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Hawaiian Islands Synoptic Discussion and Guidance

293
FXHW60 PHFO 200156
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Fri Jul 19 2019

.SYNOPSIS...
Moderate to locally breezy trade winds will continue well into next
week. Clouds and showers will favor windward and mountain areas,
especially during nights and mornings. Most leeward areas will
remain dry, except leeward Big Island, where a few afternoon and
evening showers can be expected.

&&

.DISCUSSION...
No big changes to the ongoing forecast. Trade winds will ease to
moderate levels through tonight as high pressure to the distant N
weakens slightly. Moderate trade winds will then continue into next
week, although locally breezy conditions can be still be expected,
mainly in the afternoons. The trade winds will deliver low clouds
and showers to windward areas that will occasionally spread leeward,
especially when showery cloud clusters move through. Anticipating
the arrival of such clusters is generally a low skill endeavor
beyond the short term, thus a climatological trade wind forecast
dominates into next week, with showers most active during nights and
mornings. A cloud and shower band currently bout 200-300 miles E of
the islands is moving W near 20 mph, and will likely bring a few
windward showers overnight.

A trough/convergence band to the S and SE of the islands has become
active over the past couple of days, and models have been
inconsistent in depicting various weak lows/disturbances briefly
emerging before dissipating, and little significant organization is
currently noted. Models generally agree that the axis of convection
will shift slowly northward through early next week before
weakening. GFS indicates a disturbance currently near 152W will
track NW the next couple of days, with little significant
development. However, an associated slug of moisture moves toward
(and over some of) the islands from the S/SE from Sunday through
Tuesday, while 12Z ECMWF guidance continues to keep most of the
associated moisture S of the island chain. The latest official
forecast represents a blend of the two models, indicating the bulk
of the moisture passing just S of the islands Sunday/Monday. Another
disturbance E of 140W may inject moisture into the trade wind flow
that arrives around the middle of next week, but confidence in that
is close to nil.

In the upper levels, a low aloft centered N of Kauai is supplying
instability above about 13000 feet, but a strong subsidence
inversion based between 6000 and 8000 feet is not allowing surface
based parcels to tap into this instability. The low and associated
trough may bring periods of high clouds into next week before a
ridge aloft develops over the area by the middle of the week.

&&

.AVIATION...
A high will remain nearly stationary far northeast of the main
Hawaiian islands and maintain locally strong east winds over the
area through Saturday. AIRMET TANGO is in effect for moderate
turbulence below 7,000ft and this AIRMET will likely continue into
Saturday.

Low clouds around the main Hawaiian Islands have decreased in
coverage and become more stable since morning. VFR conditions
prevail and AIRMET SIERRA for Mountain Obscuration has been dropped
for everywhere except the Lanai mountains. That AIRMET will likely
be dropped around sunset, but cloud cover over east sections of the
islands may increase enough overnight to require AIRMET SIERRA to be
issued again.

&&

.MARINE...
Small Craft Advisory (SCA) winds will continue through the typically
windier waters around the Big Island and Maui County through the
weekend. Elsewhere, the trade winds have come down a notch and the
SCA has been cancelled. A weak disturbance several hundred miles
south-southeast of the Big Island is forecast to pass south of
offshore waters into early next week.

East facing shores will continue to see rough and choppy surf in
response to the trade wind swell. As the winds ease this weekend,
the surf will drop slightly. Some small background south and
southwest swells are expected through the weekend. An increase in
surf along south facing shores is possible early next week as a
couple small longer period southwest and southeast swells arrive in
the islands.

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...Donaldson
MARINE...Eaton

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



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