Flash Flooding and Heavy Rain in Hawaii, February 17 to 19, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Tue Feb 16 2021

.SYNOPSIS...
A front approaching from the northwest will bring the chance of
heavy rain and thunderstorms to Kauai for the next few days.
Wet trade wind weather is expected for the rest of the state
into the weekend.

&&

.DISCUSSION...
No changes to the forecast this evening, and not expecting any
significant changes for the short term overnight. The cold front
to the northwest of Kauai continues to advance, and is expected to
reach the Garden Isle overnight. Some showers and thunderstorms
that popped up over Kauai this afternoon out ahead of the front
have faded. As the front moves closer and over the island
overnight, we can expect an uptick in shower activity and the
potential for some thunderstorms, including the possibility for
heavy rain. Although the front is currently advancing towards
Kauai County, the models continue to show the front stalling
overhead, which will cause showers and thunderstorms to linger in
the region. The Flash Flood Watch for Kauai County continues
through Thursday as a result.

An upper level trough pushing in just behind the front will help
to usher in colder temperatures aloft, which will help to enhance
shower activity state wide into the weekend.

While the front gets caught up over Kauai, the rest of the state
is expected to continue with easterly winds, which will focus
shower activity over the windward slopes. Wetter than normal
precipitable water along and ahead of the front is expected to
reach over the central islands. Meanwhile, the colder temperatures
aloft could spark some afternoon thunderstorms over the Big Island
slopes during the next couple of afternoons. Wet conditions looks
to remain into the weekend before tapering off Sunday into next
week.

&&

.AVIATION...
Diurnally enhanced showers and isolated thunderstorms are waning
over Kauai and MVFR clouds have subsequently eroded as well. The
island is now poised for a lull of several hours until breezy
northerly winds associated with the approaching cold front reach
the island between 10-12z / 12am-2am HST. An increase in shower
coverage, particularly over the northwestern and north-facing
slopes can then be expected for the remainder of the night along
with an increasing chance of thunderstorms. For Oahu through Big
Island, relatively quiet aviation conditions characterized by
prevailing VFR and a distinct lack of any organized shower
activity are forecast for the remainder of tonight.

For Wednesday, moderate easterlies will tend to focus showers
over the windward and mauka sections of Oahu through Big Island.
In addition, improved moisture quality will again lead to
thunderstorm potential over Big Island during peak heating,
particularly the leeward side. Showers and isolated thunderstorms
along and ahead of the front will remain focused over Kauai and
surrounding waters throughout this time.

AIRMET Sierra for mountain obscuration over Kauai has been
cancelled. Reissuance is probable with the arrival of the cold
front later tonight.

&&

.MARINE...
A front just west and northwest of the area will likely reach
Kauai tonight, where it will stall and dissipate over the next
couple of days. Light east to southeast flow will prevail ahead of
the front through Wednesday, with breezy to locally strong
northeast winds filling in behind the front. (Winds and seas may
reach Small Craft Advisory (SCA) levels in the areas with the
stronger winds.) Strengthening high pressure will be passing by
north of the area later in the week and on through the weekend.
Winds and seas are expected to reach SCA levels across more areas
by Thursday night or Friday, with these conditions continuing
through the weekend.

The current medium period northwest swell will get reinforced by
a new swell Wednesday. This swell will linger but lower gradually
through Friday. A small, but long-period, northwest swell is
expected to fill in Friday night and Saturday, with a slightly
larger northwest swell expected Monday and Tuesday. Surf heights
are forecast to remain below High Surf Advisory (HSA) levels.

With the expected strengthening of the trade winds over the
weekend, short-period choppy surf will gradually increase over
the weekend and on into early next week. Surf heights may approach
HSA levels early next week. Small, mainly background southerly
swells will continue throughout the forecast period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Thursday afternoon for Niihau-Kauai.

&&

DISCUSSION...M Ballard
AVIATION...JVC
MARINE...Kinel