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Hawaiian Islands Synoptic Discussion and Guidance

392
FXHW60 PHFO 211338
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 AM HST Wed Aug 21 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 in the trade wind flow.

&&

.DISCUSSION...
No significant changes to the forecast with the morning package.
Some minor tweaks to the winds in the first 24 hours primarily to
nudge a few areas towards higher wind speeds as suggested by some
of the high resolution models.

A 1021 mb high was analyzed about 700 miles north-northwest of the
islands overnight. The high and its associated ridge will maintain
a trade wind pattern into the weekend. A weakening trough south of
the islands brought some additional showers to the islands
yesterday and overnight, but its effects will continue to decrease
today. Meanwhile, a front more than 770 miles north- northeast of
the islands will bring an increase in shower activity to the area
near and north of Kauai Friday night.

As the high builds north of the islands today, we can expect to
see an uptick in the trade winds. The latest satellite derived
precipitable water (PW) shows the bulk of the moisture that was
carried into the islands with the above mentioned trough has moved
west of the islands. Satellite shows more typical trade wind clouds
and showers being carried in the trade wind flow to the windward
sides of the islands, as we would expect with PW values around 1.5
inches.

The front to the north-northwest will sink southward and help to
weaken the trade winds Thursday and Friday. Heading into the
weekend, the weakening front moves to the west and a ridge north
of the islands will become more established, allowing trade winds
to rebound.

&&

.AVIATION...
A moderate to locally strong trade wind regime will continue
beyond 04z as a ridge of high pressure remains north of the
islands. Satellite imagery shows a broken field of low clouds
working across windward Big Island and Maui, towards Oahu and
Kauai. The showers have brought brief MVFR conditions to especially
Hilo Airport, with ceilings at 14 hundred feet and 1 and three
quarter mile vis. Conditions appear to be improving there as the
field is gradually dispersing. Oahu has seen an increase in trade
showers in the past hour, and Kauai will follow suite in the next
hour or two. But as the morning progresses, after sunrise, these
clouds and showers will dissipating, giving way a nice afternoon.
The lee side of the Big Island will go through its norm phase of
morning sun, then afternoon clouds with a few showers. AIRMET
Tango is likely below 6k feet as the trade winds have become
stiffer and stronger as per radar and Lihue sounding.

&&

.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
through Thursday. 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 strong levels over the weekend, with SCAs likely
being required once again for the windier marine zones.

The current long-period south swell will hold steady today, then
gradually lower tonight 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 tonight 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