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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 171930

Area Forecast Discussion
National Weather Service Honolulu HI
930 AM HST Sun Oct 17 2021

Expect increasing cloud cover today as high clouds spread across
the state. Breezy trade winds will persist, pushing showers
across windward and mauka areas through tonight. Trade winds
will decrease and a drier weather pattern will return from Monday
into mid-week. Trade winds may strengthen later this week.


The pressure gradient between the subtropical ridge and equatorial
low pressure is sufficiently steep to support locally breezy trade
winds across the main Hawaiian Islands this morning. Satellite
loop shows an upper trough just northwest of the main island
chain. Thick middle and high clouds along the southeast flank of
this upper trough are moving northeastward across portions of the
state and we expect this cloud cover will increase as the day goes
on. Radar shows isolated to scattered showers favoring windward
areas of the islands. Models keep breezy trade winds in place
through tonight. Instability induced by the upper trough may
enhance cloud cover and showers, especially over windward Kauai
during the night and early morning hours.

As the upper trough moves west of the state on Monday, our
airmass will become more stable. High pressure north of the state
will weaken as it moves southward Monday through Wednesday,
causing trade winds to weaken. Showers and cloud cover will
decrease as well but continue to favor windward areas. Daytime sea
breezes may develop over leeward areas starting Tuesday, which
could trigger brief showers over leeward and interior sections of
some of the islands each afternoon.

Starting Thursday, models show surface high pressure will build
far north of the state. This will push low clouds and showers
along a remnant frontal boundary southward towards the state.
Models disagree on whether these clouds and showers will reach the
islands, with ECMWF keeping the bulk north of the islands east of
Kauai. Trade winds may strengthen starting late Thursday.


With high pressure centered far northeast of the islands, trade
winds will remain moderate to locally breezy through tonight. MVFR
in low clouds and showers will mainly affect windward and mauka
areas, particularly during the overnight and early morning hours.
A stray shower may spill over into leeward communities from time
to time as well. However, VFR will prevail overall.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. This AIRMET is
expected to remain in effect through the day.


High pressure north of the state continues to drive moderate to
locally strong trade winds, and the Small Craft Advisory for the
typically windy waters around Maui County and the Big Island
remains in effect through 6 AM HST Monday. Cold fronts passing
north of the islands will weaken the pressure gradient and
subsequent trade winds over the next several days. Trades are
forecast to decrease a bit Monday and Tuesday, then decrease more
Wednesday and Thursday. A weakening cold front entering the
northern waters Wednesday will wash out by Thursday. Late week
high pressure settling in north of the islands will re-introduce
fresh to locally strong trades Friday.

The downward trend in trades over and upstream of the islands the
next few days will allow choppy east facing shore surf to
gradually lower and hold close to seasonal levels. The ongoing
small, medium period north northwest swell will keep some small
surf in place along north facing shores today. A couple of small
early week north pulses will keep north surf to waist high within
most sets. A larger north swell is forecast to build in late
Tuesday into Wednesday with a late Wednesday into Thursday peak.
This may push mid week north surf to High Surf Advisory levels.
This swell is expected to be reinforced with an even higher, more
north to northeast swell Thursday and Friday. This swell may increase
mid week surf to advisory levels along most north facing shores.
Mainly background small, medium period south swells will hold very
small surf along southern facing shorelines. The next very small,
longer period south swell is forecast to reach the islands Monday
night and fill in through Wednesday.


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




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman