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Hawaiian Islands Synoptic Discussion and Guidance

698
FXHW60 PHFO 241331
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
331 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...
No changes to the forecast since the update overnight. A
thunderstorm did develop around midnight south of Kauai, and more
recently a couple brief thunderstorms developed northeast and
west of the Big Island on the borders with the offshore waters.
Some heavier showers more recently have been observed just east of
Maui and over the Kauai Channel. A Flash Flood Watch remains in
effect for the Big Island and Maui through tonight, in large part
because of the saturated grounds over the windward sides of those
islands. While those areas are the most likely to experience some
flooding, the unstable airmass moving over the islands could bring
some nuisance flooding to other areas of the state. The upper
level trough moving in from the west is expected to swing through
to the central islands by tonight, and edge closer to the Big
Island tomorrow.

High pressure to the far north-northeast continues to bring breezy
trade winds to the region. Breezy winds today are expected to
nudge down a notch tomorrow through Friday as the high weakens.
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 is getting an added boost today as an upper low
approaches 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.

AIRMETs Sierra and Tango are in effect 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
today. A High Surf Advisory is in effect through this evening 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 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...M Ballard
AVIATION...JVC
MARINE...Donaldson

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