Flash Flood and Heavy Rain in Hawaii, November 24 to 26, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
844 PM HST Mon Nov 23 2020

.SYNOPSIS...
Breezy trade winds will continue Tuesday with a gradual decline
during the second half of the week. Moisture riding in on the
trades will focus showers mainly over windward and mauka areas. An
upper trough moving in from the west will help to spark a few
thunderstorms through Wednesday. A weak frontal band may move
across the area late Friday or Saturday accompanied by a period of
showers. A cool and dry airmass will follow in the fronts wake.

&&

.DISCUSSION...
Anticipate an update to the forecast shortly due to a refinement
of the thunderstorm areas for the next couple of days. The
driving force for thunderstorms will be from an upper level trough
moving in from the west. Overall have expanded the area where
thunderstorms may occur based on the latest model guidance,
including some of the higher resolution models, through Wednesday.
Additionally, a thunderstorm did develop over the Kona slopes
this afternoon, and will similar conditions expected tomorrow,
have added thunderstorms there for tomorrow afternoon. In general,
the threat for thunderstorms will follow the upper level trough
axis as it swings in from the west tonight, and then moves over
the central islands Tuesday afternoon/evening. The latest runs
show the trough holding together a little more as it reaches the
Big Island, which in part explains the expansion of the
thunderstorm areas.

High pressure to the far north-northeast continues to bring breezy
trade winds to the region. Although winds are expected to
gradually decrease as the high weakens, they have been a little
slower to decrease than originally expected. Showers will continue
to be carried in on the trade winds, and in areas on Maui and the
Big Island which have received a decent amount of rain recently,
there is the possibility of flooding. A Flash Flood Watch is in effect
for those islands, with the greatest concern being on the windward
sides of the islands.

At this time, it looks like some drier weather over the islands
for Thanksgiving, in a more typical trade wind weather pattern.
Trade winds are expected to once again be on the increase over the
next weekend as high pressure builds north of the state. The
increasing trades may help to push a dissipating front over the
islands late Friday or Saturday, leading to a period of increased
in showers primarily over northeast facing slopes. A cooler and
drier airmass will follow in the fronts wake Saturday night and
Sunday.

&&

.AVIATION...
Showers embedded within the resident band of moisture focused over
the Islands will get an added boost tonight into Tuesday from an
upper low approaching from the northwest. This will favor locally
heavy showers over windward areas as well as a greater coverage of
leeward showers. Additionally, periods of MVFR ceilings are
likely over windward coasts and slopes through Tuesday morning.
Breezy trades will continue through the forecast period.

AIRMET Sierra remains in effect for mountain obscuration for
Molokai through Big Island.

AIRMET Tango remains in effect for lee turbulence for the entire
area.

&&

.MARINE...
A strong high far northeast of the area will maintain easterly
trade winds over Hawaiian waters through Thursday. The trades remain
strong. A Small Craft Advisory is in effect for most waters from
Oahu to the Big Island. Winds will subside over the next few days
as the high weakens. Winds may strengthen a bit and shift out of
the northeast over the weekend as the remnant of a front moves
south across the area.

Large waves produced by the strong trade winds will continue
overnight. A High Surf Advisory is in effect through tonight for
east facing shores of all islands except Lanai and Niihau. Surf
along the east facing shores will subside over the next few days
as the trade winds weaken.

The current moderate northwest swell will continue to subside
through Wednesday. A new moderate-sized northwest swell is
expected Wednesday and Thursday. A larger northwest swell will
build Friday, peak Saturday night, then gradually subside. A
couple more northwest swells are possible early next week. Some
of the large trade-wind swell is wrapping around into south
facing shores. Otherwise, surf along south facing shores will
remain small through the Thanksgiving Day weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through late Tuesday night for Maui-Kona-South
Big Island-Big Island North and East-Kohala-Big Island Interior.

Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 AM HST Tuesday for Kaiwi Channel-
Maui County Windward Waters-Big Island Windward Waters.

&&

DISCUSSION...M Ballard
AVIATION...JVC
MARINE...Donaldson