High Surf in Hawaii, March 13 to 15, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
329 AM HST Sat Mar 13 2021

.SYNOPSIS...
The wet pattern will continue as a cold front moving through
stalls over the eastern end of the state this weekend. Heavy
showers and a few storms along and ahead of the front combined
with saturated grounds will keep the threat for flash flooding in
place. Gusty north to northeast winds following the front will
focus clouds and showers over windward and mountain locations for
Kauai and Oahu. The wet pattern will remain possible through the
first half of the upcoming week as low pressure tracks westward
across the state.

&&

.DISCUSSION...
Heavy showers and a few thunderstorms will continue to belt the
eastern end of the state as a cold front advances down the island
chain this morning and stalls by tonight. This additional
rainfall will only aggravate the ongoing flooding issues due to
the inundated soils and swollen streams from the week's heavy
rainfall. Although the rainfall will be more widespread over the
eastern end of the state, strong north to northeast winds
following the front has shifted the rainfall coverage to windward
and northern slopes for Kauai and Oahu this morning (rainfall
rates ranging from a quarter to half of an inch per hour). In
addition to the rainfall, the gusty northerly winds combined with
the saturated soils could translate to downed trees today, which
could further delay or impact restoration efforts. As a result,
the Flash Flood Watch remains in effect statewide and a Wind
Advisory is now in effect for Kauai and Niihau today.

For the Big Island Summits, gusty winds along with a wintry mix
can be anticipated as a potent shortwave trough dives
southeastward into the area. The Winter Weather Advisory for the
summits may need to be extended through tonight.

Despite the model inconsistencies this week that have lowered
forecast confidence for later this weekend and next week, the last
few model cycles between the various solutions have been more
consistent and in decent agreement. They depict low pressure
positioned northeast of the state along the front beginning to
track back toward the islands Sunday, then through the islands as
a trough early next week. If this solution evolves as projected,
expect gusty northerly winds to hold over the western end of the
state and potentially remaining light over the eastern end. This
surface feature combined with broad upper troughing over the
region will keep the rainfall chances in the forecast. They
indicate the low opening and dampening out as it slips west-
southwest of the area with trades returning statewide Wednesday
through the second half of the week.

&&

.AVIATION...
A frontal system, currently located over Oahu, is forecast to
push southeast across the state today. Ahead of this feature,
abundant moisture and lift will generate numerous showers and
isolated thunderstorms over Molokai, Maui, Lanai, and the Big
Island. The heavier showers and storms will likely produce
periods of MVFR and IFR ceilings and visibility.

Expect breezy northeast winds behind the front. Thus, most shower
activity on Kauai and Oahu will be concentrated along the
windward coasts and slopes.

AIRMET SIERRA remains in effect for Kauai, Oahu, Molokai, Lanai,
and Maui above 2000 feet due to tempo mountain obscuration from
clouds and showers.

AIRMET ZULU remains in effect over Oahu, Molokai, Maui and
adjacent waters between 13000 and 24000 feet for tempo light
icing.

&&

.MARINE...
A cold front will stall out in this vicinity of the central
islands through the remainder of the weekend. An area of low
pressure will develop along the stalled out front a couple hundred
miles northeast of the islands on Sunday, then slowly open up
into a trough as it moves southwestward and through the central
islands Monday night and Tuesday. High pressure will then build to
the north of the islands Wednesday through late next week. The
winds will remain strongest across the western end of the state
where the gradient will be tightest through Tuesday, with a more
uniform trade wind pattern becoming re-established around the
middle of next week. A Small Craft Advisory (SCA) is now in effect
for the windward Maui waters and Kaiwi Channel westward through
Sunday. This SCA will likely need to be expanded to include most
coastal waters tonight as a large north swell moves into the
area. SCA conditions will then likely hold in place across much of
the marine area through early next week.

A new large north to north-northeast swell will rapidly fill in
from northwest to southeast down the island chain today and
tonight, hold through the remainder of the weekend, then slowly
lower early next week. A High Surf Advisory is now in effect for
north facing shores of the smaller islands through Sunday, and has
been expanded to include east facing shores of Kauai, Oahu,
Molokai, and west facing shores of Maui. This HSA will likely
need to be expanded to include east facing shores of Maui, and north
and east facing shores of the Big Island tonight. Surf should
fall below advisory levels along north facing shores Sunday night,
but could hold above the advisory threshold for east facing shores
through Tuesday. Additionally due to the northerly swell
direction, surges may occur at the entrances to the Kahului and
Hilo harbors over the weekend.

A moderate northwest swell will move through the islands
Wednesday through late next week. An east swell will gradually
lower through the weekend as trade winds upstream of the islands
diminish. A more typical short-period trade wind swell will
return during the middle to latter part of next week as trade
winds over and upstream of the islands increase. Surf along south
facing shores will receive a small boost today as a small
southwest swell moves through the islands. South shore surf will
then fall back to background levels Sunday through late next week.

&&

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

Wind Advisory through this afternoon for Niihau-Kauai.

High Surf Advisory through 6 PM HST Sunday for Niihau-Kauai
Windward- Kauai Leeward-Oahu North Shore-Oahu Koolau- Olomana-
Molokai-Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala.

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

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

&&

DISCUSSION...Gibbs
AVIATION...Thomas
MARINE...Jelsema

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
312 AM HST SAT MAR 13 2021

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

0310 AM     FLOOD            KAHULUI                 20.87N 156.46W
03/13/2021                   MAUI               HI   EMERGENCY MNGR

            SOME WATER REPORTED GETTING INTO A FEW HOMES DUE TO HEAVY
            RAIN OVERWHELMING STORM DRAINS.

&&

TSTALL