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Hawaiian Islands Synoptic Discussion and Guidance

469
FXHW60 PHFO 290640
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Fri Feb 28 2020

.SYNOPSIS...
A strong high pressure system far northeast of the islands will
produce windy conditions statewide through Sunday. An upper level
low northeast of the Hawaiian Islands will drift over the state
this weekend, keeping wet weather conditions in the forecast for
all islands with isolated thunderstorms and periods of snowfall
reaching the highest peaks of Maui and the Big Island. Unsettled
weather will continue into the first half of next week. Improving
conditions in the forecast by Wednesday, then trending towards
more unsettled weather for the second half of next week.

&&

.DISCUSSION...
A strong high pressure system will build above 1040 MB central surface
pressure this weekend, increasing local area pressure gradients
and keeping windy trade winds in the forecast through Sunday. Wind
speeds are already exceeding wind advisory thresholds in portions
of Maui and Hawaii Counties this evening. A Wind Advisory was
issued this evening for both counties.

Meanwhile in the upper levels, a cold core low pressure system is
forming northeast of the island chain. This low will drift over
the eastern half of the state by Saturday morning. Unstable
conditions associated with the cold core temperatures aloft will
lift/weaken the subsidence inversion heights/cap producing
numerous showers and isolated thunderstorms across the region
through Sunday. This evenings radar imagery shows these numerous
showers already moving into the windward slopes of all islands.
Most leeward areas will also see periods of wet weather this
weekend. Snow levels over the highest peaks of the Big Island and
Maui will fall just below the 10,000 foot level. Periods of
snow and ice are likely over Haleakala, Mauna Loa and Mauna Kea
through Sunday. A Winter Weather Watch remains in effect from
Saturday night through Sunday evening for all three mountain
peaks.

Monday through Wednesday...The upper level low slowly weakens and
drifts northward with lingering scattered to numerous showers in
the forecast trending downward through the middle of the week. The
surface high north of the islands drifts further eastward away
from the state causing wind speeds to weaken down into the
moderate to breezy range.

Thursday and Friday...High pressure builds back in north of the
islands with trade winds increasing back up to breezy to locally
windy levels across the region. A weak upper level trough will
trend shower activity higher with continued unsettled weather in
the forecast through Friday.

&&

.AVIATION...
Strong high pressure northeast of the state will keep strong and
gusty trade winds blowing across the island chain through
Saturday. Clouds and showers will favor windward and mauka areas,
although showers will reach leeward areas regularly due to the
strength of the trades. Some temporary MVFR cigs/vsbys can be
expected as showers move through windward locales, with VFR
conditions expected to prevail in leeward areas.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Oahu, Molokai, Maui, and the Big Island.
These conditions will likely continue through the night.

AIRMET Tango is in effect for low level turbulence over and
downwind of the terrain of all islands. These conditions will
likely persist through the weekend. AIRMET Tango may also be
needed for upper level turbulence over the western half of the
state later tonight.


&&

.MARINE...
Strong east-northeast trades associated with 1038 mb high pressure
northeast of the state are here to stay through Monday. Pailolo and
Alenuihaha Channels, Maalaea Bay and waters around South Point of
the Big Island will remain near gale-force levels through Sunday.
WAVEWATCH III and ECMWF-Wave guidance depict seas building into the
12-18 ft range over the weekend/early next week across a good sized
area within a couple of hundred nautical miles to the northeast of
the windward waters as the gradient tightens between high pressure
to the northeast and a trough in the vicinity the islands. As this
surface trough continues westward through the islands Tuesday
through Wednesday, there may be a brief period of time when the
local winds trend down into the light to moderate range for some
locations (low confidence being this far out).

The Small Craft Advisory (SCA) remains in effect through the weekend
for all waters due to a combination of winds and seas, which will be
extended through Monday in later packages. In addition to the rough
conditions, shower coverage will increase through the weekend as
an upper low moves into the area.

Surf along east facing shores will steadily rise through the weekend
due to the aforementioned scenario evolving over and upstream of the
islands. Heights will hold around the advisory level (8 ft) through
Saturday, then climb toward warning levels late Saturday night
through Sunday. The advisory in place through Sunday will need to be
extended through Monday in later packages.

Surf along north and west facing shores will gradually ease over the
weekend as the northwest swell that peaked earlier fades. This will
be the beginning a quiet stretch for north and west facing shores,
which may continue through all of next week. Some of the trade wind
energy, however, may wrap into exposed north facing shores each day.

Surf along south facing shores will remain small with mainly background
south and south-southwest swells moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Sunday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 AM HST Monday for Lanai-Maui Windward West-
Maui Leeward West-Leeward Haleakala-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Winter Storm Watch from Saturday evening through Sunday
afternoon for Haleakala Summit-Big Island Summits.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Gibbs

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



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