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Hawaiian Islands Synoptic Discussion and Guidance

069
FXHW60 PHFO 151402
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
402 AM HST Sun Sep 15 2019

.SYNOPSIS...
A high will persist far north of the islands. A low aloft is
making the atmosphere unstable and weakening the trade winds near
Oahu and Kauai. Over the next couple of days the low will move off
to the west. Trade winds will strengthen and the atmosphere will
become drier and more stable.

&&

.DISCUSSION...
The 2 AM surface analysis shows a 1032 mb high far north of the
islands near 42N160W. Satellite water vapor loops show a low aloft
450 miles north of Kauai near 29N160W. The low has been making
the atmosphere unstable and allowing locally heavy showers to
form around the area. Clouds and showers over the islands last
evening have dissipated, but there are spotty heavy showers
offshore. The low is also inducing a surface trough between the
islands and the high. This trough has been weakening the trade
winds around Oahu and Kauai.

The high north of the area is forecast to remain nearly stationary
through Monday before moving off to the east. The low aloft is
forecast to move away to the west. As the low moves away, the
atmosphere will become more stable. The trade winds will strengthen
and push drier air over the area from the east. Locally heavy
rain is possible through this evening. After that, trade winds
will prevail through Thursday. The trades will focus showers over
mainly windward areas, with the showers most active nights and
mornings.

Starting Friday, the forecast becomes rather murky with big
differences among the computer models. the models generally agree
that the atmosphere will become more moist, so an increase in
rainfall is expected.

&&

.AVIATION...
Trade winds are still relatively light from the east/southeast
and should stay light through most of Sunday. A lower-level
trough still lies between Hawaii and a high up to the north. As
the trough pushes to the north, expect an increase in wind speeds
as we get into Monday.

There may be a few isolated showers this morning, but outside of
that, the morning will be mostly clear. For this afternoon, we
may have a repeat of what we saw yesterday afternoon with leeward
showers possible which could be locally heavy at times. The
showers may not be quite as heavy as they were on Saturday but
still could lead to MVFR conditions over interior sections of the
islands.

There are currently no AIRMETs in effect.

&&

.MARINE...
A surface trough north of Kauai continues to move west. The
blockage of the Big Island under this slightly south of east wind
flow, has resulted in lighter winds across the leeward waters of
the smaller islands. Winds are stronger over exposed windward
waters, especially around Maui County and the Big Island. A boost
in the easterly trades is expected later this morning. A Small
Craft Advisory (SCA) is now in effect through at least late Monday
afternoon. The SCA covers some of the near-shore waters of Maui
County and waters around the Big Island, including the windward
side. This coverage of this SCA will be changing where we may see
the Big Island's windward waters be lowered. This comes about as
the trades become more east. At this time, the trades are expected
to drop off a notch on Thursday, but this may change,

A combination of small south-southeast and south-southwest swells
has peaked and will now gradually subside. But the new week
points to a series of modestly small swells from the southwest,
south, and southeast. It commences with a prolonged southwest
swell on Tuesday that will last through most of the week. The
southeast swell is slated to arrive Wednesday, then a south swell
follows in on Friday.

Small surf is expected along north, east and west facing shores
through the new week. The boost in the easterly winds will
slightly raise the surf along east facing shores. A small east
swell from Tropical Cyclone Kiko is possible as early as
Wednesday. Latest model guidance is showing a swell of 2 to 3 feet
with periods of 11 to 13 seconds.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Monday for Maalaea Bay, the
Pailolo and Alenuihaha Channel, Big Island Windward Waters and
waters south of the Big Island.

&&

$$
DISCUSSION...Donaldson
AVIATION...Chevalier
MARINE...Lau

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