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Hawaiian Islands Synoptic Discussion and Guidance

479
FXHW60 PHFO 281329
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
329 AM HST Sat Nov 28 2020

.SYNOPSIS...
Moderate to locally breezy northeast trade winds will prevail
through the weekend, with clouds and showers favoring windward and
mauka areas. Winds will become light Monday through mid week,
with daytime sea breezes and overnight land breezes expected. Dry
conditions will prevail through much of next week, with a few
showers near the coast each night and over the island interiors
each afternoon. Moderate trades and a more typical windward and
mauka shower regime may return by Friday.

&&

.DISCUSSION...
Currently at the surface, a southwest to northeast oriented ridge
of high located around 650 miles north of Kauai, is driving
moderate northeasterly trade winds across the island chain early
this morning. Infrared satellite imagery shows partly to mostly
cloudy skies in windward areas and clear to partly cloudy
conditions in leeward locales. Radar imagery shows scattered to
numerous showers moving into windward areas with a few showers
spilling leeward from time to time. Main short term concerns
revolve around rain chances and trade wind trends.

High pressure north of the state will keep moderate to locally
breezy trade winds blowing through the weekend. The high will
weaken as a cold front begins to approach from the northwest early
next week, and this should ease the trades to light and variable
levels by Monday, with daytime sea breezes and overnight land
breezes common statewide. This land and sea breeze pattern should
hold through mid week, with moderate to breezy trade winds
possibly returning Friday of next week.

As for the remaining weather details, pockets of moisture
embedded within the trade wind flow will bring passing showers to
the islands through the weekend, with showers favoring windward
and mauka areas. A shortwave trough passing overhead from
northwest to southeast may act to enhance the trade showers as it
moves through today. The latest 12z soundings from PHTO and PHLI
show significant dry air aloft, and this should keep any thunder
from occurring over the eastern end of the state, so have removed
the mention of thunder here. Dry conditions should set in
statewide Monday through Thursday, with a few showers possible
near the coast during the night and early morning hours, and over
the interior areas each afternoon. A trade wind shower regime may
return next Friday.

&&

.AVIATION...
A high pressure system north of the Hawaiian Islands will weaken
today decreasing trade wind speeds into the more moderate range
through the weekend. Enhanced shower activity will continue
through the morning hours as unstable clouds associated with an
old frontal boundary drifts over the islands from the north. These
enhanced showers should diminish from west to east starting later
this morning, with enhanced showers lingering near the Big Island
and Maui through the afternoon hours. Periods of MVFR conditions
are possible with heavier showers this morning mainly over
windward and mountain regions.

AIRMET Sierra issued for Tempo Mountain Obscuration above 025 for
north through east sections of Molokai and Maui. This AIRMET will
likely be cancelled later this morning.

AIRMET Tango remains in effect for Tempo Moderate Turbulence below
080 for all areas immediately south through west of mountain
ranges. This AIRMET will likely be cancelled later this morning as
wind speeds decrease.

AIRMET Tango remains in effect for Tempo Moderate Turbulence
between FL270-370. A passing upper level trough will keep this
AIRMET for Clear Air Turbulence in effect through the afternoon.

&&

.MARINE...
High pressure north of the islands will continue to move to the
northeast through today, and we are expecting trade winds to
weaken some into early next week. The SCA has been scaled back to
just the Alenuihaha Channel for today.

A moderate northwest swell is expected to peak today, with
another northwest swell expected Sunday which will maintain
moderate surf through Monday. A small northeast swell will
maintain moderate surf along east facing shores through the
weekend. Without support of this swell early next week and the
lighter trades, surf will be smaller along the east facing shores.
Surf will be small along south facing shores throughout the
period, where a couple of long period pulses from the south-
southwest and south are expected through next week.

A large northwest swell is expected to arrive by next Tuesday and
Wednesday, which will produce warning level surf along north and
west facing shores. Surf would likely reach advisory levels on
Kauai and Oahu on Tuesday, and warning levels by Tuesday night.
The swell will spread down the island chain, peaking Wednesday
followed by gradual decline through Friday. Tides are currently
running about half a foot above predicted levels, and high tide
Wednesday morning is predicted at 2.3 feet. The high tide and surf
arrival will further increase the possibility of runup on
roadways during this event. A Small Craft Advisory (SCA) will also
be needed as seas exceed 10 feet as early as Tuesday afternoon
for waters near Kauai, then quickly spread to other exposed waters
Tuesday night. Seas are expected to fall below SCA levels
Thursday afternoon.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Alenuihaha
Channel.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Powell

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