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Hawaiian Islands Synoptic Discussion and Guidance

045
FXHW60 PHFO 180655
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
855 PM HST Sat Aug 17 2019

.SYNOPSIS...
The ridge of high pressure north of the Hawaiian Islands will
weaken through Sunday night, resulting in lighter trades the next
couple of days. Locally strong trades will return by the middle
of next week as the ridge of high pressure ridge rebuilds north of
the area. Showers riding in with the trades will favor the
windward and mountain areas, especially during the nights and
early mornings. Rainfall amounts, though, will be light.

&&

.DISCUSSION...
The mainly moderate trades blowing this evening will be trending
lighter through Sunday night as the ridge of high pressure located
420 miles north of Kauai weakens. It appears Monday will have the
lightest winds. With the lighter trades, expect daytime sea-
breeze to arise, especially along the lee and interior areas of
the smaller islands. This will lead to some afternoon clouds over
these areas, but not a whole lot of showers are expected from
them as a mid level ridge will limit their vertical extent to no
higher than 10k feet on both Sunday and Monday afternoons, meaning
isolated and spotty at best. Leeward Big Island will get their
usual afternoon and evening clouds each day with only isolated to
scattered showers at best from these clouds.

A disturbance passing south of the islands Tuesday night into
Wednesday may bring some added trade wind moisture to especially
the eastern islands. But otherwise, this rather dry trade wind
pattern will continue.

A boost in the trades will commence as early as Monday night,
building to moderate to locally strong by Wednesday. This locally
strong trade wind regime will hold through the second half of
next week with a 1025 mb surface high sitting far north of the
islands.

&&

.AVIATION...
A ridge of high pressure north of the state will keep moderate
trade winds in place through Sunday. Low clouds and showers will
bring brief MVFR cigs/vsbys to windward and mauka areas, mainly
during the overnight and early morning hours. Predominantly VFR
conditions are expected during the next 24 hours.

No AIRMETs are currently in effect.


&&

.MARINE...
Trades are forecast to weaken into the light to moderate category
Sunday through Monday as the gradient relaxes over the state.
Localized land breezes through the overnight and early morning
periods followed by late morning and afternoon sea breezes will be
possible near the coasts through this time. Guidance remains in
good agreement Tuesday through the second half of the week and
depicts a strengthening ridge north of the islands, which supports
a return of fresh to locally strong trades (advisory-level winds
in the typically windier locations between Maui County and the Big
Island) by Wednesday.

Surf along south facing shores will remain small through Monday,
with mainly background energy continuing out of the southwest and
southeast. An upward trend is anticipated beginning Monday night,
as one of the largest south swells of the season begins to build
across the Hawaiian waters. Surface analysis charts showed a
compact storm-force low bottoming out at 956 mb southeast of New
Zealand last Monday through Tuesday within Hawaii's swell window.
ASCAT passes reflected this and showed a pocket of storm-force
winds nosing northward through this period within the 190-200 deg
bands relative to the islands. Altimeter passes within this area
showed seas ranging from 35 to 42 ft. This swell has already moved
through the nearshore PacIOOS buoy at Pago Pago, which showed
seas slightly higher than predicted by guidance. Energy levels
within the lower frequency bands closely resembled the levels
observed at this buoy for our recent July 14th warning-level
event. As a result, surf heights will likely near/reach warning
levels along south facing shores through the peak of the event
Tuesday through Tuesday night and may not drop below advisory
levels until Thursday.

The small westerly swell associated with recent typhoon activity
in the western Pacific will steadily trend down Sunday through
Monday. Surf will respond and lower along the exposed coasts from
this source.

Surf along north and west facing shores may slightly trend up
through the second half of the week as a small northwest swell
builds down the island chain. Guidance remains in good agreement
and depicts a gale evolving around 2000 miles northwest of the
islands near and just west of the Date Line early this week.

Surf along east facing shores will trend up by midweek as the
trades strengthen locally and upstream of the islands.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


H Lau...Jelsema...Gibbs

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