Flash Flooding and Debris Flow in Hawaii, March 11 to 15, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
906 PM HST Wed Mar 10 2021

.SYNOPSIS...
A wet pattern could linger into the weekend as deep tropical
moisture pools northward over the state in response to low
pressure to the west. Saturated grounds and elevated water levels
in streams will keep the threat for flash flooding up across the
state through Friday, especially over the western end. Light to
moderate southerly winds will hold through Friday, then shift out
of the north-northeast and become strong over the western end of
the state by Saturday as high pressure builds to the north behind
a cold front.

&&

.DISCUSSION...
Deep tropical moisture pooling northward across the state due to an
upper trough and surface low positioned to the west will keep the
threat for heavy rainfall and flash flooding in the forecast through
Friday. Although the threat for localized flooding will remain in
place statewide, the best chance will remain over Oahu and Kauai
where the better instability is. As a result, the Flash Flood Watch
(FFA) remains in effect through 6 PM HST Friday statewide. In
addition to the rainfall, the aforementioned instability associated
with the upper trough will be enough to trigger thunderstorms
periodically.

Guidance depicts the low-level flow remaining in the light to
moderate range out of the southerly direction through Thursday, then
shifting out of the southwest Friday as the surface low to the west
begins to lift off to the northeast. Winds should be light enough to
keep the land and sea breeze pattern in place through this time.
Although heavy showers and storms will remain possible into the
overnight hours for Kauai and Oahu due to the instability and moist
convergent flow in place, more of a diurnal trend will be likely
over the eastern end of the state through Friday (best chance for
showers being through the afternoon hours as sea breezes develop).

The extended guidance shows the upper and surface low lifting off to
the northeast this weekend as a shortwave trough dives southeastward
into the area and a cold front passes to the north. Strong high
pressure (1040 mb) building to the north in the wake of the front
will potentially translate to strong north-northeast winds over the
western end of the state beginning Friday night/Saturday. Light
winds, however, may linger over the eastern end of the state as the
tail-end of the front stalls nearby to the north of the Big Island.
Forecast confidence diminishes for early next week due to large
differences shown between model solutions. The ECMWF solution is
more bullish and depicts another strong shortwave trough dipping
southeastward into the area with a cold front passing to the
north. If this materializes, the wet pattern could hold into next
week.

&&

.AVIATION...
Poor aviation conditions are entrenched over Kauai and especially
Oahu where areas of heavy rain are ongoing and periods of MVFR to
IFR have been observed. This has particularly been the case along
the southern coast of Oahu, especially PHNL where nearby pockets
of slow-moving shower and thunderstorm activity persist. Although
an occasional lull in activity will be possible, an elevated
threat will MVFR and thunder will persist in the vicinity of PHNL
for at least a couple more hours. Further, moist onshore flow will
likely result in an expansion of showers and low clouds over the
windward and southeast-facing slopes of Big Island. Winds will
remain light and out of the southeasterly quadrant through the
forecast period.

AIRMET Sierra for mountain obscuration remains in effect for
Kauai, Oahu and portions of Big Island. Sierra will be cancelled
for Maui and Molokai.

&&

.MARINE...
A low pressure system over the western offshore waters will
produce southerly and variable winds across the waters through
Friday. This low, along with an approaching cold front, will
continue to produce enhanced showers and thunderstorms into the
weekend. A cold front will move into the northern offshore waters
Friday into Saturday. Strong north to northeast winds associated
with high pressure building to the north trailing this front will
be possible beginning Friday night into Saturday for the Kauai
and Oahu waters.

A Small Craft Advisory (SCA) remains in effect for windward waters
of Oahu, Maui County, and Big Island, including the Alenuihaha
Channel and waters southeast of Big Island through 6 AM HST
Thursday. Seas and winds are expected to subside below SCA levels
Thursday. A High Surf Advisory (HSA) remains in effect for the
east-facing shores of Kauai, Oahu, Molokai, Maui and the Big
Island through 6 AM Thursday.

Fading east swells Thursday will lower surf heights along exposed
east-facing shores. North swells will remain small through Friday
with small surf along north and west-facing shores. New large
north to northeast swells will arrive Friday night and pass across
the island waters through Saturday. Surf heights along north and
east-facing shores may reach warning levels both Saturday and
Sunday. These swells will slowly decline from Sunday night into
Tuesday. Small background southwest to south swells will keep surf
small along most south-facing shores with a small bump expected
from Thursday into Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for all Hawaii
islands.

High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

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

Small Craft Advisory until 6 AM HST Thursday for Oahu Windward
Waters-Kaiwi Channel-Maui County Windward Waters-Alenuihaha
Channel-Big Island Windward Waters-Big Island Southeast Waters.

&&

DISCUSSION...Gibbs
AVIATION...JVC
MARINE...Bohlin/Blood

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
828 PM HST WED MAR 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0745 PM     TSTM WND DMG     HONOLULU                21.31N 157.86W
03/10/2021                   HONOLULU           HI   EMERGENCY MNGR

            POWER LINES DOWN ON KAPAHULU AVENUE.

&&

RCB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
831 PM HST WED MAR 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0753 PM     FLOOD            HONOLULU                21.31N 157.86W
03/10/2021                   HONOLULU           HI   EMERGENCY MNGR

            CARS STALLED ON FLOODED HOBRON LANE IN WAIKIKI.

&&

RCB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
931 PM HST WED MAR 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0620 PM     FLASH FLOOD      4 NNE HALEIWA           21.65N 158.09W
03/10/2021                   PHZ114             HI   EMERGENCY MNGR

            PARTIAL ROAD CLOSURE AT WAIMEA BAY DUE TO FALLEN BOULDER.

&&

JGIBBS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
937 PM HST WED MAR 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0634 PM     LIGHTNING        HONOLULU                21.31N 157.86W
03/10/2021                   HONOLULU           HI   EMERGENCY MNGR

            WIDESPREAD POWER OUTAGES FROM HONOLULU TO PORTIONS OF THE
            WINDWARD SIDE UP TO HAUULA.

&&

JGIBBS