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Hawaiian Islands Synoptic Discussion and Guidance

569
FXHW60 PHFO 210628
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Tue Aug 20 2019

.SYNOPSIS...
A surface ridge north of the area will maintain locally breezy
trade winds over the islands. Expect showers to remain focused
over windward and mountain areas.

&&

.DISCUSSION...
No changes to the forecast expected this evening.

A surface ridge north of the islands will maintain a moderate to
locally breezy trade wind pattern over the islands. A weakening
trough south of the Big Island continues on its westward trek.
This trough is partly responsible for the increase in moisture,
particularly over the southern end of the state. Satellite derived
precipitable water (PW) shows lower values should start filtering
into the islands later tonight. As a result, shower activity over
windward Maui and the Big Island should be on the decrease
overnight.

The trade winds are expected to increase a bit more tomorrow
(Wednesday) as the high associated with the ridge continues to
build. The models remain in good agreement with a front far north
of the islands moving southward through Thursday. This will help
to disrupt the trade wind flow a bit during the second half of the
week. The front will move to the west at the end of the week, and
trade winds will rebound for the weekend. The bulk of the
moisture associated with this front is expected to remain to the
north of the islands, however some increase in shower activity is
possible over Kauai Friday.

&&

.AVIATION...
A moderate to locally strong trade wind regime will continue into
Wednesday. Most of the cloudiness lies over the Alenuihaha Channel,
the waters off the NE facing Big Island coast, and over the lee
areas of the Big Island. Otherwise, skies are fair with minimal
shower activity. Cloud bases are estimated above 3k feet with
tops at 6k feet but as high as 9k feet with the showers.

Although there are minimal showers now, we anticipate an increase
overnight, not enough to warrant an AIRMET Sierra, though. But
will bring some brief periods of MVFR ceilings and vis to
especially the windward and mountain areas. A few late night
showers may also develop along the Kona coast as well. Otherwise,
VFR conditions will prevail overnight and into Wednesday morning.

AIRMET Tango is not in effect as conditions are marginal at best
for low level turb, which should hold through Wednesday. But we
will monitor the situation closely and issue it if needed.

&&

.MARINE...
High pressure building northwest of the state will keep a moderate
to locally strong trade wind flow in place over the next couple
days. As a result, a Small Craft Advisory (SCA) remains in effect
for the typically windy waters around Maui and the Big Island. A
front dropping southward and closer to the island chain late in
the week, will ease the trade winds slightly Thursday night
through Friday night. A new high building northeast of the islands
will then strengthen the trades back to moderate and locally
fresh levels over the weekend, with SCAs likely being required
once again for the windier marine zones.

The current south swell will peak tonight, then gradually lower
late Wednesday into the weekend. A High Surf Advisory remains in
effect for south facing shores of all islands through 6 AM
Thursday. A series of small long-period south and southwest swells
will move through Sunday through early next week. A larger
southwest swell is possible Tuesday through the middle of next
week.

Small northwest swells will linger over the next few days
providing some out of season surf along north facing shores.
Strengthening trade winds will give a boost in short period
choppy surf along east facing shores through Wednesday night and
again over the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai
Leeward-Lanai Makai-Kahoolawe-Maui Leeward West-Maui Central
Valley-Leeward Haleakala-Kona-South Big Island-Big Island North
and East.

Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...H Lau
MARINE...Jelsema

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



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