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Hawaiian Islands Synoptic Discussion and Guidance

115
FXHW60 PHFO 190125
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
325 PM HST Fri Oct 18 2019

.SYNOPSIS...
Trade winds will continue through the weekend and into next week,
periodically delivering clouds and showers to windward areas,
although fewer showers and lighter trades are expected over the
weekend. High clouds of varying thicknesses will periodically move
over the islands the next couple of days.

&&

.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. High
clouds will periodically move over the islands from the southwest
over the next couple of days.

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, that is gradually moving away and weakening, with
increasing stability expected over the islands. Afternoon soundings
show that the subsidence inversion has lowered somewhat today, and
is now based around 7-8 thousand feet. While this boundary layer
depth will still allow showers to form, 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.

Southwest to westerly flow aloft is expected to bring periods of
high clouds over the islands beginning Saturday, potentially
lingering for several days. Overall impacts will be limited, and it
is difficult to know how thick the clouds will get, but forecast
grids have been modified to increase sky cover for the next several
days.

&&

.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 a slight 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 into this evening, before land breezes push them back
offshore overnight.

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

&&

.MARINE...
he north swell which had been running a little higher than guidance
is still lingering, so have continued to carry that into tomorrow
morning in the forecast. Otherwise no significant changes to the
marine grids with the afternoon package.

High pressure northeast of the state will maintain the fresh to
strong easterly trade winds through tomorrow afternoon, so the Small
Craft Advisory (SCA) continues 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. As mentioned above, the
north swell is lingering just a bit longer than previously forecast,
but the trend is down, and it is expected to fade by tomorrow
morning. 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