Flash Flooding and Heavy Rain in Hawaii, January 17 to 20, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
408 PM HST Sun Jan 17 2021

.SYNOPSIS...
A vigorous front will move down the island chain tonight, then
stall around Maui and the Big Island between Monday and Tuesday.
Behind the front, we will see strong northerly winds of 20 to 30
mph with stronger gusts. The combination of strong northerly
winds, and an upper level low aloft, will bring the risk of Flash
Flooding to parts of Oahu, Maui County and the Big Island. The
greatest risk will be along windward and mountain areas,
especially on Maui Monday afternoon and Monday night. Moderate to
strong easterly trade winds return to the region during the
second half of the week with a bit of a wet flavor.

&&

.DISCUSSION...
The leading edge of a vigorous cold front is now moving through
Kauai and is expected to move through Oahu and Maui County later
tonight. Numerous impacts are expected with this front, including
the potential of flash flooding and strong northerly winds behind
the front. A Flash Flood Watch has been expanded to include the
Big Island with this forecast package. Confidence is increasing
for flash flooding Monday into Monday night especially around
windward areas of Maui County and the Big Island. Latest model
guidance are in fairly good agreement with the front stalling
between Maui and the Big Island on Monday. The combination of the
strong northerly winds and an upper level low aloft will enhance
precipitation and will allow rainfall to anchor over the terrain.
Confidence is increasing for select areas to receive over 10
inches of rainfall during this event. The most likely areas to see
heavy rain will be over windward Maui and the Hamakua Coast on the
Big Island between Monday afternoon and Monday night.

Also due to the strong northerly winds expected behind the front,
a Wind Advisory has been issued for the smaller islands with
sustained winds of 20 to 30 mph with gusts of over 45 mph
expected.

Although the front looks to dissipate on Tuesday, moisture
associated with the front will continue to linger across the state
and is expected to slowly drift westward through Wednesday. The
strong northerly winds should taper off by then, and breezy trades
should return by the middle of the week. A trade wind pattern is
expected for the second half of the week but due to lingering
moisture, we will likely see a more of a wet trade wind pattern.

&&

.AVIATION...
A front near Kauai will move down the island chain through
tonight. While clouds and showers are associated with the frontal
cloud band, showery prefrontal cloud areas are affecting some of
the islands now. TAF sites will begin to experience MVFR
conditions from time to time, at least across the smaller islands,
as the frontal cloud band passes by.

AIRMET Sierra for tempo mountain obscuration above 2500 ft is in
effect for the western islands and may need to be expanded
eastward later tonight and into Monday. The arrival of the cold
front is expected around 06z/8pm HST for Oahu and around 15z/5am
HST for Maui and will be marked by a greater coverage of showers,
some locally heavy, followed by an abrupt shift to gusty
northerly winds.

&&

.MARINE...
A cold front is passing into the nearshore Kauai waters late this
afternoon and is timed to make it into Oahu's nearshore waters
late this evening. The front will slow along its southeastward
progression and will likely stall out over the central islands
going into early Monday. The approach, passage and eventual
stalling of this frontal boundary will produce a few days of
showers, some locally heavy, and isolated thunderstorms across the
Hawaiian waters. The western upper trough across the waters
surrounding Kauai, along with the weak convergence along the near
stationary surface boundary, will be the impetus to maintaining
higher shower and thunderstorm chances through mid week.

The High Surf Warning (HSW) has been extended in time and is now
in effect through 6 AM Tuesday morning for all smaller island
exposed north and west-facing shorelines. Despite this afternoon
into evening's brief lowering of surf to near or below warning
level criteria, the HSW will remain in effect to account for the
next large northwest swell's arrival during the overnight Monday
morning hours. This powerful 14 to 16 foot, near 14 second period
northwest swell will push surf heights back to over warning level
thresholds along all smaller island north and west-facing shores.
Peak surf heights during the day Monday into early Tuesday are
not expected to be quite as high as they were during this most
recent weekend XL surf event. A High Surf Advisory (HSA) is in
effect for the north and west-facing Big Island shorelines as
surf peaks out around HSA criteria tomorrow afternoon. Strong
north to northeast winds in association with this early week cold
front will result in lesser quality wave shapes during the passage
of the highest swell. While these most recent swell directions of
around 330 degrees are not traditionally favorable for
significant impacts within both Kahului and Hilo Bay Harbors (as
with a more north swell direction), there is a slight chance that
there could be some minor harbor surge impacts.

A Gale Warning is in effect for the Alenuihaha Channel and westward
waters as strong post-frontal north to northeast winds will frequently
gust to greater than gale force. Heightened passing northwest swells,
along with very rough seas in the wake of the cold front, will produce
greater than 10 foot significant seas across most nearshore waters
through Tuesday night. A Small Craft Advisory (SCA) for high seas
remains in effect for the windward Big Island waters tonight. This
SCA will be expanded to include the leeward Big Island waters
tomorrow.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Tuesday afternoon for Oahu-Molokai-
Lanai-Kahoolawe-Maui.

Wind Advisory until 6 PM HST Monday for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala.

High Surf Warning until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

Flash Flood Watch from Monday morning through Tuesday afternoon
for Big Island.

High Surf Advisory until 6 AM HST Tuesday for Kona-Big Island
North and East-Kohala.

Small Craft Advisory from 6 AM to 6 PM HST Monday for Big Island
Leeward Waters.

Small Craft Advisory until 6 PM HST Monday for Big Island
Windward Waters.

Gale Warning until 6 PM HST Monday for Kauai Northwest Waters-
Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo
Channel-Alenuihaha Channel.

&&

DISCUSSION...Kino
AVIATION...Powell
MARINE...Blood