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Hawaiian Islands Synoptic Discussion and Guidance

166
FXHW60 PHFO 231934
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
934 AM HST Fri Oct 23 2020

.SYNOPSIS...
A dissipating front just northwest of the state will bring
scattered to numerous showers to the western portion of the state
today, while drier conditions are expected over Maui County and
the Big Island. An area of moisture moving in from the east will
increase showers across windward sections of Maui County and the
Big Island this weekend, while shower coverage decreases over the
western end of the state. Another front may move into the islands
during the middle of next week. This could lead to a very wet
pattern across portions of the island chain, with the potential
for heavy rainfall, flash flooding and even thunderstorms through
much of next week.

&&

.DISCUSSION...
A dissipating front is nearly stationary just northwest of Kauai,
while an upper low resides about 300 miles north-northwest of
Kauai. Light southerly flow is noted over Kauai, while light to
moderate southeasterly flow prevails over the rest of the state.
Satellite loop shows overcast low clouds across Oahu, with
scattered low clouds to clear skies across the rest of the state.
Radar depicts a northeast to southwest band of showers, some
heavy, moving northeastward through the Kauai Channel, with
lighter isolated showers over the southern half of Oahu.
Elsewhere, very little shower activity is occurring this hour.

The front northwest of Kauai is expected to dissipate today,
while strong high pressure remains northeast of the islands. The
upper low north-northwest of the state will gradually open up and
begin to lift slowly northward. Convergent southerly flow over the
western islands should continue to promote scattered to numerous
showers, favoring south and southwest facing slopes and coasts.
Meanwhile, relatively dry conditions should continue over the
eastern end of the state, with a few showers affecting windward
and southeast facing slopes this morning, and a few daytime heat-
driven showers possible during the afternoon hours over interior
and leeward sections.

While the upper low lifts out, convergent southerly flow is
expected to continue near Kauai. Fueled by lingering moisture
associated with the front, scattered showers should continue to
favor south and southwest facing slopes and coasts at night and
during the early morning, and interior sections during the
afternoon hours. A band of deeper moisture will reach the eastern
end of the state tonight and Saturday, which should bring an
increase in shower coverage to windward sections of Maui and the
Big Island. A few daytime heat-driven showers are possible over
interior and leeward sections as well.

Models show the potential for a very wet pattern over the islands
next week, depicting an upper trough pushing another front
southeastward toward and possibly into the islands by the middle
of next week. Convergent flow ahead of the front may draw deep
tropical moisture northward, increasing the potential for heavy
rainfall, flash flooding and even thunderstorms across portions of
the state. We will continue to monitor this pattern closely and
adjust the forecast accordingly as details become more clear.

&&

.AVIATION...
A stacked low pressure system and associated stationary front
northwest of the state is producing light southeast winds
statewide and numerous heavy showers with isolated embedded
thunderstorms across the western half of the state today. AIRMET
Sierra is posted for tempo mountain obscuration due to showers
over Oahu that will likely continue into this afternoon. Light
icing will also be possible around the heavy showers and
thunderstorms. Unstable conditions will continue into the weekend
for the western half of the state.

Maui County and Big Island are mostly sunny this morning. Daytime
heating will produce low clouds and isolated showers over these
islands this afternoon, but VFR conditions will prevail. A
convergence band moving in from the east will bring showers and
deteriorating conditions tonight for windward Big Island.

&&

.MARINE...
A weakening front and upper level low near Kauai are generating a
line of scattered to locally heavy showers in the Kauai channel
with embedded thunderstorms. This line of convection is expected
to remain through the rest of the day. A stationary surface ridge
over Maui is producing light to moderate southerly winds over the
western end of the state and southeasterly winds over the eastern
end of the state. Southeasterly winds will increase to moderate to
fresh range over windward waters of the Big Island and Maui this
weekend as the ridge moves north. The current upper level trough
will weaken and move north of the area through tonight. A new
upper level trough and front will move in from the northwest which
could bring a more widespread weather event next week.

Surf along north and west facing shores will remain almost flat
through tonight, then gradually increase Saturday for Kauai and
Oahu as a long- period, northwest swell moves down the island
chain. This source will continue to fill in Saturday night and
peak through the day Sunday. Surf will near, if not reach,
advisory levels for north and west facing shores of Kauai and Oahu
on Sunday. Surf will hold into Monday, then slowly ease Tuesday
through midweek as the swell lowers.

Surf along south facing shores will remain small with mainly a mix
of background, long-period southerly and short-period southeasterly
swell moving through.

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

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...Foster
MARINE...Almanza/Morrison

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

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