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Hawaiian Islands Synoptic Discussion and Guidance

829
FXHW60 PHFO 141930
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
930 AM 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 morning, while
an upper ridge keeps our airmass stable. Satellite and radar data
show patchy broken low clouds with isolated to scattered 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 weakening into a broad surface trough as it reaches the
islands Wednesday, then drifting 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 will allow for moderate easterly trades to
persist through this afternoon. 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...
A ridge north of the state will maintain a moderate to locally
breezy trade wind pattern through tonight. A Small Craft Advisory
(SCA) remains posted 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.

An upper level low forming to the northeast of the islands during
this time 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. 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, keeping surf from going
flat along south facing shores.

A small long-period west swell is expected to fill in today,
peaking tomorrow and then dropping off Wednesday. This swell was
generated by former Tropical Cyclone Hagibis. 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