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Hawaiian Islands Synoptic Discussion and Guidance

077
FXHW60 PHFO 201932
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
932 AM HST Thu Feb 20 2020

.UPDATE...
The forecast remains on track, featuring locally windy trade
winds and a drying trend during the afternoon. The previous
forecast discussion follows.

&&

.SYNOPSIS...
Gusty trade winds will continue today, then gradually decrease
tonight through Saturday as a front approaches from the
northwest. Although rainfall will focus over windward and mauka
locations, strong winds will push showers into leeward areas at
times as well. Winds will become light and variable by Sunday as
the front nears the islands, with sea and land breezes expected
statewide. Showers will favor interior and mountain areas during
the day and locations near the coast at night. Trade winds are
expected to return early next week, with rather wet conditions
possible over the western islands as the front stalls out in this
vicinity.

&&

.PREV DISCUSSION.../ISSUED 303 AM HST Thu Feb 20 2020/
Currently at the surface, a 1033 mb high centered around 900
miles north-northeast of Honolulu is driving strong and gusty
trade winds across the island chain early this morning. Infrared
satellite imagery shows the back edge of a swath of low clouds
approaching the islands from the east, with mostly cloudy skies
present across much of the state at the moment. Radar imagery
shows numerous showers pushing into windward areas from Maui to
Kauai, with coverage beginning to diminish over windward Big
Island. Considerable leeward spillover continues into leeward
sections of the smaller islands, while leeward Big Island is
mainly dry. Main short term concerns revolve around trade wind
trends and rain chances through the remainder of the work week and
into the weekend.

Strong high pressure will slide eastward and weaken slightly over
the next few days as a cold front approaches from the northwest.
Strong and gusty trades will continue across the island chain
today, and as a result, the Wind Advisory is now in effect for
most areas of the state through 6 PM this evening. The trades are
expected to trend lower tonight and Friday, then shift around to
the east-southeast on Saturday as the front moves closer. Light
and variable winds will likely develop by late Saturday across the
western islands while moderate trades hold on across the eastern
end of the state. The gradient will slacken further Saturday
night, allowing a land breeze pattern to develop statewide, with
this sea/land breeze pattern holding in place through at least
Sunday night. Model solutions are in fairly good agreement showing
the front or its remnant band of moisture pushing into the
western islands Monday, then stalling out in this area through the
first half of next week before dissipating. As the front stalls,
a new high will slide by well to the north of the island chain.
Trade winds look to return Monday, with moderate to breezy trades
taking hold Monday night and continuing through the middle of
next week.

As for weather details, the back edge of the showery low clouds is
expected to push westward through the islands over the next few
hours, with a much drier airmass overspreading the island chain
around or shortly after daybreak. This should bring a more
typical trade wind shower pattern to the local area through
Friday night, with showers favoring windward and mauka areas. A
few showers will continue to spread leeward, although there will
likely be a reduction in leeward shower coverage given
precipitable water values dropping to around 0.75 inches.

Over the weekend, a transition over to a sea breeze/land breeze
pattern will keep showers favoring areas near the coast in an
east-southeast to southeast boundary layer flow at night, and
interior and mauka areas during the afternoon/early evening. A
transition over to a trade wind pattern will bring a return of
more windward/mauka showers and the occasional leeward spillover
early next week. Rather wet conditions could develop over the
western islands during the first half of next week however, as the
front or its remnants stall out in this vicinity.

&&

.AVIATION...
Stable stratocumulus clouds with isolated showers tracking in
with breezy trade winds will bring occasional MVFR ceilings across
windward slopes/waters today. No AIRMETs are in effect for
mountain obscurations around the state this morning. Elsewhere,
mostly sunny to partly cloudy skies will prevail. AIRMET Tango
remains in effect for tempo moderate low level turbulence over and
downwind of the terrain of all islands due to the breezy trades.

&&

.MARINE...
A strong surface high pressure system is moving steadily east to
the far north of the state with a very tight pressure gradient
south of it. This is producing strong to near-gale force easterly
trade winds over Hawaiian waters, with gales in areas where the
wind accelerates around the islands. A Gale Warning remains in
effect for the typically windier waters adjacent to the islands
of Maui County and the Big Island through this afternoon, at which
point it will likely be downgraded to a Small Craft Advisory
(SCA). An SCA also remains in effect through early Friday morning
for all other Hawaiian Waters, but will likely be scaled back
later tonight to include fewer areas, as winds and seas slowly
diminish Friday through Friday night.

Trades will weaken by this weekend as the high moves farther away
to the east and a front approaches the state from the northwest.
This will also allow the winds to begin shifting out of the east-
southeast over most areas by Saturday. The front is expected to
move into the far northwest Hawaiian offshore waters Sunday, with
increasing northeasterly trade winds returning behind the front.

A High Surf Advisory remains in effect through Friday afternoon
for east facing shores of the islands, due to the strong trade
winds maintaining rough elevated surf. This surf will subside over
the weekend as the trades weaken.

The current small northwest swell will continue through this
evening, then gradually lower later tonight through Friday. A
slightly larger, long-period northwest swell arriving late Friday
will likely peak on Saturday, then gradually lower Sunday through
Monday. A large northwest swell is possible early next week.
Elsewhere, a series of small south and south-southwest swells
will maintain small background surf along south facing shores
through this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Kauai-Oahu-Molokai-
Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-Maui
Central Valley-Windward Haleakala-Leeward Haleakala-South Big
Island-Big Island North and East-Kohala.

High Surf Advisory until 6 PM HST Friday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Gale Warning until 6 PM HST this evening for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

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

&&

$$

UPDATE...Wroe
PREV DISCUSSION...Jelsema
AVIATION...Foster
MARINE...TS

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



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