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Hawaiian Islands Synoptic Discussion and Guidance

777
FXHW60 PHFO 031340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Wed Mar 3 2021

.SYNOPSIS...
Breezy trades will persist today, then lower to moderate levels
Thursday and Friday. Fairly typical trade wind weather will
persist through Thursday, with showers favoring windward and mauka
areas, particularly at night and into the early morning hours. A
surface trough will develop in the vicinity of the islands
Thursday night, then slowly shift westward Friday and Friday
night. This will bring wet trade wind conditions particularly to
windward sections of the smaller islands. As the trough shifts
westward, a more typical but slightly wetter than normal trade
wind pattern will return over the weekend, with trades increasing
back to breezy levels. The trades will strengthen to windy levels
early next week, with the wetter than normal trade wind
conditions persisting.

&&

.DISCUSSION...
Currently at the surface, a 1042 mb high is centered around 1700
miles northwest of Honolulu, and is driving moderate to breezy
trades across the island chain early this morning. Infrared
satellite imagery shows a band of clouds moving through the region
resulting in mostly cloudy conditions across much of the state,
although some decrease in cloud cover is already evident over much
of Maui County behind the cloud band. Radar imagery shows
scattered to numerous showers moving through Kauai and Oahu, as
well as windward sections of the Big Island, with only a few
showers with the trades blowing a few showers into leeward
sections affecting windward sections of Maui. Main short term
concerns revolve around trade wind trends and rain chances during
the next couple days.

High pressure northwest of the islands will gradually weaken as it
settles southeastward during the next few days. Breezy trades are
expected today, with the trades then diminishing to moderate
levels Thursday through Friday. A weak surface trough will
develop in the vicinity of the islands on Friday, then shift
westward Friday night as a new strong 1040+ mb high builds north
of the state. This strong high will slowly settle southeastward
through early next week, with breezy conditions expected over the
weekend, and windy conditions developing for the first half of the
next week.

As for the remaining weather details, fairly typical trade wind
weather will prevail through Thursday, with showers favoring
windward and mauka areas, particularly at night and through the
early morning hours. We should see things dry out nicely behind
the band of clouds and showers early this morning, paving the way
for a really nice day with only a few windward showers. Another
band of clouds and showers which can be seen in satellite imagery
further upstream to the northeast of the state, appears on track
to move through the islands tonight bringing another increase in
shower coverage. Expect conditions to dry out similarly on
Thursday, although quite a bit of high cloud cover will be
overspreading the islands as an upper level trough digs to the
northwest of the state.

A weak surface trough will develop in the vicinity of the islands
Thursday night along the tail end of an old front. This surface
trough will then slide slowly westward Friday and Friday night.
Moisture from the surface trough combined with added instability
from the trough aloft will lead to an increase in shower coverage
and intensity across the islands during this time, with the
potential for some locally heavy rainfall over windward areas. At
the moment, the deepest moisture appears to target the smaller
islands, particularly Kauai and Oahu. Once the surface trough
shifts west of the state, we should see a fairly typical but
slightly wetter than normal trade wind pattern develop for the
weekend, which will continue into early next week.

&&

.AVIATION...
Early this morning, a band of clouds and showers has been moving
across the state. This has resulted in brief periods of moderate
to heavy rain and MVFR level clouds for Oahu and Kauai. As a
result, AIRMET Sierra was issued to account for tempo mountain
obscuration for Oahu and Kauai this morning, though clouds and
showers are expected to begin breaking up over the next few
hours.

A surface high to the northwest of the state will continue to
support breezy to locally strong easterly trade winds today,
but speeds will gradually wane. AIRMET Tango remains in effect
for moderate to severe low-level turbulence near and downwind of
mountains, as well as moderate mid-level turbulence between FL220
and FL300. With winds expected to east throughout the day, AIRMET
Tango for surface winds greater than 30kts for Maui and the Big
Island will likely be allowed to expire later this morning.

&&

.MARINE...
The Gale Warning for Alenuihaha and Pailolo channels, Maalaea Bay
and for waters south of the Big Island has been downgraded to a
Small Craft Advisory (SCA). This SCA, along with the SCA in effect
for all other waters, will remain in effect through this
afternoon due to a combination of strong trade winds and elevated
seas. The SCA may be scaled back briefly to typically windy
waters around Big Island and Maui County tonight. Building high
pressure to the northwest of the state will boost trades first
over waters west of Kauai, then across remaining waters Thursday
into the weekend. This may warrant an expansion and continuation
of the SCA.

Large trade wind-driven seas will maintain east shoreline surf
at or above the 8 foot High Surf Advisory (HSA) threshold through
this afternoon. As trade winds over and upstream of the islands
weaken, surf along east facing shores will decrease, becoming
small by Thursday. Increasing trade wind swell and an arriving
northerly swell at the end of this week and into the weekend will
provide a bump in east shoreline surf.

A northwest swell will begin arriving this afternoon and evening,
peaking surf along north and west-facing shores late Thursday
into Friday. The secondary north swell will arrive Friday and, in
addition to the returning trade wind swell wrap, may boost surf
heights to, or slightly above, HSA heights from the end of this
week into the weekend.

Surf along south-facing shores continues to pull in some trade
wind swell wrap, with an expected decline today with the easing
trades. A southwest swell will fill in late today through
Thursday before slowly declining through the weekend, which will
keep south shore surf from going flat.

&&

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

Small Craft Advisory until 6 PM HST this evening for all
Hawaiian waters-

&&

$$

DISCUSSION...Jelsema
AVIATION...Vaughan/JVC
MARINE...Powell

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

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