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Hawaiian Islands Synoptic Discussion and Guidance

623
FXHW60 PHFO 290150 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
350 PM HST Mon Sep 28 2020

.SYNOPSIS...
Light and variable winds will become widely established by
tomorrow, and will then stick around through the rest of the
week. This will lead to land and sea breezes, with a concentration
of clouds and showers over the interior of the islands during the
afternoon hours. Some clearing is likely during the overnight
hours.

&&

.DISCUSSION...
No changes to the forecast with the afternoon package. Showers
have lingered over the leeward areas of Kauai, but started to
dissipate over the last hour or so. Some showers formed over the
leeward areas of Oahu midday, but only a few isolated showers are
still around at 3pm.

The surface trough associated with this moisture we've seen over
Kauai and Oahu today has moved west of Kauai, and is expected to
continue to track west and weaken overnight. Some clouds are
immediately upstream of Maui County, but largely few clouds are
found upstream of the islands. As such expect some clearing
tonight and limited shower activity.

The ridge to the north is weakening, and winds have been lighter
today than yesterday, with some local sea breezes setting up
today. Expect weaker winds to become more widespread over the next
24 hours, with land and sea breezes into Saturday. Some
indications that we could see some trades return later on
Saturday, which is reflected in the forecast.

The next batch of moisture is about 300 miles upstream of the Big
Island. How much of this moisture will reach the islands is still
in question. The models continue to have it dissipate and lift
north, but some of it could reach the islands Wednesday night
through Thursday morning, which could provide a few more showers
during the normally drier period of a land and sea breeze pattern.


&&

.AVIATION...
Prevailing VFR and light to moderate trades will continue through
Tuesday with further slight weakening of the gradient expected
during this time. This will allow land and sea breeze circulations
to become established over leeward areas which will be supportive
of inland cloud buildup and isolated shower activity. Nothing more
than very brief MVFR expected within any heavier windward showers.

No AIRMETS in effect.

&&

.MARINE...
Gentle northeast winds will continue through Thursday then
will back easterly Friday and Saturday. A return of moderate to
fresh trade winds is expected by Sunday.

The current south swell will gradually lower through midweek
resulting in surf heights decreasing. A mix of southeast trade
wind swell and background small, southerly energy should be enough
to keep the surf along south facing shores from going flat into
next weekend.

For the long range, the active trend continues down south around
New Zealand within Hawaii's swell window. A broad area of gale-
to storm force winds over our swell window from the Tasman Sea to
south southeast of New Zealand. This will generate a late season
south swell that should arrive locally early next week (Oct. 5th
time frame). More details will follow later in the week once the
swell moves through the nearshore PacIOOS buoy in American Samoa.

Surf along north facing shores will remain small through Tuesday,
then trend up once again Tuesday night through next weekend as
back-to-back north northwest swells arrive. The first one is from
a compact storm-force low developing around 1500 nm north-
northwest of the islands as it tracks eastward. This source will
arrive Tuesday night and gradually become more northerly by
Thursday as it peaks near the advisory level (15 ft faces). The
second swell is forecast to arrive Thursday night, which should
keep the surf near the advisory level through the day Friday. This
swell combined with a smaller, medium period northwest arriving
Friday night into Saturday will keep the surf up through next
weekend.

Surf along east facing shores will increase tonight up as a
small, easterly swell from former tropical cyclone Lowell in the
eastern Pacific arrives and moves through the islands. This swell
will decrease on Wednesday.

The large daily tide cycles will gradually trend down toward the
monthly minimum each day through the first week October.

&&

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

&&

$$


DISCUSSION...M Ballard
AVIATION...JVC
MARINE...Foster

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



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