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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 151342

Area Forecast Discussion
National Weather Service Honolulu HI
342 AM HST Thu Apr 15 2021

A surface low and trough located east and northeast of the Big
Island will maintain moderate to locally strong northeast winds
across the area through today. The trades weaken to light to
moderate tonight, and continue into the weekend. A dissipating
front will bring a boost to windward and mauka showers over the
weekend. A more typical east- northeast trade wind flow returns
Saturday night at which point temperatures will be not be as cool.
Humidity will be on the rise early next week as southeast winds
take over.


A northeast trade wind flow will maintain cool temperatures,
especially at night, through Saturday. This wind flow is brought
on by a slow moving surface low and trough located some 1000 miles
northeast of Oahu, and a broad surface high centered far northwest
of Kauai. The northeast flow weakens from moderate to locally
strong today, to light to moderate tonight and Friday.

As this low/trough ejects out to the northeast tonight, a new
surface low forms 500 miles further west along 150W on Friday,
resulting in our light to moderate northeast winds becoming more
northerly. This new low gets kicked out to the northeast on
Saturday, and a weak surface high follows in Saturday night
resulting in the return of a more typical east- northeast trades.
Between Sunday night and Monday, the trade wind gives way to
light southeast winds. Under this wind regime, the weather pattern
changes from trades to convective of daytime sea breeze and
afternoon showers, and nighttime land breeze.

A front is expected to reach Kauai Friday morning, then on to Oahu
in the afternoon. The front stalls and dissipate over Maui County
Saturday. As the trade winds return to its usual east- northeast
component Saturday night, trade showers will return to all islands.

The windward and mountain sections of most islands are experiencing
passing mainly light showers. The lee areas of the smaller islands
are feeling a few drops from time to time as well. We anticipate
the frequency of these showers to ease off toward late morning,
becoming isolated by this afternoon. Scattered showers will pick
up once again after the sun sets.


Moderate trade winds will persist through this morning and allow
for scattered showers to affect mainly the windward coasts and
slopes. Brief periods of MVFR ceilings and visibility may be
associated with this activity. Otherwise, expect partly cloudy
skies and VFR conditions for the leeward locations.

No AIRMETS are currently in effect and none are expected today,


A trough of low pressure east of the Big Island and broad surface
high pressure northwest of the state will maintain moderate and
fresh northeast trade winds through today. Last night's ASCAT
satellite pass verified locally strong winds through the
Alenuihaha Channel and waters south of the Big Island. Thus, have
extended the Small Craft Advisory for those windier waters of
Maui County and south of the Big Island through 6 PM HST tonight.
Trades will begin to ease tonight and into Friday in response to
the northeast trough moving further away from the islands and a
developing new low north-northeast of the islands. A weakening
front will reach the northern offshore waters this afternoon. The
front will dissipate upon reaching Maui County Saturday. A new
surface high follows in behind this front Saturday night, ushering
in a light to moderate east-northeast trade wind flow. This wind
flow changes to a more east to southeast direction Sunday night
and Monday. Early to mid week winds will become light and variable
as surface high pressure ridging orients itself west- to- east
just north of the islands.

Surf will remain small along north and west facing shores through
Thursday. A series of moderate, short period north swells will
fill in beginning Friday and last through the weekend. This will
increase surf along north facing shores through the weekend with
an expected slow decline early next week. Surf from these north
swells will remain well below high surf advisory levels. East
facing shore surf will also remain small through Thursday. North
swell may wrap around into some eastern exposures and provide a
small bump in east facing shoreline weekend surf. As any remaining
south southwest swell fades through Thursday, south facing shore
surf will stay small into early Friday. Distant long period swell
energy from the southern hemisphere arriving in a couple of days
will provide a small boost to southern shoreline surf from Friday
into the weekend.


Small Craft Advisory until 6 PM HST this evening 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
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