Flash Flood in Hawaii, November 4 to 5, 2024

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST Mon Nov 4 2024

.SYNOPSIS...

A plume of tropical moisture will bring the potential for locally
heavy rain and a few thunderstorms through Tuesday, particularly
over Maui and the Big Island. A cold front will approach the
islands and progress part-way through the state on Wednesday into
Thursday morning, then stall and dissipate as a new high builds
to its north. This will bring a period of wet trades and breezy to
locally windy conditions to the islands later this week into the
weekend.

&&

.DISCUSSION...

Radar and satellite imagery show a weak trough moving into the
eastern end of the state this morning, with moderate to heavy
showers impacting south and east facing coasts and slopes of the
Big Island and Maui. Elsewhere along the island chain, trade wind
showers can be seen moving into windward areas.

The main forecast concern in the short-term is the potential for
heavy rain and localized flooding, particularly for windward Big
Island and Maui, today through tomorrow as this surface trough
moves through the island chain. CIMSS MIMIC precipitable water
imagery continues to show a broad area of 2+ inches of
precipitable water around this trough that will move into the
eastern end of the state today and on up through the western end
tomorrow. As the surface trough and its plume of moisture moves
west-northwestward across the island chain, a weak trough aloft
will also set up over the state and increase atmospheric
instability. The low level trough, deep tropical moisture, and
upper level trough look to all line up and bring the best chance
of heavy rain and even a slight chance of thunderstorms to
windward Big Island and Maui today through Tuesday. A Flood Watch
remains in effect for Maui and the Big Island through Tuesday
afternoon. Oahu and Kauai may see a brief period of heavy rain
tomorrow before the trough dissipates, but confidence is not high
enough to expand the Flood Watch to these islands. Otherwise,
moderate to locally breezy trades are expected today, then trades
will weaken tomorrow with the approach of a cold front from the
northwest.

Model guidance is coming into slightly better agreement on the
progression of the next cold front that will approach the state
from the northwest midweek. Both the ECMWF and the GFS have this
front move down to somewhere near Maui County or the Big Island
Wednesday evening into Thursday morning. Showers are expected
along and ahead of this front, especially due to the lingering
tropical moisture associated with the surface trough pushing off
to the west at that time. The surface high pushing in behind the
front will lead to a resurgence in the trades, with breezy to
locally windy conditions developing by late Wednesday and holding
into the weekend. Moisture caught up in these trades will bring
periods of showers to mainly windward and mauka areas.

Models begin to diverge more significantly this weekend heading
into early next week. For now, have kept a wet breezy to strong
trade wind pattern through the end of the forecast period.

&&

.AVIATION...
Moderate to locally breezy trade winds expected today. A surface
trough will move westward across the state and stall near Kauai
and Oahu Tuesday. Clouds and heavy showers associated with this
feature have moved onto windward and southeast Big Island this
morning. AIRMET Sierra for IFR conditions is posted for the
mentioned areas of the Big Island, which is expected to continue
through the day. Expect reduced visibility and lower ceilings
around the eastern portion of the state today.

Clouds and showers will spread to Maui County later today, and
then Oahu and Kauai late tonight into Tuesday. AIRMET Sierra (for
mountain obscurations at a minimum) is likely follow the general
westward progression of the trough. Isolated thunderstorms will be
possible across the eastern half of the state through Tuesday.
Turbulence and icing should only be significant in or around the
heaviest showers or thunderstorms, and should not be enough
coverage to warrant an AIRMET or SIGMET through Tuesday.

&&

.MARINE...

A weak surface trough passing over the Hawaiian Islands will keep
the pressure gradient over the eastern half of the state tight
enough to maintain small craft advisories for waters surrounding
the Big Island, Maui, and Molokai through tomorrow morning. The
trough will also usher in southeast winds and isolated
thunderstorms over Hawaiian coastal and offshore waters today and
tomorrow.

Beginning Tuesday night, a wintry cold front will approach the
state from the northwest. This will have the effect of shifting
local winds back to east-northeast across the state. Trade winds
will become increasingly wet and breezy as the front passes just
north of the state Wednesday night and Thursday. The pressure
gradient behind the front may be strong enough to support fresh
trade winds into the coming week.

A series of gale force lows progressing across the north pacific
will send multiple northwest and north swells towards the Hawaiian
Islands. The current medium-period, moderate size northwest (330
degree) swell will gradually subside through tomorrow, but could
still produce locally high surf along exposed reefs through this
afternoon. A relatively short period north-northwest swell is
expected Thursday, peaking below High Surf Advisory heights. A
longer period northwest swell is expected Friday and Saturday,
potentially reaching advisory levels, with an even larger swell
possible Sunday and Monday.

Short-period wind waves will increase later in the week in
response to the strengthening trade wind flow.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Leeward-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai
North-Molokai Windward-Molokai North-Molokai West-Maui Central
Valley North-Windward Haleakala.

Flood Watch through Tuesday afternoon for Maui Windward West-
Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island
Interior-Maui Central Valley North-Maui Central Valley South-
Windward Haleakala-Kipahulu-South Maui/Upcountry-South Haleakala-
Big Island South-Big Island Southeast-Big Island East-Big Island
North.

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

&&

DISCUSSION...Farris
AVIATION...Foster
MARINE...Bedal/Birchard