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Hawaiian Islands Synoptic Discussion and Guidance

785
FXHW60 PHFO 140641
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Thu Aug 13 2020

.SYNOPSIS...
A ridge of high pressure north of the state will maintain moderate
trade winds through at least early next week. Some day to day
fluctuations are expected, with a slight increase in trade wind
speeds expected Friday night and Saturday. Otherwise, our typical
trade wind weather is expected with showers favoring windward and
mountain 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 1031 mb high is centered around 1600
miles north-northeast of Honolulu, and is driving moderate trade
winds across the island chain this evening. Infrared satellite
imagery shows partly cloudy skies in most areas, although a bit
more cloud cloudiness is present over leeward sections of the Big
Island and Maui. Radar imagery showers scattered showers over
windward slopes and coasts, with a few lingering leeward showers
in the more sheltered areas of Maui and the Big Island. 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 tonight and Friday. The trades then are expected to
increase slightly Friday night 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 tonight 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 over the next 24 hours.
Passing showers may produce brief MVFR conditions with scattered
shower activity mainly over windward and mountain areas.

No AIRMETs are in effect and none are expected.

&&

.MARINE...
Moderate to fresh easterly trades will continue through Friday,
then briefly increase into the fresh to strong category Friday
night 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 Friday 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, mostly 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 down near New Zealand
arrives. The PacIOOS nearshore Pago Pago buoy reflects 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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