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Hawaiian Islands Synoptic Discussion and Guidance

011
FXHW60 PHFO 150135
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 PM HST Mon Oct 14 2019

.SYNOPSIS...
Locally breezy trade winds will weaken beginning Tuesday as a
front approaches from the north. This approaching front, combined
with an upper-level disturbance moving into the area, will
support increasing rainfall chances by Wednesday, which could
linger into Thursday. Drier trade wind weather will resume Friday.

&&

.DISCUSSION...
High pressure far northeast of the islands continues to drive
moderate to breezy trades across local waters this afternoon,
while an upper ridge keeps our airmass stable. Satellite and radar
data show scattered to patchy broken low clouds with isolated
showers mainly across windward areas. Models depict this pattern
holding into Tuesday.

Showers will increase Tuesday night through Thursday as the upper
ridge shifts west of the area and is replaced by an upper low.
Trades will trend down, potentially enough for a land and sea
breeze regime to become established in some areas, as a
dissipating front approaches from the north. Models suggest this
front will weaken into a broad surface trough as it reaches the
islands Wednesday, then drift west of the islands by Thursday
night.

The front, combined with the upper low and increased moisture,
will support better shower coverage Wednesday through Thursday,
especially through the afternoon and evening periods. Daytime
heating, combined with a relatively cool pool aloft, may be
enough added instability to trigger a thunderstorm or two.

Expect a return to a more typical trade wind weather pattern as
the upper low moves west of the state and the atmosphere
stabilizes late this week. Trades may briefly shift out of a more
east to southeast direction over the weekend.

&&

.AVIATION...
Surface high pressure north of the state will allow for moderate
east easterly trades to persist through this afternoon and into
the evening. Thus, expect scattered showers to be mostly
concentrated along the windward slopes and coasts. Isolated
activity may periodically push over the elevated terrain and onto
the leeward sides.

No AIRMETs are currently in effect.

&&

.MARINE...
No significant changes to the marine forecast this afternoon.
Expect moderate to locally breezy trades to continue tonight with
a ridge north of the state. Winds are expected to remain above Small
Craft Advisory (SCA) levels for the typical windier waters near
Maui County and the Big Island through tonight. A front north of
the ridge is expected to push south, stalling north of the waters
by midweek. The trades are expected to begin to weaken tomorrow,
and last through Thursday. Still expecting winds to drop below SCA
levels by tomorrow, so the SCA currently expires at 6am Tuesday
morning.

An upper level low forming to the northeast of the islands Tuesday
is forecast to move to the southwest, and by Thursday be southwest
of the islands. This upper level feature will help to enhance
shower activity, and could provide enough instability for some
thunderstorms. Currently we are carrying the slight chance for
thunderstorms in the offshore waters Tuesday night through
Thursday. Initially the focus would likely be over the northeast
waters, and then shifting over the northwest waters. Will be
monitoring future model runs to be more specific with areas, and
to see if the possibility is there for development over the
coastal waters.

High pressure is expected to settle in north of the islands behind
the surface front, and should allow for strengthening trade winds
for the end of the week.

Surf is expected to remain below advisory levels along all shores
into the weekend.

A series of south and south-southwest swells will continue to
move through the islands. A small long-period west swell generated
by former Tropical Cyclone Hagibis is expected to peak tomorrow
and then drop off Wednesday. Some of the swell energy could wrap
into some northern exposures.

A couple small long-period northwest swells will build Wednesday
night, peak Thursday, then gradually lower through the end of the
work week. A smaller long-period northwest swell is expected over
the weekend.

Rough and choppy surf will continue along east facing shores
through tonight, before trending down Tuesday through Thursday as
the trades ease. East shore surf will then trend of toward the end
of the week and over the weekend as the trades increase once
again.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Powell
AVIATION...JT
MARINE...M Ballard

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



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