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Hawaiian Islands Synoptic Discussion and Guidance

997
FXHW60 PHFO 010658
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
858 PM HST Sun Feb 28 2021

.SYNOPSIS...
Windy trade winds will persist through Monday night before they
begin trending down. This comes about as the strong surface high
far north of Kauai starts weakening on Monday. Calmer trade winds
are instore for the islands during the second half of the week. A
more stable air mass is forecast to spread over the islands later
tonight and Monday as the upper level trough weakens and drift
south of the Big Island late. A boost in trade wind showers will
come during the second half of the week with a series of old
fronts affecting the islands.

&&

.DISCUSSION...
Brief, but frequent heavy showers still pose a threat for local
flooding along the Hamakua coast including the Hilo and Puna
districts of the Big Island, and the windward side of Maui this
evening. A Flash Flood Watch (FFA) remains in effect for most of
the Big Island and windward Maui to 6 am HST Monday morning, or
the rest of tonight. The threat of flooding is expected to start
lessening after midnight tonight as a more stable air mass slowly
spread across the island chain from the north. The upper trough,
which has been stationary over the eastern part of the island
chain for the past couple of days, is forecast to weaken and shift
just south of the Big Island by Monday evening. Hence, the FFA
could be lowered when the 4 am HST forecast package goes out.

The weather system responsible for the windy trades, a strong
1043 mb surface high, is located 1300 miles north of Kauai this
evening. Wind Advisory is effect for all islands but Oahu and
Kauai. We will be assessing the need to expand this advisory to
the remaining islands for Monday. We expected the peak of the
wind to occur Monday afternoon due to subsidence from the stable
air, leading to acceleration over the mountain ridge and leeward
slopes. The advisory could be extended into Tuesday despite the
weakening high. But trades are expected to tone down to moderate
to locally strong by Wednesday night.

During the second half of the week, models are in agreement for a
couple of bands of clouds/showers to push through the islands,
leading to a boost in trade showers.

Also, a new surface high, though, located far northwest of the
islands late in the week, may shift the stronger trades to the
west end of the island chain toward Friday and the upcoming
weekend.


&&

.AVIATION...
Strong high pressure to the distant N will support very strong
trade winds while a trough aloft keeps the island atmosphere
unstable, most notably near Maui and the Big Island. This will
ensure a steady stream of fast-moving briefly heavy downpours
moving through windward areas. These windward showers will
produce periods of MVFR VIS/CIG in +SHRA, then gradually diminish
as they spread leeward on the smaller islands. Shower coverage and
intensity over windward Big Island is sufficiently widespread to
warrant AIRMET SIERRA for TEMPO IFR, while SIERRA is also posted
for mountain obscuration on Maui. Coverage over the other islands
may require an expansion of SIERRA. AIRMET TANGO for moderate lee
turbulence will remain posted, and may need to include mention of
ISOL severe turbulence. TANGO has also been updated to include
mention of localized strong surface winds of 30 kt.



&&

.MARINE...
Strong high pressure located far north of the state will produce
areawide strong to local gale force trade winds across the near-
shore waters. Persistent, strong trades over the past several
days have generated fully- developed rough sea conditions around
the Hawaiian Islands and this rough sea state will hold through
Tuesday. Trades achieving or frequently gusting to gale through
the windier channels and bays surrounding Maui County and the Big
Island has a Gale Warning (GLW) posted for the Pailolo and
Alenuihaha Channels, Maalaea Bay and waters south of the Big
Island. A Small Craft Advisory (SCA) remains in effect for the
remaining coastal waters due to the combination of strong trades
and high seas. These conditions will likely persist into Tuesday
as high pressure dominates north of the islands. The high will be
weakened by a North Pacific cold front late Tuesday and Wednesday.
This will result in a short term decline in both trades and
lowering seas with a cancellation of the long standing SCA over
most waters by Wednesday evening. As high pressure moves in from
the west and travels north of the islands late in the week, trades
will restrengthen to moderate to locally strong magnitudes late
Thursday into the weekend over the western waters.

Large trade wind-driven seas will maintain high surf along east
facing shores the next couple of days. Although the east swell is
subtly lowering, these swells in tandem with easterly wind waves
will continue to justify the ongoing High Surf Advisory (HSA) for
the east-facing shores of Kauai, Oahu, Molokai, Maui and the Big
Island. Trades will maintain strong to near gale force magnitudes
through Tuesday. Thus, the HSA will remain up to account for 8 to
12 foot surf heights along most eastern exposures through Tuesday
evening or early Wednesday morning.

Surf will remain below HSA criteria on all other shores through
much of the week. Small, moderate period northwest swells have
peaked today and will decline Monday and Tuesday. A larger
northwest swell arriving late Wednesday will lift north and
west-facing shoreline surf to near advisory levels Thursday and
Friday. These swells will shift more north and decline through
next weekend. Surf along south-facing shores will remain at or
slightly above winter averages through the week due to a combination
of both small south-southwest swells and trade wind wave and
swell wrap.


&&


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

Flash Flood Watch until 6 AM HST Monday for Maui Windward West-
Windward Haleakala-Haleakala Summit-South Big Island-Big Island
North and East-Big Island Interior.

Wind Advisory until 6 PM HST Monday for 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.

Winter Weather Advisory until 6 AM HST Monday for Big Island
Summits.

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

Small Craft Advisory until 6 PM HST Tuesday 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.


&&

$$


H Lau/Birchard/Blood

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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