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Hawaiian Islands Synoptic Discussion and Guidance

754
FXHW60 PHFO 240620
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
820 PM HST Fri Oct 23 2020

.SYNOPSIS...
The persistent wet and humid pattern in place over the western
end of the state is expected to linger through much of next week
as another cold front and upper disturbance approach and move into
the area later this weekend through next Wednesday. Although the
shower coverage may briefly decrease over the weekend as the
upper disturbance in the area lifts off to the north, showers will
remain in the forecast, especially through the afternoon hours. A
modest increase in moisture moving in from the east will support
a slight increase in shower activity for Maui County and the Big
Island over the weekend. A more widespread rainfall event will be
possible next week, which will translate to an increasing concern
for flash flooding given the recent heavy rainfall on Kauai and
Oahu.

&&

.DISCUSSION...
Water vapor imagery depicted an amplified upper pattern over the
northern Pacific with persistent upper troughing over the central
Pacific - north and northwest of Hawaii. The upper low positioned
nearby to the north is opening up and beginning to lift off to
the north and away from the area. Some drier middle- to upper-
level air and rising upper heights will be anticipated briefly as
this feature pulls away from the area overnight into the weekend.
Although this will translate to more of a stable environment
compared to the past several days, spotty showers will remain in
forecast due to the persistent plume of moisture in place and
light southerly low-level flow. The best chances for showers will
remain over Kauai and Oahu through the afternoon hours as sea
breezes develop. A modest increase in moisture, however, filling
in from the east will support a slight increase in shower coverage
over Maui County and the Big Island this weekend where very
little rainfall has fallen.

Guidance has initialized well with the current pattern and shows
the progressive pattern continuing early next week as a potent
shortwave trough digs southeastward toward the region driving
another cold front into the area Tuesday through Wednesday. This
will result in the wet and humid conditions persisting. Given the
recent periods of heavy rainfall and moistening soils, the main
concern with this scenario will be the threat for heavy rainfall
and flash flooding - especially over Kauai and Oahu. Confidence
in the details will follow in later packages this weekend as this
scenario and unfolds.

&&

.AVIATION...
Light convergent flow out of the southerly quadrant will continue
through the forecast period bringing periods of showers and MVFR
to leeward areas, especially during mid to late day Saturday once
sea breezes become established. A band of deeper moisture
immediately east of the state per regional satellite imagery will
work westward during Saturday resulting greater inland shower
coverage for Molokai through Big Island compared to recent days.

AIRMET Sierra remains in effect for Kauai and Oahu in the near
term but conditions are forecast to improve overnight.

&&

.MARINE...
Strong high pressure northeast of the state and a trough of low
pressure to the northwest, will keep a fairly stagnant pattern
in place across the marine area through the weekend. Moderate to
fresh southeast winds will prevail over the eastern islands,
while winds remain light and variable over the western end of the
state. An approaching front may bring increasing southerly winds,
heavy rainfall, and thunderstorms to much of the area Tuesday
though the middle of next week.

Surf along north and west facing shores will remain very small
though Saturday morning, then gradually increase late Saturday
afternoon as a long-period northwest swell arrives. This swell
will fill in Saturday night, with surf possibly approaching
advisory levels along north and west facing shores of Kauai and
Oahu as it peaks on Sunday. North Shore surf will gradually lower
through the middle of next week, then hold at small levels
heading into next weekend.

Surf along south facing shores will remain small with a mix of
long-period south and short-period southeast swells moving
through.

Surf along east facing shores will remain small through the
weekend, then trend up early next week as a small northeast swell
arrives.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Gibbs
AVIATION...JVC
MARINE...Jelsema

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



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