Flash Flood and Heavy Rain in Hawaii, October 11 to 12, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
331 PM HST Mon Oct 11 2021

.SYNOPSIS...

Strong trades will bring widespread windward showers tonight into
early Wednesday, especially over Big Island and Maui where there
is a slight chance of thunder and locally heavy rainfall is
possible. Trades will gradually trend toward moderate by this
weekend.

&&

.DISCUSSION...

The primary change to the forecast for tonight was to add a slight
chance of thunder for Windward Big Island and Maui and adjacent
waters. This change was prompted by isolated but persistent deep
convection anchored on the nose of the approaching moisture plume,
or right around 20N/145W and translating due west. A brief flare
up of this activity resulted in numerous lightning strikes well
offshore between about 130pm and 230pm HST. At press time, deep
convection persists although lightning appears to have diminished
for now. Model analysis suggests mesoscale convergence at the apex
of the 1000-700mb plume beneath a broader area of conditional
instability has been supporting this activity and will steadily
advance westward toward the Big Island this evening. Therefore,
a slight chance of thunder was added for the first half of
tonight before said convergence begins to decrease. Otherwise, no
significant changes were needed to the overall forecast reasoning
as the approaching low-level trough will supply strong trades with
ample low-level moisture tonight. A high coverage of steady
moderate-to-locally heavy showers will develop over Windward Big
Island and Maui this evening, eventually spreading to the
remaining smaller islands overnight albeit in a diminished
capacity. The official forecast calls for 2-4" in the wetter
upslope areas of Big Island & Maui with locally 5" possible,
especially if any thunderstorms develop over the windward slopes.
The high res guidance is very aggressive with QPF and is likely
keying in on the aforementioned conditional instability as
moisture is forced upslope by strong winds. However, given that
tonight's rainfall will be occurring against a background of
building ridging aloft, increasing subsidence, and a tendency for
mid-level drying above the incoming moisture field, the
expectation is for thunder remain very isolated. Overall, expect
the event to be characterized by steady moderate rain with
brief/transient higher rates within heavier showers, mainly over
the eastern end of the state.

Showers will continue over Windward Big Island well into Tuesday
before diminishing in coverage during the afternoon as deeper
moisture exits to the west. There remains a very low potential
for thunder over the leeward Big Island slopes on Tuesday
afternoon, although warm air aloft characterized by 500mb
temperatures around -6C will make destabilization difficult. The
remainder of the week will feature weakening trades caused by
deamplification of the pattern over the north Pacific and upper
ridging centered south of the state. The steady relaxing of the
gradient will allow breezy trades to take hold during mid-week and
then weaken to moderate by Saturday.

&&

.AVIATION...

A strong high pressure system far northeast of the state will result
in breezy trade winds over the next several days across Hawaii.
Due to the breezy winds, AIRMET Tango remains in effect for areas
over and immediately south through west of mountains on all
islands for tempo moderate turbulence below 8000 feet.

Incoming deep tropical moisture will bring an uptick in clouds
and showers this evening across the state. AIRMET Sierra is posted
for windward slopes of Oahu, Molokai, Maui and the Big Island for
tempo mountain obscurations above 2500 feet. Kauai will likely be
included in AIRMET Sierra later this evening as the showers track
westward. MVFR ceiling and visibility will also occur around
showers and windward terrain. Leeward areas should also expect
brief/isolated MVFR conditions and mountain obscurations.

This weather pattern will continue statewide on Tuesday before
moving to the west Wednesday.

&&

.MARINE...

No major changes with the marine forecast with regards to winds,
seas or surf this afternoon, but have added the slight chance of
thunderstorms to some of the waters near Maui and the Big Island
to start the night. An upper level low to the northeast of the
islands, combined with a weak surface trough to the south is
helping to provide some instability with the moisture riding in on
the strong trade winds. There have been some thunderstorms in the
eastern offshore waters today, and expect a few more through
Tuesday night in the east and southern waters. The upper level low
will be lifting to the north during this time. The main threat
over the coastal waters will be tonight, while the threat will
linger a little longer over those offshore waters.

Strong high pressure to the north of the islands, combined with a
weak trough to the south of the islands will continue to produce
fresh to strong trade winds across the coastal waters. The surface
trough is expected to move westward and be to the southwest of
the islands by tomorrow morning, but little change in the pressure
gradient over the islands is expected during this time. These
strong trade winds are contributing to seas in excess of 10 feet
in many waters, and are bringing advisory level surf to east
facing shores.

A Small Craft Advisory (SCA) remains in effect through Tuesday
night for all coastal waters due to the winds and/or seas, and a
High Surf Advisory (HSA) remains in effect through Tuesday
afternoon for east facing shores of Kauai, Oahu, Molokai, Maui and
the Big Island. The SCA will likely be extended into the latter
part of the week for at least the typical windier areas near Maui
and the Big island. As winds weaken just a bit midweek, surf along
the east facing shores should subside to just below advisory
levels.

South swells through midweek will keep surf along south facing
shores surf above normal, before dropping to mostly background
levels Friday through next weekend. Surf along north facing shores
will remain very small through Thursday. A small long-period
northwest swell is expected to arrive Thursday night, and will
give a small boost to the surf along north and west facing shores
Friday through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Tuesday for Olomana-Kauai East-
Kauai South-East Honolulu-Koolau Windward-Koolau Leeward-Molokai
Windward-Molokai Southeast-Windward Haleakala-Kipahulu-South
Haleakala-Big Island Southeast-Big Island East-Big Island North.

Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian
waters-

&&

DISCUSSION...JVC
AVIATION...Foster
MARINE...M Ballard