Special Marine Warning 71, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
408 AM HST Mon Dec 6 2021

.SYNOPSIS... A kona low will linger just west northwest of Kauai
this morning drifting slowly westward through Wednesday. A large
band of heavy rain associated with this low will also track slowly
westward across the state. Expect widespread heavy rainfall with
this system, especially under the large heavy rain band, capable
of producing catastrophic flooding, and strong gusty southerly
winds through Wednesday. The heaviest rainfall currently over Maui
County this morning will slowly drift westward into Oahu and
Kauai later this morning. Improving weather conditions will start
over the eastern side of the state by late Tuesday with drier air
slowly spreading into the western islands on Wednesday. More
typical trade wind weather will return from late Wednesday onward.

&&

.DISCUSSION...
This mornings satellite imagery shows a kona low roughly 300 to
400 miles west of the island of Kauai with a large plume of clouds
around 300 miles wide extending from Kauai eastward to the Big
Island. Deep convective showers continue to develop within this
tropical moisture plume producing widespread rain, heavy at times,
across the island chain. Radar imagery this morning shows bands of
moderate to heavy showers developing and moving northward in the
southerly winds. Strong and gusty winds are also observed in the
20 to 35 mph range with gusts up to 50 mph in some areas. The
latest rainfall observations in Maui County show from 2 to 10
inches of rain has fallen over the past 24 hours with more
rainfall expected today and tomorrow.

The main threats with this kona low will be continued widespread
heavy rain and strong southerly winds. Widespread storm total
rainfall amounts in the 10 to 15 inch range are forecast with
isolated areas of 20 to 25 inches of rainfall. Rain events of this
magnitude can produce catastrophic flooding and affect areas that
are typically drier, such as the south and west slopes of each
island. Isolated thunderstorms also remain in the forecast into
Wednesday.

Some additional impacts include dangerous driving conditions due to
near zero visibility. Many roads could become impassable due to
severe runoff and flooding. Numerous land slides are also expected
in areas with steep terrain. Debris in streams and gulches may clog
bridges and culverts resulting in dangerous flooding. Urban areas,
especially on Oahu, could have severe flooding. Isolated highways,
such as the Belt Highway in the Big Island's Kau District, and
Piilani Highway on Maui, may be closed in one or more locations,
resulting in long detours or even the isolation of some communities.

Strong and gusty south to southwest winds will continue today.
Wind Advisories remain in effect for all Hawaiian Islands
including the summits areas of Maui and Hawaii Counties. These
strong southerly winds may produce damaging downslope winds over
areas along the northern slopes of mountain ranges. Oahu and
portions of Molokai may be especially vulnerable to these
downsloping gusty winds.

A Winter Weather Advisory was issued this morning for the summits
of Mauna Kea and Mauna Loa on the Big Island above 12,000 feet
elevation and will remain in effect through 6 PM HST.

Weather conditions are forecast to improve from east to west
starting on Wednesday, as the kona low drifts further westward away
from the main Hawaiian Islands. Higher pressure will build in north
of the state allowing an easterly trade wind weather pattern to
return with more typical passing windward and mountain showers.

&&

.AVIATION...
Early this morning, a kona low located west northwest of Kauai is
resulting in southerly flow across the island chain. Latest
satellite and radar imagery show bands of showers, some of which
are producing heavy rain, streaming across the island chain from
the Kauai Channel to the Big Island. MVFR conditions are
widespread across the state with IFR and even some LIFR
visibilities and ceilings observed within the heaviest showers.

As the axis of heaviest showers continues to slowly drift westward
this morning, periodic breaks are likely across Maui County
before additional showers develop later this morning and this
afternoon. Regardless, there will be numerous impacts for aviation
interests throughout the forecast period and beyond including
frequent MVFR conditions with periods of IFR and LIFR. Winds will
be rather strong as well in some locations, with gusts in excess
of 40kts possible. While isolated thunderstorms remain possible
throughout the period, the main threat will be heavy rainfall that
will result in low visibility and ceilings.

Numerous AIRMETs remain in effect this morning. AIRMET Sierra has
been issued for tempo IFR conditions across Maui, Lanai, Molokai,
Oahu, and portions of the Big Island. Additionally, tempo
mountain obscuration AIRMETs remain in effect for the rest of the
Big Island and Kauai, though these may need to be adjusted later
this morning based on latest satellite trends. AIRMET Sierra for
mountain obscuration and/or IFR conditions are expected to
continue over the next two days.

In addition, AIRMET Tango remains in effect north and east of the
mountains of all major Hawaiian Islands for mechanical low-level
turbulence. For now, AIRMET Tango also remains in effect for
sustained wind speeds greater than 30kts statewide, though wind
speeds have started to diminish across the Big Island early this
morning.

&&

.MARINE...
A slow-moving kona low located about 300 nm west northwest of
Kauai is producing a large convergence bands of heavy rain, with
an occasional isolated thunderstorm, across most local waters.
These moderate to strong bands of rain, in tandem with the wind
acceleration around Mauna Kea, Mauna Loa and Haleakala, have
produced strong winds frequently gusting to gale force as verified
by a late Sunday evening ASCAT-C pass. As the low moves westward
the next several days, convergence bands of precipitation will
generally spread more westward over Oahu today and eventually in
the proximity of Kauai later today or early Tuesday. Bands of
primarily showers with isolated storms will likely linger over the
western end of the island chain Tuesday and then move west of the
islands Wednesday as high pressure building in northeast of the
state reinstates a moderate to fresh trade wind regime.

To account for strong rain convergent bands passing across more
western islands today, a Gale Warning is now in in effect for the
coastal waters west of the Kaiwi Channel including the channel. A
Small Craft Advisory (SCA) is in effect for the remaining eastern
Hawaiian nearshore waters due to a combination of these high
winds and seas that still remain elevated due to the passage of a
large north swell. The greatest potential for the strongest
maritime winds the next few days will be within any band of
showers that may or may not contain embedded thunderstorms.
Strengthening trade winds will maintain SCA conditions, especially
for those typically windier waters around Maui and Big Island
Tuesday night or Wednesday.

The most recent swell observations at the far northwest offshore
buoys are beginning to show that the large north swell that peaked
yesterday has begun a slow and steady decline. Swell at both offshore
NOAA 001 and 101 buoys are observing a near 10 foot, 12-14 second
swell (versus near 20 feet, 15 seconds a mere 24 hours ago). Surf has
fallen below High Surf Warning heights and therefore have lowered
flags to a High Surf Advisory (HSA) that is in effect for all north
facing shorelines through the day. The HSA for west facing shores has
been allowed to expire as surf heights have fallen below 12 feet (8
feet Big Island). North shore surf will likely fall below HSA levels
tonight and will fade to small heights from late Tuesday into the
weekend. Strong southerly winds within and near recently strong
convergent overnight rain bands developing far east of a kona low
northwest of Kauai have produced periods of rough, short period
chop that will be impacting south facing shores today. The latest
PacIOOS buoy at Pearl Harbor has shown a very significant spike in
significant wave heights from 3 feet a few hours ago to near 10
feet at 0300 HST. As high pressure builds in from the northeast
later this week, strengthening trade winds will result in elevated
rough wind wave surf along most east facing shores from mid week
onward. By Wednesday, increasing trade winds will produce elevated
rough wind wave surf along most east facing shores.

Abnormally high ocean levels are coinciding with seasonal peak
high tides the next couple of days. Although the north swell is
declining, some elevated coastal run up may occur at peak high
tide times early this morning. Even in areas not affected by
swell, ocean levels will be elevated and some coastal inundation
may occur, especially during periods of heavy rainfall. (Urban)
flooding such as what was experienced in the Waikiki area
yesterday has prompted a Coastal Flood Statement to primarily
increase potential nuisance flooding awareness during high tides.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Tuesday afternoon for all Hawaiian Islands.

High Surf Advisory until 6 PM HST this evening for north facing
shores of all islands.

Wind Advisory until 6 PM HST this evening for all Hawaii Islands.

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

Small Craft Advisory until 6 PM HST this evening for all Maui
County and Big Island waters.

Gale Warning until 6 PM HST this evening for all Kauai and Oahu
waters including the Kaiwi Channel.

&&

DISCUSSION...Bohlin
AVIATION...Vaughan
MARINE...Blood