Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 201939
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
939 AM HST Thu Jul 20 2017

.SYNOPSIS...
A persistent ridge to our north will keep breezy trades blowing
through Friday. The trades will ease this weekend as the ridge
weakens. Tropical moisture will overspread the islands this
weekend, leading to higher humidity and an increase in showers.
The remnants of Fernanda are expected to pass just north of the
islands on Monday, likely weakening our winds, and introducing
muggy conditions.

&&

.DISCUSSION...
A ridge of high pressure extends from a 1031 mb high near 40N180W
through 35N155W and 33N140W, far north of the main Hawaiian
islands. This ridge has been a fixture with our weather for the
past few days, keeping trades on the breezy side and pushing
clouds and showers across windward and mauka areas. We expect this
pattern to persist through Friday. Troughing north of the ridge
will dig southward by this weekend, putting a dent in the ridge
between 160W and 180W, which will result in weakened trades for
the islands.

Tropical storm Fernanda is about 1000 miles east of Hilo as of
this writing, weakening as it is poised to cross 140W. Satellite
loop shows skies across the main Hawaiian Islands are clearing
somewhat this morning, but increased tropical moisture ahead of
Fernanda is noted east of about 152W. Fernanda is forecast to pass
north of the islands on Monday, but the eastern edge of the low
cloud field which merges with that of Fernanda`s will reach the
eastern islands tonight. Also, models show an increase in deep
moisture for the eastern end of the state on Friday which will
bring an increase in trade wind showers there, with dry trade
wind weather expected to continue across the central and western
islands.

While trade winds are forecast to decrease Friday night through
the weekend, we still expect increased clouds and showers
associated with Fernanda to favor windward and mauka areas.
However, leeward showers may develop at times during the
afternoons if local sea breezes form.

As the remnants of Fernanda pass by to the north of the islands
on Monday, trade winds will weaken further and sea breeze
development is expected across most areas. This light wind regime,
with overnight land breezes and daytime sea breezes, may continue
into Tuesday before the remnants of Fernanda pass by and high
pressure strengthens north of the state. Breezy trade winds will
then return by the middle of next week. Models show the main
area of moisture associated with Fernanda will pass north of the
islands on Monday but, with the humid conditions and light winds
in place, afternoon and evening showers are expected particularly
over the interior of the islands during the first half of next
week. A drier, less humid, and more stable airmass is then
expected to return by the middle of next week.

&&

.AVIATION...
Moderate to breezy trade winds will persist as a high pressure
ridge remains far to the north of the islands. Brief MVFR
conditions, lower ceilings and passing showers, will mainly
affect windward areas.

With the occasionally strong trades, an AIRMET is currently posted
for moderate low level turbulence over and lee of all mountains.

&&

.MARINE...
Small Craft Advisory (SCA) conditions can be expected for waters
south of the Big Island, all waters around Maui County, and waters
south of Oahu through early friday morning, and the typically
windier waters around Maui County and the Big Island through early
Saturday morning.

A rapidly weakening Tropical Storm Fernanda is forecast to pass
just north of the islands from Sunday through late Monday. The
wind forecast at this time continues to carry light to moderate
trade winds.

A High Surf Advisory is in place for the east facing shores of
the Big Island and Maui through this afternoon due to a mid- to
long- period east swell associated with a captured fetch from when
Fernanda was a major hurricane. Guidance shows this swell
subsiding later today, but continues to indicate reinforcing east
swells from Fernanda arriving along the east facing shores from
as early as Friday. High Surf Advisory conditions for the east
facing shores will become likely again for the Big Island Friday
or Saturday and spread west to the smaller islands. The current
High Surf Advisory will need to be revisited later today.

The largest high tides of the month (king tides) will occur each
afternoon during the next several days. Note that these abnormally
high tides combined with wave run up due to the east swell
produced by Fernanda may result in coastal flooding in some areas,
especially from Friday into Monday.

Elsewhere, no significant swells are expected. Small south swells
will continue to produce small background surf along south facing
shores into next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 AM HST Friday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maui County
Leeward Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Kinel
MARINE...Eaton


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