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Hawaiian Islands Synoptic Discussion and Guidance

691
FXHW60 PHFO 242007
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1007 AM HST Tue Nov 24 2020

.SYNOPSIS...
Breezy trade winds are expected today, along with showers riding
in on the trades, which are expected to be enhanced due to an
unstable airmass moving in from the west. This unstable air could
trigger some isolated 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 is then
expected to follow.

&&

.DISCUSSION...
It's been a rainy morning for most windward areas across the
state, especially over the island of Maui and Kauai where several
Flood Advisories have been issued. Rainfall intensities have then
since tapered off but showers will continue to ride in with the
breezy trades today. The enhancement of the trade wind showers are
due to an upper level trough that is slowly moving west to east
across the state. Rainfall amounts over the past 12 hours show
that most of the rain fell over the smaller islands. Windward
Kauai saw widespread 1 inch of rain with isolated amounts of up
to 5 inches over Mount Waialeale. Windward Oahu also saw many
areas with 1 inch of rainfall with over 3 inches reported over the
Nuuanu upper gauge. Windward Maui saw over 4 inches of rain
recorded over West Wailuaiki and Puu Kukui over the past 12 hours.

For rest of the day today and into tonight, a Flash Flood Watch
will remain in effect over Maui County and the Big Island due to
the combination of an unstable atmosphere and the saturated
grounds over windward areas from recent rainfall. Showers will
continue to persist across windward areas and with daytime
heating, we could potentially see some heavy showers this
afternoon into the evening, including leeward areas as well. Flash
Flooding will also be a possibility over Kauai, but at this time
confidence is not high enough to issue a watch over Kauai.

The atmosphere should begin to slowly stabilize late Wednesday as
the upper trough moves east of the state and an upper level ridge
builds north of the state.

For the long Thanksgiving weekend, we should see a drier weather
pattern with moderate to locally breezy trade winds. Trades will
remain out of our typical east-northeast direction on Thursday,
then gradually shift toward the northeast on Friday. A weak front
is expected to push through the state late Friday into Saturday
and will likely bring some moderate showers especially across
windward areas. Behind this front, we should see cool
northeasterly trades with dewpoints dropping down into the low 60s
Saturday night.


&&

.AVIATION...
Strong High pressure over the Northeast Pacific with a ridge
passing north of Hawaii will keep moderate to locally breezy trade
winds in the forecast through 24 hours and beyond. Brisk winds and
summit level combined with weak inversion aloft around 9 kft will
continue to yield mechanical turbulence to the lee of higher
terrain. AIRMET Tango for low level turbulence remains necessary.
Conditions may improve later this evening across the western half
of the state but are expected to persist across the eastern half.

Multiple bands of thick layered clouds embedded in the trade wind
flow will continue to spread light showers and occasional MVFR
conditions across the islands. AIRMET Sierra for mountain
obscuration along north and east facing slopes and coasts of all
islands will likely remain necessary through at least late
afternoon. Some temporary improvement is expected in the early
evening hours, but nocturnal cooling should keep the islands
shrouded in clouds through at least Wednesday morning.

The air mass over the state remains moderately unstable. Locally
heavy showers or thunderstorms remain a possibility statewide
through Wednesday. The greatest potential for occurrence will be
over Kauai and Interior Big Island this afternoon and early
evening.


&&

.MARINE...
A strong high far northeast of the area will maintain locally
strong, easterly trade winds over Hawaiian waters through Thursday.
A Small Craft Advisory (SCA) is in effect for most waters from
Oahu to the Big Island. Winds will slowly subside over the next
few days as the high weakens, allowing some waters to be dropped
from the SCA. 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 today.
A High Surf Advisory is in effect through this evening for east
facing shores of all islands except Lanai and Niihau. Surf will
subside over the next few days as the trade winds weaken. 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.

The current 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.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening 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 tonight for Maui-Kona-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters.

Small Craft Advisory until 6 PM HST this evening for Kaiwi
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Big Island Windward Waters.


&&

$$


DISCUSSION...Kino
AVIATION...Bedal
MARINE...TS

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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