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Hawaiian Islands Synoptic Discussion and Guidance

316
FXHW60 PHFO 210657
AFDHFO

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

.SYNOPSIS...
The wet pattern in place over the western end of the state is
expected to linger through early next week as deep tropical moisture
continues to lift northward into the area ahead of a persistent
upper disturbance in the area. Periods of heavy showers that
become focused over any particular area could translate to
localized flooding concerns - with the best chance remaining over
Oahu and Kauai. Although drier conditions will prevail over the
eastern end of the state, a few afternoon showers associated with
the sea breezes will remain possible each day.

&&

.DISCUSSION...
Latest surface analysis and satellite imagery showed a diffuse
frontal boundary that remains stalled near Kauai and a weak ridge
extending southwest into the eastern end of the state. Water vapor
imagery depicted an amplified upper pattern over the northern
Pacific characterized by a broad upper trough extending from north
to south between Midway and Kauai. A plume of deep tropical
moisture pooling northward over the islands ahead of this feature
combined with a sufficient amount of lift and instability are
fueling heavy showers and a few thunderstorms over and around Oahu
and Kauai this evening. Expect this trend to persist overnight
into Wednesday as this boundary and upper low continue to
influence the local weather pattern. Showers should remain
isolated over the eastern end of the state through Wednesday with
the best chance of development through the afternoon hours when
sea breezes push ashore.

Guidance has initialized well with the current pattern and shows the
wet and humid conditions continuing into the second half of the week
as the upper low to the northwest drifts southeastward toward the
area. This combined with plenty of moisture in the area will
continue to support active convective spells over the western end
of the island chain. Given the recent periods of heavy rainfall
and moistening soils, the main concern with this scenario will be
the threat for heavy rainfall and localized flooding.

For the extended (weekend through early next week), guidance shows a
similar pattern unfolding as the upper pattern amplifies and
another upper trough and frontal boundary approach Sunday into
early next week. This will result in the wet and humid conditions
persisting with light and variable winds.

&&

.AVIATION...
A stalled front north of Kauai will continue to bring periods of
convective showers and isolated thunderstorms over Kauai, Niihau
and Oahu over the next 24 hours. More stable weather conditions
are forecast over the eastern half of the state from Molokai to
the Big Island. Periods of MVFR conditions are expected over the
western islands with brief isolated IFR coverage possible in the
heaviest shower bands. Mostly VFR conditions will prevail over the
eastern half of the state. This weather pattern will continue
through Wednesday.

AIRMET Sierra remains in effect for Tempo Mountain Obscuration
over south through west facing slopes of Kauai and Oahu.

&&

.MARINE...
A stalled boundary will linger across the western waters the next
several days. Generally light to gentle southwesterly to variable
breezes along and west of this boundary with more moderate east
to southeast winds over much of the state east of the boundary.
Periods of fresh breezes within the channels and local bays
surrounding Maui County are possible through late Wednesday. As
the parent low northwest of Kauai weakens and lifts north through
early Thursday, along with northeastern high pressure pulling
further away from the state, east to southeast winds will further
weaken. Weakened large scale winds will likely allow for the
development of late week into weekend coastal land and sea
breezes. The next cold front will approach the far western waters
early next week. The latest guidance has the front traveling
through the northwestern waters and veering moderate winds around
to the northeast into next Wednesday.

A combination of peak monthly tides and nearly a foot higher-
than-predicted water levels has produced nuisance coastal flooding
and this will continue for another day. Impacts are anticipated
around daybreak during peak daily high 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.

Surf along east-facing shores have trended and will stay up
through Thursday as an east-northeast swell arrives from a mid
week northerly fetch region forming off the Northern California
coast. This will be focused at the islands and be timed to arrive
this weekend. Otherwise, weak east to southeasterly winds will
continue to produce low surf.

Surf along south-facing shores will remain small through the week
as a result of both south and south-southeast background swell
energy.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Bohlin
MARINE...Blood

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



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