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Hawaiian Islands Synoptic Discussion and Guidance

993
FXHW60 PHFO 210207
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
407 PM HST Tue Oct 20 2020

.SYNOPSIS...
A stationary front will linger across Kauai over the next couple
of days. Clouds and passing showers will favor the west end
of the island chain with drier conditions across Maui County and
Big Island. An upper low will approach the northwest end of the
state and may strengthen the front Thursday and Friday with
increasing showers mainly for Kauai.

&&

.DISCUSSION...
Visible satellite and surface analysis show a stationary front
situated over Kauai this afternoon, while gentle to locally
moderate southeast winds prevail elsewhere across the rest of the
state. The light wind flow allowed for cloud build ups across Maui
and the Big Island this afternoon with some showers popping up on
Oahu. Radar shows scattered showers associated with the front
across Kauai and the Kauai Channel, while earlier thunderstorms
near Kauai are now over 50 miles north of Oahu this afternoon.

The stationary front is expected to slowly weaken tonight into
Wednesday resulting in more diffuse shower coverage along the
western end of the state. A slight chance for thunderstorms is in
the forecast for Kauai through Wednesday morning. Land breezes
should also help to keep clouds and showers offshore. Maui county
and the Big Island will remain mostly dry.

A cutoff low will form north of the state on Wednesday and will
dig south towards Kauai through Friday, then open up to a trough
and lift away to the east this weekend. The upper low will induce
a surface low northwest of the state, which will keep us in a
light, southeasterly flow pattern for at least the next 5 or 6
days. The instability associated with upper low may invigorate the
moisture band along the front, and could bring a period of more
showers and thunderstorms across the western end of the state
Thursday and Friday. There is some small differences in the GFS
and ECMWF for the position of the lows and where the
shower/thunderstorm band will eventually set up.

As the front intensifies, a pre-frontal convergence band will set
up east of the state and move into windward Big Island on
Saturday. On Sunday, the low weakens and lifts away to the east
this weekend, some of the moisture associated with the front and
convergence band northward. Meanwhile, a cold front will approach
from the northwest early next week and pass over the islands
around the middle of next week.


&&

.AVIATION...
Convergence along a weak stalled front near Kauai has triggered
heavy showers and thunderstorms around Kauai and somewhat close to
Oahu today. Thunderstorms were more active over and south of
Kauai this morning and have shifted north of Oahu this afternoon.
Expect continued moist and unstable conditions to maintain a
threat for periods of MVFR VIS/CIG in SHRA and possibly
thunderstorms for the western end of the state tonight and
Wednesday. AIRMET Sierra remains posted for south through west
slopes of Kauai and Oahu through the afternoon and may be needed
tonight.

A surface ridge over Maui County is supporting a drier and more
stable E-SE flow over the eastern half of the island chain. A
subsidence inversion holding around 6000 feet will keep a cap on
clouds and limit shower potential. Little overall change to this
pattern is expected through Wednesday.


&&

.MARINE...
A stalled boundary will linger across the western waters the next
few days, with generally light to gentle southwesterly to variable
breezes along and west of it and moderate south to southeast over
much of the rest of the state. Periods of fresh breezes within
the channels and local bays surrounding Maui County are possible
the next day or so as well. As the parent low northwest of Kauai
weakens and lifts north around midweek and high pressure to the
northeast backs further away from the state, southeast winds will
weaken further and allow coastal land and sea breezes to develop.
An upper low will approach the northwest end of the state and may
strengthen the front Thursday and Friday with increasing showers
mainly for Kauai and moist, southeasterly flow continuing
elsewhere.

A combination of peak monthly tides and nearly a foot higher-
than-predicted water levels has produced nuisance coastal flooding
and will continue to do so for the next couple of days. Impacts
are anticipated around daybreak during peak daily tides. See the
latest Special Weather Statement for more details regarding specific
impacts. Daily tidal ranges will lower through the second half of
the week.

Surf along north and west facing shores will continue to lower
through mid week as the weekend northwest swell fades. Outside of
a small, short-period east to northeast swell that will affect
exposed north-facing shores in the next day or so, very little
activity is anticipated. A developing gale over the far northwest
Pacific is forecast to send a long-period northwest swell into
the local waters, arriving late Saturday into Sunday. This swell
would produce surf heights similar to what was experienced this
past weekend along north and west facing shores with High Surf
Advisories needed.

Surf along east-facing shores will remain elevated through Thursday
as an east-northeast swell arrives this weekend. Surf along
south facing shores will remain small through the week.


&&

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

&&

$$


DISCUSSION...Foster
AVIATION...Wroe
MARINE...TS

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



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