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Hawaiian Islands Synoptic Discussion and Guidance

059
FXHW60 PHFO 170159
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 PM HST Sat Nov 16 2019

.SYNOPSIS...
A trough continues to approach the Big Island from the east while
a front lingers near Kauai. Expect scattered showers to become
more numerous after tonight as trade winds return. Showers may
become heavy at times through this weekend, with thunderstorms
possible. Expect a wet moderate trade wind pattern for the first
half of next week as an upper low moves over the state.

&&

.DISCUSSION...
A diffuse and hard-to-find front near Kauai continues to spread
isolated to scattered showers across Kauai and Oahu. Isolated
thunderstorms are noted northeast of Kauai, Oahu and the Big
Island. Showers will move over windward slopes of the islands
through this evening. Models show the front and the trough will
converge over the state through the weekend, increasing rainfall
probabilities.

Clouds and showers will expand eastward across Oahu and the
islands of Maui County as trade winds return through tonight.
Isolated to scattered showers will become more numerous across
the area after tonight. On Sunday, models show a nearby upper
trough deepening into an upper low as surface high pressure builds
north of the islands and trade winds strengthen. However, there
is already enough instability associated with the existing upper
trough to raise the possibility of heavy showers and a few
thunderstorms across the state tonight, with an elevated risk for
localized flooding in slow moving shower bands on Sunday. The
highest chances for showers from Sunday onward will favor windward
areas of each island. On Monday, the upper low will be fully
developed and will linger over the islands in a moisture rich
environment through the first half of next week.

Weather conditions, including the locations and amounts of
potential rainfall rainfall across each island, will strongly
depend upon the location and strength of the forecast upper low.
For now, we will continue to forecast wet trade wind weather for
the first half of next week. The upper low should weaken on
Wednesday, with shower activity expected to trend lower afterwards
through Friday.


&&

.AVIATION...
A front will dissipate near Kauai tonight. Northeast trade winds
will return as high pressure builds in to our north. Clouds and
passing showers will be most active over north and northeast
facing slopes. Some MVFR conditions are possible tonight and
Sunday morning and an AIRMET is currently posted for mountain
obscuration over Kauai. An unstable airmass developing across the
area will allow for a slight chance of thunderstorm over portions
of the forecast area.

&&

.MARINE...
A High Surf Advisory (HSA) is in effect for most north and west
facing shores of the smaller islands because of an incoming large
northwest swell. This swell may be large enough to cause surf to
approach the High Surf Warning thresholds along some shorelines
during its expected peak from tonight into Sunday. In addition,
surf may reach the HSA criteria along west facing shores of the
Big Island starting tonight. Combined seas will also reach the
Small Craft Advisory (SCA) criteria this weekend over waters
exposed to this large swell. Looking ahead to the middle of next
week, a new long period northwest swell arriving early Wednesday
may cause surf to reach the HSA criteria along most north and west
facing shores of the smaller islands from late Wednesday through
Thursday.

A weak and diffuse front in the vicinity of Kauai is nearly
stationary, while a surface trough just east of the Big Island is
moving slowly west. Winds are shifting to the northeast across
most of the state in response to these two features. The
northeasterly flow will gradually strengthen as the front
dissipates and the trough moves over the eastern islands through
tonight. A combination of increasing moisture associated with the
front and trough, and unstable atmospheric conditions due to an
upper level low developing over the area, will cause cloud cover
and showers to increase, along with the possibility of
thunderstorms over portions of the coastal waters from this
weekend into Monday. Easterly trade winds will gradually
strengthen from Sunday through early next week. The trades may
eventually approach SCA levels over the typically windier waters
adjacent to the islands of Maui County and the Big Island by
Tuesday or Wednesday.

Small south swells will maintain small background surf along south
facing shores this weekend. Surf will remain small along east
facing shores into early next week due to the relatively light
winds over and upwind of the state. The surf will increase
slightly along east facing shores by late Monday or Tuesday as the
trades strengthen. As the trades continue to strengthen later
next week, expect choppy surf to increase along east facing
shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Big Island Windward Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Burke
MARINE...Kinel

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