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Hawaiian Islands Synoptic Discussion and Guidance

181
FXHW60 PHFO 141319
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
319 AM HST Fri Aug 14 2020

.SYNOPSIS...
A ridge of high pressure north of the state will maintain
moderate trade winds through early next week. Some day to day
fluctuations are expected, with a slight increase in trade wind
speeds expected tonight and Saturday. Otherwise, typical trade
wind weather will prevail with showers favoring windward and
mauka areas, especially during the early morning and overnight
hours. A trough of low pressure aloft could bring an increase in
shower coverage and intensity Tuesday through late next week.

&&

.DISCUSSION...
Currently at the surface, a 1029 mb high is centered around 1550
miles north-northeast of Honolulu, and is driving moderate trade
winds across the island chain early this morning. Infrared
satellite imagery shows clear to partly cloudy skies in most
areas, with a bit more cloud cover in some windward locales.
Radar imagery showers scattered showers moving into windward
slopes and coasts, with a stray shower spreading leeward from time
to time. Main short term concern revolves around rain chances and
trade wind trends during the next couple days.

High pressure north of the state will keep moderate trade winds in
place today. The trades are then expected to increase slightly
tonight and Saturday as the gradient tightens in response to a
broad area of low pressure passing by to the south of the state.
The trades should trend back to moderate levels Sunday through
Tuesday as the ridge north of the islands weakens. The gradient is
forecast to relax further Wednesday through late next week, with
trades lowering into the light to moderate range and allowing for
sea breezes and land breezes in the more sheltered leeward areas.

As for weather details, fairly typical trade wind conditions will
continue today through Monday with bands of moisture moving
through the islands bringing scattered windward showers and the
occasional leeward spillover. Showers will be most prevalent
during the night and early morning hours during this time frame.
Troughing aloft is forecast to build southward over the state
Tuesday through late next week, and this should elevate inversion
heights and bring an increase in shower coverage and intensity
statewide. Showers will continue to favor windward slopes and
coasts during this time, with an increase in leeward and interior
shower activity during the afternoon and evening hours.

&&

.AVIATION...
A high pressure ridge north of the Hawaiian Islands will keep
moderate trade winds in the forecast today increasing into the
moderate to breezy range later tonight into Saturday. Passing
showers may produce brief MVFR conditions mainly over windward
areas with scattered showers today becoming more numerous showers
in the overnight to early morning time periods.

No AIRMETs are in effect. AIRMET Sierra for Tempo Mountain
Obscuration may be needed by the overnight time period over the
north and east slopes of the Big Island and Maui.

&&

.MARINE...
Moderate to fresh easterly trades will continue today, then
briefly increase into the fresh to strong category tonight
through late Saturday as the pressure gradient tightens over the
islands in response to low pressure passing far to the south. This
will likely translate to a Small Craft Advisory being issued for
the typical windier waters between Maui County and the Big Island
by or come this afternoon. Guidance shows the gradient quickly
relaxing Sunday into early next week as this area of low pressure
to the south continues westward and a weakness in the subtropical
ridge forms far north of the state as a backdoor front drops
southward toward 30N.

Surf along east facing shores will hold steady into the weekend
near or just under the advisory level of 8 ft (faces) as a
moderate northeast swell moves through from a batch of strong
breezes that setup within the 040-050 degree band off the west
coast earlier this week. Surf will gradually ease Sunday into
early next week as the winds relax locally as the swell drops.
Although confidence remains low, a small easterly swell will
remain possible from distant Elida in the eastern Pacific over the
weekend, especially for the eastern end of the state.

Surf along south facing shores will remain small through Saturday,
then gradually trend up Sunday into early next week as a small,
south-southwest swell from recent activity near New Zealand
arrives. The PacIOOS nearshore Pago Pago buoy reflected this
small source moving through over the past 36-Hrs. WAVEWATCH III
guidance shows a reinforcement out of the same area Monday night
through Tuesday, which should hold through midweek before easing.
For the long range, another similar pulse will become a
possibility through the third week of the month as low pressure
systems continue to move through Hawaii's swell window southeast
of New Zealand.

Surf along north facing shores will remain near the summertime
average through the period, except for locations exposed to the
previously discussed northeast swell.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs

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



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