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Hawaiian Islands Synoptic Discussion and Guidance

217
FXHW60 PHFO 171944
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
944 AM HST Sat Aug 17 2019

.SYNOPSIS...
The high pressure ridge north of the Hawaiian Islands will weaken
today with light trade winds forecast on Sunday and Monday. Expect
increasing trade winds by the middle of next week along with
elevated humidity levels and some enhanced shower activity as two
low level troughs pass south of the state.

&&

.DISCUSSION...
Shower activity was enhanced this morning as a weak low level trough
continues to drift through the islands. Numerous showers were
observed over the typical windward and mauka areas with isolated
to scattered showers developing over leeward slopes. This low
level trough will drift west of the islands later today with
decreasing showers through the afternoon hours.

A mid-latitude trough passing through the Central Pacific will
weaken the high pressure ridge north of the islands today,
decreasing trade wind speeds on Sunday and Monday. Onshore sea
breezes caused by day time surface heating will develop over each
island with offshore land breezes each night. Drier trends are
forecast for the next few days under the subsidence from a strong
upper level ridge. The trade wind subsidence inversion heights
will range from 5000 to 7000 feet through Monday afternoon.

By Monday evening the weather pattern begins to change as
additional low level troughs move into the Hawaii region. The
first of these trough arrives from Monday night through Tuesday
morning with some enhanced showers developing mainly along
windward slopes of each island. A second tropical disturbance is
now forecast to pass south of the Big Island on Tuesday night with
potentially less impacts on rainfall and humidity levels across
the islands by Wednesday. Trade wind speeds will increase starting
on Tuesday, as the ridge builds north of the islands and this
trough passes to the south of the state. Moderate to locally
breezy trades are forecast from Wednesday onward. Higher chances
for elevated moisture levels and showers will linger over the Big
Island from Tuesday to Thursday with lower precipitation impacts
for the rest of the state. We will probably trend precipitation
chances downward, in the afternoon forecast grids, across the
western islands for the Wednesday through Thursday time periods.

Moderate to locally breezy trades are forecast to continue on Friday
and next Saturday as the high pressure ridge builds in north of
the islands. An upper level cut off low drops in from the north
likely enhancing shower regional shower activity especially in the
overnight and early morning hours.

&&

.AVIATION...
A 1024 mb surface high far to the northeast of the state will
bring light to moderate easterly trade winds through this
afternoon. This flow will help generate isolated showers along
the windward slopes and coastal areas this afternoon, which are
forecast to become scattered after sun down. Brief MVFR conditions
will remain possible tonight due to reduced visibility and
mountain obscuration from low clouds and showers.

AIRMET Sierra remains in effect through 20Z for islands east of
Kauai due to tempo mountain obscuration above 2000 feet, though
conditions will likely improve by late this morning.

&&

.MARINE...
A surface high pressure system far northeast of the state will
gradually weaken this weekend, which will cause the trade winds
to diminish slightly through early next week. A new surface high
pressure system building far north of the state by the middle of
next week is expected to cause the trade winds to gradually
strengthen starting next Wednesday. We may see the return of Small
Craft Advisories for the typical windy areas around Maui and the
Big Island during the second half of next week.

Small south and southwest swells will maintain background
summertime surf along south facing shores this weekend. A large,
long-period south-southwest swell arriving Monday is expected to
peak Tuesday, then gradually lower through the rest of next week.
Latest observations from the Pago pago buoy is indicating the
swell running higher than the WaveWatch 3 model. This suggests
surf heights may near High Surf Warning levels for a brief time
during its peak along south facing shores on Tuesday.

The current west swell produced by tropical cyclones in the
Northwest Pacific will continue to slowly decline through Monday.
Surf heights produced by this lingering swell will be largest
along shorelines with a westerly exposure. A small northwest swell
will be possible from next Thursday through Friday. Modest surf
will persist along east facing shores today, then will slowly
subside from Sunday through early next week as the trade winds
weaken.

For additional details on swells and surf, please see the latest
Collaborative Nearshore Swell and Wind Forecast for Oahu (SRDHFO)
product.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...JT
MARINE...Kino

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



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