Heavy Rain in Hawaii, February 26 to 28, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Fri Feb 26 2021

.SYNOPSIS...
Strong and gusty trade winds will continue into early next week, as
a trough aloft keeps the island atmosphere unstable. Clouds and
showers will focus over windward areas, with some occasional heavy
downpours. Increasing moisture near Maui and the Big Island will
bring a more widespread threat of heavy windward showers, and a
slight chance of thunderstorms.

&&

.DISCUSSION...
Very strong (1047 mb) surface high pressure to the distant NE will
gradually weaken the next couple of days, but a new strong high
(near 1044 mb) will build far N of the islands Saturday night and
Sunday. As a result, strong and gusty trade winds will persist over
the islands through early next week, potentially easing thereafter
as a large low pressure in the NE Pacific pushes the high westward,
leading to a relaxation of the local pressure gradient. A Wind
Advisory for parts of the state through today may need to be
extended, but current forecast anticipates a slight weakening of the
wind that would allow it to be cancelled.

The island atmosphere will remain unstable into next week, due to
the presence of a persistent trough aloft, and its associated cold
pool. Moisture riding in on the trade winds will be enhanced,
leading to higher than normal cloud tops, which will lead to larger
rain drops, bringing heavier downpours than might be otherwise
expected with trade winds in place. While these showers will be
progressive and will not linger over any one area for a prolonged
period, high-resolution model guidance anticipates their repeated
development over the windward slopes of Maui and the Big Island over
the next couple of days. This is expected to lead to increasing
ground saturation, and a Flash Flood Watch (FFA) has been issued for
these areas. An expansion of the FFA to windward portions of the
other islands is not anticipated as guidance indicates the deepest
moisture (and instability) over the Big Island occasionally
spreading to windward portions of Maui.

The cold temperatures aloft and deeper-than-normal moisture means
that the Big Island Summits can expect periods of wintry weather,
with the primary threat in the short term coming in the form of
freezing rain. As moisture and instability increase over the
weekend, a few snow showers may mix in as well. A Winter Weather
Advisory is in effect through Monday morning.

&&

.AVIATION...
Strong easterly trade winds will continue today, sustained by an
area of very strong high pressure located northeast of the main
Hawaiian Islands. AIRMET Tango remains in effect for low-level
mechanical turbulence over and immediately south through west of
mountains. Additionally, since strong winds are forecast to persist
throughout the day, AIRMET Tango is also in effect for surface winds
greater than 30 kt in the Alenuihaha Channel and south of the Big
Island. Low-level wind shear could also be a concern at a few
locations today, such as PHOG.

As an upper-level low moves into the area today, an uptick in shower
activity is expected, particularly over eastern sections of the
islands. MVFR conditions are likely in heavier showers, especially
for the Big Island today and could include Maui and other islands
later tonight and Saturday. AIRMET Sierra is currently in effect for
windward areas of the Big Island and will likely be
extended/expanded later tonight. Isolated thunderstorms are also
possible on leeward slopes of the Big Island this afternoon.
Otherwise, VFR conditions will prevail throughout the period.

&&

.MARINE...
Large, strong high pressure located off the mainland west coast has
and will continue to maintain a tight enough pressure gradient
across the eastern Pacific to produce strong trades over the local
Hawaiian Island waters. The early week eastern migration of a
secondary northern Pacific high will keep a taut pressure gradient
back across the islands and sustain strong trades through Tuesday.
East Pacific high pressure will weaken in response to another area
of mid week strong high pressure passing to the north of the state.
A cold front leading this high will back moderate to fresh trades
winds a touch north late next week. Rough sea conditions will remain
elevated under persistent strong trades into the foreseeable future.
Near gale to gale force winds through those notoriously windier
channel waters has a Gale Warning in effect for the Alenuihaha
Channel and the Big Island Leeward Waters through this afternoon. A
Small Craft Advisory (SCA) remains in effect for the remainder of
the Hawaiian Coastal Waters through the weekend.

ASCAT satellite-derived winds verify a large belt of strong to near
gale trades focused over and well upstream of the state. This very
long fetch of robust trades will sustain rough, elevated seas and
high surf along most eastern exposures. Some of this trade wind
swell will also wrap around into north and south facing shorelines.
The High Surf Advisory (HSA) will therefore remain in effect the
next several days for larger island east-facing shorelines. As
current northwest swells fade through the day, a backing northwest
swell will fill in this weekend and peak near or slightly above head
high surf late Saturday into early Sunday. The next scheduled
northwest swell will be the larger of the two and peak near HSA
level surf along northern and western exposures late Wednesday into
early Thursday. A small, Southern Hemispheric background swell will
keep small southern-facing shore surf around through early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for East facing shores of
Kauai Oahu Molokai Maui and the Big Island.

Wind Advisory until 6 PM HST this evening for Molokai-Kahoolawe-
Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Leeward Haleakala-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Flash Flood Watch from 6 PM HST this evening through Sunday
afternoon for Maui Windward West-Windward Haleakala-Haleakala Summit-
South Big Island-Big Island North and East-Big Island Interior.

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Alenuihaha Channel-Big Island Leeward Waters.

Gale Warning until 6 PM HST this evening for Alenuihaha Channel-Big
Island Leeward Waters.

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-Maui County Leeward Waters-Maalaea Bay-
Pailolo Channel-Big Island Windward Waters-Big Island Southeast
Waters.

&&

DISCUSSION...Birchard
AVIATION...TS
MARINE...Blood