
islands. Wednesday night and Thursday, moisture associated with a
dissipated frontal boundary (seen moving S of 30N in latest
satellite images) is expected to arrive over the islands from the
NE, likely fueling an increase in showers, again primarily windward.
This increased moisture may linger near the islands into the
weekend, helping to fuel increased showers as winds weaken.
Forecasting the timing and location of showers at that time is
uncertain due to the (potential for) light winds, and the weekend
forecast has limited confidence at this time.

&&

.AVIATION...
A strong high pressure system in the Central Pacific will
maintaining breezy to locally strong trade winds across the Hawaiian
Islands into Thursday. Expect isolated to scattered showers favoring
windward and mountain areas for the next few days.

Breezy trade winds will continue to cause low level mountain wave
turbulence over and immediately south through west of mountains on
all islands below around 080 through the first half of this week.
AIRMET Tango remains in effect for moderate mountain wave turbulence
over and immediately south through west of all islands.
No other AIRMET's in effect or expected.

&&

.MARINE...
Strong to near gale-force trades are expected across the Hawaiian
waters Tuesday through Wednesday due to strong high pressure
positioned north of the state. Seas are forecast to respond and
build to around the advisory level (10 ft) over the exposed waters.
The current small craft advisory will likely be expanded across all
waters later tonight to account for these conditions expected. Winds
and seas will quickly trend back down through the second half of the
week and into the upcoming weekend as the area of high pressure
weakens and shifts eastward in response to a cold front passing to
the north. Confidence begins to lower through the second half of the
weekend into next week as the pattern shifts across the northern
Pacific to a more progressive setup. The GFS solution indicates the
ridge axis will remain overhead locally, which would keep more of a
light and variable pattern in place. The ECMWF, however, shows this
ridge remaining positioned farther north, which would favor light to
moderate trades holding.

A reinforcement out of the northwest is filling in at the Hanalei
and Waimea nearshore PacIOOS buoys this evening and is remaining in
line with guidance. Expect this source to peak late tonight through
Tuesday, then fade through midweek. Surf should remain well below
the advisory levels along north and west facing shores as it peaks.
A gradual downward trend is expected through the second half of the
week.

For the extended, guidance is reflecting a large and broad gale- to
storm-force low setting up across the far northwest Pacific in
response to the aforementioned pattern shift Thursday night through
the weekend. Seas are forecast to climb into the 40-50 ft range
within the associated fetch region focused at the islands within the
300-320 directional band. Although there remains some noticeable
differences between the various solutions as this feature evolves
through this time, all depict the first significant northwest swell
reaching the islands late Sunday through early next week. Specifics
will come later in the week as this scenario evolves and confidence
increases.

Surf along east facing shores will near advisory levels Wednesday
through Thursday due to fresh to locally strong trades expected
locally and just upstream of the state. Surf should slightly lower
Friday into the weekend as the trades weaken.

Small surf will continue along south facing shores with mainly
background southern Pacific pulses moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM Wednesday for Kauai Leeward
Waters, the Kauai Channel, Oahu Leeward Waters, the Kaiwi
Channel, Maalaea Bay, the Pailolo Channel, the Alenuihaha
Channel, Big Island Leeward Waters and Big Island Southeast
Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Bohlin
MARINE...Gibbs

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



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