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Hawaiian Islands Synoptic Discussion and Guidance

465
FXHW60 PHFO 060636
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
836 PM HST Fri Mar 5 2021

.SYNOPSIS...
A high to the north will maintain breezy trade winds over the area
through the next several days. For the next couple of days, the
most active showers will be over windward areas of Kauai, Oahu and
Maui County. Starting late Sunday, heavier showers will move over
the Big Island, then spread west through mid-week.

&&

.DISCUSSION...
A high to the north will maintain breezy trade winds over the area
through the middle of next week. The trade winds are pushing
remnants of a front over the islands. These remnants will focus
showers over mainly windward areas from Kauai to Maui. A trough
aloft is making the atmosphere over the islands a bit unstable, so
the frontal remnants are taller than typical trade-wind clouds.
The clouds moving over the islands are tall enough, and the trade
winds are strong enough, to carry showers leeward across the
smaller islands.

The trough aloft is forecast to deepen over the next few days.
That will make the atmosphere more unstable and help pull deeper
moisture over the islands from the southeast starting on Sunday.
It is too soon to know the details of this developing pattern, but
it looks as if rainfall will become heavier and more widespread
starting on the Big Island by late Sunday, then spread northwest.
Heavy rain and thunderstorms will be possible on the Big Island
with ice and snow over the upper elevations. As this pattern
develops, a Flash Flood Watch and a Winter Weather Watch may need
to be issued.

There are some large differences from model run to model run and
between the GFS and ECMWF models next week. That reduces the
confidence we have in the long-term forecasts.

&&

.AVIATION...
High pressure far north-northwest of the islands is helping to
maintain an easterly low level flow near the islands. The remnants
of a front to east of the islands, combined with an upper level
trough over the island, are helping to usher in clouds and
showers to the islands from the east. Most of the activity is
focused over the windward and mountain areas, however many of the
showers are traversing the islands. The high pressure system is
expected to move to the east over the next day or so, with the
upper level feature staying in place, so expect little change in
the forecast. Most areas will continue to experience VFR
conditions, most of the time.

AIRMET SIERRA continues for tempo mountain obscuration over Kauai
and Oahu due to clouds and showers. Anticipate this will be in
place most of the night, and could be expanded to additional
islands. Winds have been strong enough over Kauai to lead to
AIRMET TANGO for low level turbulence to the lee of the mountains.
Will be monitoring this overnight, but its possible that the winds
may weaken some during the second half of the night.

&&

.MARINE...
A strong high pressure system far north of the Hawaiian Islands
will drift slowly southeastward over the next several days.
Trade wind speeds will remain the fresh to strong range through
Saturday, and into the strong to near gale range from Sunday
through Wednesday. Gale conditions may develop over the windier
eastern waters and channels next week.

The current Small Craft Advisory (SCA) includes most island
waters due to a combination of strong trade winds and elevated
seas. A SCA for most coastal waters will likely continue through
much of next week due to increasing trade winds.

A north swell is elevating surf heights along exposed shores.
This north swell is larger over the eastern waters with a high
surf advisory in effect for north facing shores of Molokai, Maui
and the Big Island. This swell may produces surges in north facing
harbors, including Hilo and Kahului harbors. This swell will
diminish Saturday afternoon into Sunday. The forerunners of the
next northwest swell will arrive on Sunday with overlapping
northwest and north swells producing continued medium size surf
along north and west facing shores through Monday, then decreasing
to smaller size surf from Tuesday through Thursday.

Strengthening trade winds will increase surf along east facing
shores this weekend with a building trade wind swell. Surf may
reach HSA levels along exposed east facing shores on Tuesday and
Wednesday.

Background south swells will provide small surf to south facing
shores through Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM Saturday for north facing shores of
Molokai, Maui, and the Big Island.

Small Craft Advisory until 6 AM Sunday for all Hawaiian waters
except Maui County Leeward waters and Oahu Leeward waters.

&&

$$
DISCUSSION...Donaldson
AVIATION...M Ballard
MARINE...Jelsema/Bohlin

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



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