High Surf in Hawaii, December 3 to 6, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
1023 AM HST Fri Dec 3 2021

.SYNOPSIS...

Periods of heavy rain potentially resulting in flooding will be
possible through Saturday. Widespread heavy rain capable of
causing more significant flood impacts is then expected Sunday
into the middle of next week.

&&

.DISCUSSION...

Today and tonight...
A wide coverage of moderate to occasionally heavy showers is
loosely organized along a convergent wind shift boundary over the
central portion of the state. These showers prompted a flood
advisory over Molokai earlier and will continue to bring periods
of heavy rain mainly to Maui and the Big Island during the next
several hours. Isolated thunderstorms may develop over these
islands as diurnal heating causes destabilization heading into
this afternoon. In general, this band of convection will be
sufficiently progressive to limit significant flooding concerns
but the potential for flooding can certainly not be ruled out.
Despite the areas of heavy rain evident on radar imagery, the
wettest spot during the last hour has been on the north shore of
Kauai within the onshore/northerly flow where the Kilohana rain
gauge has recorded 0.61".

The cold front itself lags the wind shift by a few hours and is
on the doorstep of Kauai. Its passage will occur with little
fanfare aside from increasing stability, dropping dewpoints, and
cooler temperatures. In the wake of the front, dewpoints will fall
well into the 50s for Kauai through Maui tonight with overnight
lows following suit. A crisp, cool night and Saturday morning is
in store for these areas with forecast lows in the low to mid 60s.
Likewise, high temperatures on Saturday will top out in the mid
to upper 70s for the same locations. The front is forecast to be
east of the Big Island by Saturday morning.

Saturday through Sunday morning...
Saturday will represent a transition day from the initial, and
mostly benign, cold frontal passage to the heart of the event.
During the day Saturday, the closed upper low is forecast to be
centered near 35N approximately due north of Kauai with the
surface cold front positioned east of the Big Island. As the final
key piece of 185kt jet energy is injected into the upstream side
of the trough, it will quickly evolve southwestward. As the cold
upper low nears the islands, southwesterly flow over the island
chain will deepen from the top down, culminating in the
development of a surface low west of Kauai by Saturday evening. As
this occurs, showers over the western end of the state will tend
to deepen but should still remain rather benign. However, a marked
increase in shower coverage and intensity over central portions
of the state will be possible late Saturday evening into late
Saturday night. This will occur within a broad corridor of
developing isentropic ascent, a consequence of deepening southwesterly
flow, and increased orographic forcing focused over leeward
zones. The lack of a mechanism to force showers to organize into a
band appears to be a limiting factor for significant flooding
concerns for these locations during this time period.

Over the Big Island, uncertainty still exists with respect to the
ultimate eastward position of the front, but at this time it
expected to be east of the area Saturday into Sunday afternoon.
However, with the upper low and associated 130+ upper jet parked
over the western end of the state, right entrance dynamics will be
increasing in a big way during this time. Thus, although the
front should remain east of the Big Island at the surface, it will
slope backwards such that the 700mb front resides somewhere over
Maui County. The global models indicate numerous waves of
potentially organized showers moving from southwest to northeast
over the Big Island and Maui Saturday through Sunday morning.
Although the moisture associated with this activity will initially
be limited, the strengthening mid-level frontal circulation and
increasing instability aloft will still be sufficient to cause
periods of high rain rates.

Sunday afternoon through Wednesday...
The heart of the event will be driven by a deep, strong frontal
circulation maintained by nearly stationary and intense right
entrance dynamics associated with the upper jet near Kauai. As the
the plume of deep moisture east of the Big Island begins its
westward slide across the state, it will come under the influence
of increasing frontal forcing which will allow for greater
organization and heavier rain rates over the Big Island. Then, as
it progresses toward the central islands, including Oahu, the
surface front will tend to reorganize further west directly
beneath the strongest dynamics. The resultant front will be
vertically steep and the surface front & deeper moisture will be
much closer to the strongest forcing and cooler temperatures
aloft.

The peak of the event is therefore expected to initially feature
widespread heavy rain with embedded isolated thunderstorms and
intense rain rates over the Big Island beginning sometime Sunday
afternoon and persisting through Sunday night. Several inches of
rain can be expected and the attendant flash flooding potential will
be maximized over the Big Island during this time period.

The latest data suggests that as the moisture band works
northwestward Sunday night through Monday, it will encounter an
increasingly favorable environment in terms of instability aloft
and deep layer forcing. As a result, the potential for banding and
the propensity for training cells containing intense rain rates
may actually increase within the Oahu-Maui corridor. Significant
flooding impacts will be possible over Oahu, Molokai and
potentially Maui during this time.

No changes to the inherited headlines at this time. The Flash
Flood Watch remains in effect for the forecast reasoning outlined
above. The Blizzard Warning likewise remains in effect as intense
mid- level winds associated with the developing low will bring
gusts in excess of 100 mph to the Big Island summits. Moisture
will be sufficiently deep during this time to bring periods of
snowfall amounting to a foot or greater. Snow levels will then
rise above summit level as the band of heavy rainfall moves over
the island on Sunday.

&&

.AVIATION...

A deep upper level trough moving in from the northwest will
destabilize the air mass over Hawaii today, creating conditions
conducive for locally heavy rain and possible thunderstorms. Bands
of showers along and ahead of the corresponding surface front can
be seen on radar continuing their slow march eastward across the
state. AIRMET Sierra for Mountain obscuration is currently in
effect for the islands of Oahu, Molokai, and Maui and may need to
be expanded to include Maui and north facing slopes of the Big
Island later today.

As of 8am HST, AIRMET Tango was in effect for low level
mechanical turbulence over and downstream of the higher terrain of
Kauai and Oahu. Conditions may spread eastward across the remainder
of the islands later today. Lastly, strong winds behind the front
could necessitate an additional AIRMET for widespread strong
surface winds this evening.

&&

.MARINE...

A vigorous cold front is beginning to move through the state from
the northwest this morning. Winds have been turning toward the north
and are increasing in speed as the front progresses. The front is
expected to stall near the Big Island tonight and Saturday as a
strong low aloft develops near Kauai. This low will spawn a surface
kona low just west of Kauai Saturday night, which will linger near
the islands for several days. A convergence band of showers east
of the low will develop over the islands, with a strong potential
for an extended period of unsettled weather. Although confidence
is not all that high, generally expecting low pressure to linger
west of Kauai through the middle of next week, keeping an unstable
southerly flow in place over the area.

As the front initially moves down the island chain, strong and
gusty winds are expected, especially in and near any heavy showers
or thunderstorms. A Gale Warning is posted for waters between
Kauai and Niihau as northerly winds surge today, with Small Craft
Advisory (SCA) winds and seas for most other marine zones. Although
winds will ease Saturday, combined seas will be well above the 10
foot threshold for SCA for most waters through the weekend. Winds
will increase again Sunday, out of the south and southwest this
time, as the surface low develops northwest of Kauai.

Locally generated wind waves, plus a building north-northwest swell,
are expected today. This swell will shift to the north and continue
to build through the weekend as the wave period increases. This
fairly long-lived, large north swell will quickly build surf heights
close to 30 feet tonight along north facing shores and holding through
this weekend. Therefore, a High Surf Warning has been issued for
north facing shores of Niihau, Kauai, Oahu, and Maui, with a High
Surf Advisory for west facing shores of Niihau, Kauai, and Oahu as
well. As the swell becomes more northerly Saturday night and Sunday,
high surf could affect north shores of the Big Island as well. High
tide early Saturday and Sunday could lead to wave runup in some
coastal areas, as well as significant harbor surges in both Kahului
and Hilo harbors. A Marine Weather Statement has been issued for
those hazards.

Expect minimal surf along east facing shores today, with some
increase this weekend as the large north-northwest swell turns
more northerly, allowing some swell energy to wrap around the east
sides of the islands. Surf along south facing shores will rise
slightly over the weekend and early next week due to locally-
generated wind waves, as well as a small, long-period south swell
beginning Monday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Monday afternoon for Niihau-Kauai Leeward-
Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central
Oahu-Waianae Mountains-Lanai Mauka-Kahoolawe-Maui Windward West-
Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island
Interior-Kauai North-Kauai East-Kauai South-East Honolulu-
Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward-Molokai-
Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley
North-Maui Central Valley South-Windward Haleakala-Kipahulu-
South Maui/Upcountry-South Haleakala-Big Island South-Big Island
Southeast-Big Island East-Big Island North.

Wind Advisory until 6 PM HST this evening for Niihau-Kauai-

High Surf Warning until 6 AM HST Monday for north-facing coasts.

High Surf Advisory until 6 AM HST Monday for Kauai Leeward-
Waianae Coast-Molokai West.

High Wind Warning from 6 PM this evening to 6 AM HST Monday for
Big Island Summits.

Blizzard Warning from 6 PM this evening to 6 AM HST Sunday for
Big Island Summits.

Wind Advisory from 6 PM this evening to 6 AM HST Monday for
Haleakala Summit.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Kauai Leeward Waters.

Gale Warning until 6 PM HST this evening for Kauai Leeward
Waters.

&&

DISCUSSION...JVC
AVIATION...Bedal
MARINE...TS