Heavy Rain in Hawaii, February 9, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Tue Feb 9 2021

.SYNOPSIS...
A fast-moving cold front is expected to move over the Big Island
today. Some clouds, showers and the change for thunderstorms will
accompany the front. Cool, breezy northerly winds will spread
across the islands behind the front. Winds will become moderate
trade winds Wednesday through Friday, bringing gradually
increasing windward showers. Southeast winds may bring a further
increase in moisture starting next weekend.

&&

.DISCUSSION...
The front has continued to progress down the island chain
overnight and is edging closer to the Big Island. The cloud band
associated with the front has started to fall apart overnight,
with fewer showers evident on radar over the central islands. That
being said, some thunderstorms have still developed to the
southwest of the islands over the last couple of hours, speaking
to the instability introduced to the region by the front and the
upper trough on the heels of the front.

Temperatures at the higher elevations of Haleakala are dropping
closer to freezing, so any precipitation falling there is likely
in the form of freezing rain and/or snow. With the front near
Maui, the threat for winter weather continues, so the Winter
Weather Advisory for Haleakala will continue for now, but may be
able to be dropped later today, depending on how quickly the drier
air moves over Maui.

Relatively clear and dry conditions were seen on the Big Island
overnight, but as the front approaches over the next few hours,
clouds and showers are expected. Temperatures atop Mauna Kea and
Mauna Loa are at or below freezing, so the winter weather advisory
continues there as well, however it looks like it could still be
a few hours before any precipitation reaches the summits. As
such, they will likely remain under the advisory through much of
the day.

Northerly winds are filling in over Kauai, Oahu and parts of Maui
County behind the front, and dewpoints have dropped into the mid
50s along the coast, and lower inland. However wind speeds have
not been too strong. Suspect we will see an uptick after daybreak,
so have left winds as they were, with a few areas approaching
advisory levels.

There is good model agreement in the short term with the front
moving over the Big Island today, and then moving to
southeast/east tonight. Leftover moisture from the front will
linger off to the east for a few days. Meanwhile, winds are
expected to become moderate trades Wednesday, and then hold on
into the weekend. As the trades become established, some of that
leftover moisture will ride in on the trades, which could boost
some of the trade wind shower activity.

The models remain in good agreement into the weekend, but start to
diverge Sunday. Both the GFS and ECMWF show a front or boundary to
the northwest of the islands, which will bring southeast winds to
the islands again for start of the new week. Beyond that
confidence is low. The PoPs and associated grids are weighted
towards the GFS solution in the extended at this time.

&&

.AVIATION...
The quick moving cold front passing over the state has begun to
deteriorate, but is still expected to bring low clouds, rain, and
possible thunder and lightning to Maui County and portions of the
Big Island this morning. Kauai and Oahu which now lie in the wake
of the front are experiencing scattered light showers and locally
breezy north to northwest winds.

AIRMET Sierra remains in effect for current or expected mountain
obscuration across Maui County. Conditions could spread to
Windward Big Island early this afternoon, but should end across
Molokai and Lanai by late morning.

Winds will veer from north northwest to east northeast Tuesday
evening as the front exits the area. Expect winds to be breezy to
locally strong immediately along and behind the front. A return
to dry light to moderate trade wind flow is expected Tuesday
night.

AIRMET Tango remains in effect for low level mechanical
turbulence to the lee of higher terrain. Tango currently includes
Kauai and Oahu but will likely need to be expanded to include
Maui County by late morning.

The front is being strengthened by a digging upper level trough
that trails the surface feature. An AIRMET for moderate upper
level turbulence has been issued for Maui County and adjacent
waters. Conditions are not expected to persist past 1600Z as the
trough is expected to weaken as it moves eastward.

&&

.MARINE...
A cold front will continue moving southeastward through the Big
Island waters this morning. The front will move into the far
eastern waters tonight. Periods of showers are forecast along the
front with isolated thunderstorms. Breezy to locally strong
northerly winds this morning will shift to northeasterly trade
winds this afternoon.

A Small Craft Advisory (SCA) remains in effect for all coastal
waters due to increasing winds and elevated seas with a rising
northwest swell. This SCA will decrease in coverage over the
western waters on Wednesday as trade winds decrease and seas
lower from west to east. The SCA will likely be cancelled for all
waters by Thursday morning. Another large northwest swell will
arrive on Friday with a SCA likely going up again for Hawaiian
waters exposed to this swell.

The northwest swell arriving this morning will continue to build
today, peaking from this afternoon through tonight. Surf heights
are expected to rise into advisory levels along most north and
west facing shores of the smaller islands from this afternoon
through Wednesday morning. This swell will lower from Wednesday
into Thursday as the forerunners of a new large northwest swell
arrives by early Friday morning.

A new long period northwest swell will rise quickly on Friday.
Surf heights will likely reach High Surf Warning levels late
Friday through Sunday. The swell and surf heights will then slowly
lower through the first half of next week.

Surf along east and south facing shores will remain on the small
side through the next seven day forecast period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from noon today to 6 AM HST Wednesday for
Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Olomana-Molokai-Maui Windward West-Windward
Haleakala.

Winter Weather Advisory until 6 PM HST this evening for
Haleakala Summit-Big Island Summits.

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

&&

DISCUSSION...M Ballard
AVIATION...Bedal
MARINE...Bohlin