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Hawaiian Islands Synoptic Discussion and Guidance

914
FXHW60 PHFO 181945
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Fri Oct 18 2019

.SYNOPSIS...
Trade winds will continue over the islands through the weekend and
into next week, periodically delivering clouds and showers to
windward areas, although fewer showers are expected this weekend.

&&

.DISCUSSION...
To summarize, trade winds will prevail over the islands through the
next week, albeit with some variations in speed, and some subtle
variations in direction. The somewhat showery weather pattern of the
last couple of days should trend a little drier over the weekend as
a low aloft continues to distance itself from the islands.

To elaborate, high pressure to the distant northeast of the islands
will drift east the next couple of days, move little for a couple of
days, and then drift toward the north in about 5 to 7 days from now.
This will place the islands on the southwest periphery of the high's
circulation, meaning that the prevailing moderate to locally breezy
easterly trade winds will be occasionally weakened and somewhat
veered to the east-southeast, especially on the western end of the
island chain. This effect will be most pronounced when a front
passes north of the islands Sunday/Monday, and potentially again
toward the end of next week.

Water vapor imagery shows a broad trough aloft to the northwest of
the islands, with early morning soundings showing an elevated
subsidence inversion based between 8 and 10 thousand feet. Radar
shows quite a few showers soaking windward areas, falling from open-
celled showery cumulus. A trend toward fewer trade showers is
expected over the weekend as the low aloft drifts away, a mid-level
ridge builds nearby, and incoming PWAT diminishes slightly. An
increase in trade winds early next week may bring a subtle increase
in windward shower coverage, but significant changes are not
expected as moderate trades prevail. Long range guidance indicates
the potential for much lighter winds in about a week, as the high
weakens and a trough/low develops north of the islands.

&&

.AVIATION...

Moderate to locally breezy trade wind flow will persist through
Saturday. Ragged clouds and scattered light showers will focus
mainly across windward areas, with some increase in coverage
overnight. Afternoon sea breezes across Leeward Big Island will be
weak, but sufficient for some cumulus development and a few showers
possible late this afternoon and early evening.

No AIRMETs are in effect or anticipated at this time. Brief MVFR
ceilings and visibilities are possible in showers. Otherwise, VFR
conditions will prevail all areas.

&&

.MARINE...
High pressure northeast of the state will maintain the fresh to
strong easterly trade winds through tomorrow afternoon. The Small
Craft Advisory (SCA) continues through tomorrow afternoon for the
typical windier waters near Maui County and the Big Island.

As a front moves closer to the state from the north late Saturday
night, winds are expected to shift to the east-southeast,
particularly over the southern end of the state. The front is
expected to stay north of the islands, and will likely weaken the
winds over the western islands. Expecting winds to shift back to a
more easterly direction Monday.

Surf is expected to remain below advisory levels through the
forecast period. A combination of a west-northwest and north swell
will hold surf along north facing shores. The north swell is
expected to fade later today. A new small long-period swell expected
Saturday will help maintain the surf along exposed shores.

A gale low is expected to develop near the Kurils Saturday night
into Sunday, and then track to the east toward the Date Line Sunday
into Monday. This would generate a northwest swell that could reach
the islands late next week.

A recent storm-force low far southeast of New Zealand could bring a
long-period south swell that would peak over the weekend, and then
fade into early next week.

The fresh to locally strong trade winds will maintain rough surf
along east facing shores through tomorrow. As the winds weaken
Sunday, expect some improvements.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...TS
MARINE...M Ballard

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