Heavy Rain in Hawaii, November 29 to 30, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
1012 AM HST Mon Nov 29 2021

.SYNOPSIS...
Wet and unsettled weather will remain this week. Gusty winds
today will ease Tuesday and remain at moderate speeds through
midweek before increasing Friday. Windward and mountain areas
will have the best chances of rain over the next few days before
widespread rainfall and the potential for flash flooding pushes
over the islands Friday and over the weekend.

&&

.DISCUSSION...
A strong high pressure ridge north of the state is producing gusty
trade winds, while showers riding those trades are bringing quick
hitting downpours to windward and mountain areas. Some of these
showers are reaching leeward locations as well. An upper low
sitting east of the state is providing some instability which will
allow isolated thunderstorms to form mainly in the vicinity of the
Big Island and Maui County through Tuesday night. By Wednesday,
the upper low is expected to weaken and the chance of
thunderstorms decreases, but local downpours will likely remain
as the low sits in place over the islands.

Moderate easterly trade winds will remain through Wednesday before
a cold front approaching from the northwest disrupts the trades
Thursday, with forecasted gusty north winds reaching the western
half of the state Friday. Global models remain fairly consistent
on the track of the front with southerly winds developing across
the state from Saturday to Sunday. This front will be accompanied
by a potentially potent upper level low that, if it comes in as
advertised, will bring widespread rainfall and the potential for
flash flooding Friday and over the weekend.

&&

.AVIATION...
An upper low will track over the Big Island and Maui today, and
then stall south of Maui over the next several days. An associated
surface trough will move east to west across the islands today.
Increasing moisture and instability will enhance shower activity
today, especially for north through east facing sections of the
islands. MVFR ceilings and visibility within periods of heavier
showers are expected across windward sides of the islands at
times. Additionally, as stability decreases this afternoon and
evening, isolated thunderstorms will be possible from Maui to the
Big Island. AIRMET Sierra is posted for tempo mountain obscurations
above 2500 feet for Oahu through the Big Island along windward
facing slopes.

AIRMET Tango is posted for tempo moderate turbulence below 7000
feet due to the breezy trade winds. Winds are expected to taper
off tonight as the surface trough weakens and moves west of the
state.

&&

.MARINE...
A surface trough approaching the state will bring an increase of
showers and potentially some isolated thunderstorms around the Big
Island today. Locally strong NE trade winds are in place this
morning, and we should see the winds veer toward the east tonight
and weaken slightly as the trough moves through the state. A
Small Craft Advisory (SCA) is posted for zones through the day
today, and may need to be extended in time for some areas,
depending on the evolution of the trough.

As the trough weakens and moves southwest of the state over the
next several days, winds will weaken further but remain at fresh
speeds for many areas. Some lingering instability from the upper
level low could produce some isolated thunderstorms through
Tuesday night primarily over the eastern half of the state.  By
Wednesday we should see moderate to locally fresh easterly trade
winds.

Towards the end of the week, guidance is indicating a powerful
cold front moving through the state around Friday. Strong north to
northwest winds will follow behind the front and we could see some
unsettled weather ahead and along the front. In addition, a large
north swell will move in with this front and will create rough
ocean conditions Friday through the weekend.

A new north-northwest swell filled in last night and is now on
the decline. This swell will steadily decrease through Tuesday. A
series of small northwest swells are expected Tuesday through
Thursday. A much larger north-northwest swell is expected over the
weekend and will likely bring warning level surf along north and
west facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Kauai Leeward-
Kauai Mountains-Waianae Coast-Waianae Mountains-Lanai Mauka-
Kahoolawe-Maui Windward West-Maui Leeward West-Haleakala Summit-
Kohala-Kauai North-Kauai South-East Honolulu-Honolulu Metro-Ewa
Plain-Koolau Leeward-Molokai-Lanai Windward-Lanai Leeward-Lanai
South-Maui Central Valley South-Kipahulu-South Maui/Upcountry-
South Haleakala-Big Island North.

High Wind Warning until 6 PM HST this evening for Big Island
Summits.

Small Craft Advisory until 6 PM HST this evening for all
Hawaiian waters-

&&

DISCUSSION...Morrison
AVIATION...Foster
MARINE...Kino