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Hawaiian Islands Synoptic Discussion and Guidance

658
FXHW60 PHFO 151356
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
356 AM HST Fri Nov 15 2019

.SYNOPSIS...
A light wind regime will continue as a cold front approaches Kauai
from the northwest and a low level trough moves into the Big
Island from the southeast. Expect land and sea breezes for all
islands today and tonight with increasing shower activity over
Kauai and the Big Island through the afternoon hours. Returning
trade winds will help to spread clouds and showers to Oahu and
Maui County on Saturday. Expect wet unsettled weather to linger
into the middle of next week with moderate trade winds in the
forecast.

&&

.DISCUSSION...
This mornings satellite imagery shows a low level trough moving
into the windward Big Island and a cold front slowly approaching
Kauai from the northwest. Convergent shower bands are developing
ahead of this frontal boundary near Kauai this morning. Expect
increasing clouds and showers over Kauai and Hawaii Counties
through the day.

A trend towards wet and unsettled weather arrives today as two
low pressure systems begin to converge over the Hawaiian Islands
today and tomorrow, spreading scattered to numerous showers
across the island chain through the weekend. Deeper moisture and
instability will move in with two bands of Precipitable Water (PW)
ranging from 1.5 to 2.0 inches. Weak southerly winds will develop
near Kauai today with prefrontal shower bands favoring the
southern and western slopes of the island. Big Island showers will
start along the windward slopes this morning then spread into
interior and leeward sections later this afternoon and tonight.

Unstable wet weather will spread into Oahu and Maui County from
Saturday afternoon into Sunday with scattered to numerous rain
showers forecast statewide by Sunday morning. Moderate trade winds
also return to the region and higher rainfall amounts will favor
windward and mountain slopes on Sunday. Enough instability aloft
will produce periods of heavy rainfall and a few thunderstorms.
Persistent and slow moving heavy rain bands may develop,
increasing the threat for localized flooding.

By Sunday night cold air aloft will produce an upper level low
over the state. Colder temperatures will keep an unstable
environment anchored over the region with wet trades in the
forecast through Tuesday night. Low chances for a few
thunderstorms remain in the forecast mix through Monday, much
will depend upon the strength and location of this upper level
low.

&&

.AVIATION...
A light wind regime will continue across the island chain during
the next 24 hours, with overnight land breezes and daytime sea
breezes expected statewide. The airmass will remain generally dry
and stable across Oahu and Maui County, with only a few isolated
showers expected. Deeper moisture associated with an approaching
front west of the state and an approaching trough of low pressure
east of the islands, could bring some more shower activity to
Kauai and the Big Island this afternoon through tonight.

No AIRMETs are currently in effect.

&&

.MARINE...
The current north-northwest swell has been 1 to 3 feet higher
than the Wave Watch III Model guidance according to overnight
observations from buoys 51001, 51101, and 51000, as well as the
Waimea Bay and Pauwela buoys. Therefore, a High Surf Advisory
(HSA) remains in effect through this afternoon for north facing
shores of most islands, as well as west facing shores of Kauai
and Niihau. In addition, a Small Craft Advisory (SCA) remains in
effect for all waters exposed to the large north-northwest
swell, including the Kauai and Kaiwi Channels.

As the current north-northwest swell gradually fades during the
next 24 hours, an even larger west-northwest swell is expected to
arrive late tonight. This will cause surf to again reach the HSA
criteria along most north and west facing shores of the smaller
islands starting Saturday. This swell may be large enough to
approach the High Surf Warning thresholds along some shorelines
from Saturday night into Sunday. In addition, surf may reach the
HSA criteria along west facing shores of the Big Island starting
Saturday night. Combined seas are also forecast to reach the SCA
criteria over waters exposed to this swell this weekend. Looking
ahead to next week, another large 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 front approaching Kauai from the northwest will cause the winds
to shift to a more southerly direction today. The winds will
eventually become northeast late tonight and Saturday. A
combination of increasing moisture and unstable atmospheric
conditions is expected to cause increasing clouds and showers,
along with the possibility of thunderstorms developing over
portions of the coastal waters this weekend. Mariners should be
aware that any thunderstorms they encounter may be capable of
producing locally gusty winds, reduced visibilities in heavy
downpours and dangerous cloud to surface lightning. Looking ahead
to early next week, easterly trade winds will gradually
strengthen, and may approach SCA levels over the typically windy
waters adjacent to the islands of Maui County and the Big Island.

Small southerly swells will persist through this weekend. Surf
will also remain small along east facing shores into early next
week due to the light winds over and upstream of the state. The
surf will increase slightly along east facing shores by late
Monday or Tuesday as the trades strengthen.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Big Island North and East.

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Houston

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