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Hawaiian Islands Synoptic Discussion and Guidance

425
FXHW60 PHFO 060704
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
904 PM HST Sun Dec 5 2021

.SYNOPSIS... A kona low will linger just west of Kauai this evening
with a slow westward drift lasting 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 southwest winds through
Wednesday. The heaviest rainfall currently over Hawaii and Maui
Counties this evening will slowly drift westward into Oahu later
tonight, and into Kauai on Monday. 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...
Satellite imagery shows a strong kona low roughly 300 to 400 miles
west of the island of Kauai this evening with a large plume of
clouds around 300 miles wide extending from Oahu eastward to Maui
and the Big Island. Deep convective showers are developing within
this broad cloud band producing widespread heavy rain across the
central and eastern Hawaiian Islands. Radar imagery this evening
shows the heaviest shower activity over all islands in Maui County
with periods of heavy showers continuing over the Big Island of
Hawaii. The highest radar estimated rainfall rates are falling at
two to three inches per hour under this band. Surface observation
reports over the past 24 hours show widespread rainfall over Maui
and Hawaii Counties with the highest rainfall totals ranging from 2
to 6 inches.

The main threats with this kona low will be continued widespread
heavy rain and strong gusty 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 are already causing power
outages in some locations in Maui and the Big Island. Wind
Advisories remain in effect for the island of Oahu, including Maui
and Hawaii Counties. These strong winds will continue through the
night over the eastern half of the state and then move westward into
Oahu and possibly Kauai on Monday. These strong southerly winds may
produce damaging downslope winds over areas north and east of
mountain ranges. Oahu and portions of Molokai may be especially
vulnerable to these downsloping winds and a High Wind Warning will
be considered for these areas by Monday morning. The Wind Advisory
may also be extended westward to cover Kauai and Niihau with the
morning forecast package.

A High Wind Warning remains in effect for the summits Mauna Kea and
Mauna Loa on the Big Island, and over Haleakala on Maui through
Monday morning. Wind speeds were observed near 90 mph by employees
of the National Park Service on Haleakala earlier today.
These strong summit wind speeds will trend lower from Monday into
Tuesday.

A Blizzard Warning remains in effect for the highest summits of
Mauna Kea and Mauna Loa on the Big Island through Monday morning.
Warm air advection, produced by the strong southerly flow, has
lifted the snow level height to near the 13,000 foot level just
below the peaks of both mountains. Snow will melt and turn to rain
below this snow level height.

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...
This evening, low pressure located northwest of Kauai is resulting
in southerly flow across the island chain. Latest satellite and
radar imagery show a band of heavy rainfall currently streaming
across the state from east Oahu to the Big Island with the heaviest
rainfall focused over Maui County. This will continue throughout the
night with the axis of heaviest rain slowly shifting westward
towards Oahu by sunrise and spreading to Kauai on Monday. Therefore,
numerous impacts for aviation interests are expected throughout the
forecast period and beyond including frequent MVFR conditions with
periods of IFR visibility and ceilings in heaviest showers. Winds
could be rather gusty at times as well, with PHKO already reporting
gusts as high as 46kts this evening. 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 evening. AIRMET Sierra has
been issued for tempo IFR conditions across Maui, Lanai, Molokai,
and portions of the Big Island. Additionally, tempo mountain
obscuration AIRMETs remain in effect for the rest of the Big Island
and across Oahu and Kauai. Although some spacial and temporal
adjustments may be needed later tonight or early tomorrow morning,
AIRMETs for mountain obscuration and/or IFR conditions are expected
to continue over the next two days.

In addition, AIRMET Tango remains in effect downwind of the
mountains of Maui and the Big Island for mechanical low-level
turbulence. An AIRMET Tango also remains in effect for winds greater
than 30kts at the summits of Maui and the Big Island.

&&

.MARINE...
A slow-moving kona low located about 300 nm west of Kauai is
producing a large convergence band of heavy rain, with an occasional
isolated thunderstorm, across the waters surrounding Maui County and
Big Island. These moderate to strong bands of rain, in tandem with
the wind acceleration around Mauna Kea, Mauna Loa and Haleakala, are
producing strong winds frequently gusting to gale force over the
majority of the surrounding Big Island and Maui waters through early
Monday morning. As the low moves westward the next several days,
this convergence band of precipitation will generally spread more
westward over Oahu tonight and Monday and eventually in the
proximity of Kauai late Monday 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 re-
instates a moderate to fresh trade wind regime.

A Gale Warning is in effect for the coastal waters surrounding Big
Island and Maui (sans the waters southeast of Big Island) through 6
AM Monday to account for frequent wind gusts to gale. A Small Craft
Advisory (SCA) is in effect for the remaining Hawaiian waters due to
a combination of high winds and seas that remain elevated due to a
passing 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. An all water SCA
currently runs from 6 AM to 6 PM Monday, but will almost certainly
be extended in time for those waters experiencing strong
southeasterly winds Tuesday. Strengthening trade winds will maintain
SCA conditions, especially for those typically windier waters around
Maui and Big Island Tuesday night or Wednesday.

Today's large north swell has peaked this afternoon and will begin
to decline through Monday. A High Surf Warning (HSW) remains in
effect for the north facing shores of all islands, as well as the
west facing shores of Big Island north of Keahole Point, through
tonight. A High Surf Advisory (HSA) is in effect for most west
facing shores from Maui to Kauai. A Marine Weather Statement is also
in effect through tonight for surges moving into north facing
harbors, in particular Kahului and Hilo Harbors. The HSW for north
shores will likely be lowered to an HSA early Monday with west
facing shores dropping below HSA levels. North shore surf will
likely fall below HSA levels Monday afternoon and will fade to small
heights from mid week into next weekend.

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 late tonight and early Monday 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
today has prompted a Coastal Flood Statement to primarily increase
potential nuisance flooding awareness during high tides.

Strong southerly winds within and near strong rainfall bands will
produce periods of rough, short period chop along south facing
shores through Monday. By Wednesday, increasing trade winds will
produce elevated rough wind wave surf along east facing shores.
&&

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

High Surf Warning until 6 AM HST Monday for Niihau-Oahu North
Shore-Olomana-Maui Windward West-Kohala-Kauai North-Koolau
Windward-Molokai Windward-Molokai North-Maui Central Valley
North-Windward Haleakala-Big Island East-Big Island North.

High Wind Warning until 6 AM HST Monday for Haleakala Summit-Big
Island Summits.

Blizzard Warning until 6 AM HST Monday for Big Island Summits.

Wind Advisory until 6 AM HST Monday for Oahu, including Maui and
Hawaii Counties.

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

Gale Warning until 6 AM HST Monday for Maui County Windward
Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters.

Small Craft Advisory until 6 PM HST Monday for all Hawaiian
waters.

&&

$$

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

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