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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 011951

Area Forecast Discussion
National Weather Service Honolulu HI
951 AM HST Sat Oct 1 2022

An early season cold front moving toward Hawaii from the north,
will generate light trades and local sea breeze showers across
some central and eastern islands this weekend. Widespread rainfall
is expected across the northwestern half of the state tonight,
before overspreading the smaller central islands Sunday as the
front weakens and drifts slowly eastward. Moderate trade winds
will develop across Kauai and Oahu Sunday as the front exits
eastward. The boundary will further weaken and slow over the
central islands through Tuesday, before dissipating. Light to
moderate trade winds are expected statewide next week.


Current radar shows isolated to scattered showers across the
islands this morning, with the most persistent showers favoring
the central smaller islands in low level convergent flow.
Satellite shows limited cloud cover across the state this morning,
while a band of much denser cloud cover, associated with the
approaching boundary, drops slowly southward.

The moisture gradient will vary greatly across the state through
this evening, with deeper moisture and higher inversion heights
over the western end of the state near Kauai (PHLI sounding
inversion height around 10k ft), trending much lower moisture
depths and inversion heights over the Big Island (PHTO sounding
inversion height 6k ft). Overall expect limited areal coverage of
showers across much of the state through the afternoon, as
satellite based estimates show dry layered precipitable water
(1.15 to 1.35 inches) streaming in from the east. Though light
background trades will allow for sea breeze showers across some
leeward areas and island interiors this afternoon and evening.

The areal coverage of showers will increase across the western
islands tonight through Sunday, as the boundary drops southward.
The areal coverage of showers will continue to expand slowly
eastward across the central smaller islands Sunday evening into
the beginning of next week, as the weakening boundary nearly
stalls as it advances eastward. Deeper layered moisture
overspreading the island chain this weekend, will begin to dry
across the western end of the state by Monday while deeper
layered moisture lingers over the central and eastern islands into

Some heavier rainfall is possible across the islands, as moisture
and instability increase in advance of the approaching boundary.
Current model trends show a band of increased precipitable water
values (1.75 to 1.95in) drifting across Kauai tonight before
stalling across the central smaller islands through Monday.
Additionally, a few thunderstorms are also possible as current
lightning data shows a few strikes occurring in the enhanced bands
of clouds north of the state, in an area of weak instability
(Showalter Index 0 - 2). The latest model trends show this band of
instability dropping south across Kauai this evening, before
becoming anchored over the central smaller islands Sunday and

Slightly drier and more stable air returns later next week with
light background trade wind flow. This pattern will allow for
another period of locally driven afternoon sea breeze showers
across leeward areas and island interiors, along with showers
across favored windward areas, especially overnight and mornings.


A light to moderate easterly trade wind pattern is expected today
with localized sea breezes across select leeward areas. Scattered
showers could produce brief MVFR ceilings this afternoon. Winds
will back to a more northeasterly direction and showers will
increase in coverage over the western end of the state tonight as
a frontal boundary encroaches on the islands from the north. As
the frontal boundary moves into Kauai and Oahu sometime during the
day Sunday, MVFR conditions are expected with brief periods of
IFR conditions possible under heavier showers.

No AIRMETs are in effect


Light to moderate easterly trades will hold into tonight, then
trend up and shift out of the northeast over the Kauai and Oahu
waters Sunday as a cold front drifts southward into the area.
Winds may near the Small Craft Advisory level in the windier
areas around Kauai Sunday. Recent ASCAT pass showed a good sized
area of 25 knot northeast winds trailing the front. Similar winds
will fill in over the Oahu and Maui County waters late Sunday
into Sunday night. The front will stall out and dissipate over the
state Monday through Tuesday, with light to moderate northeast
to easterly trades generally prevailing through midweek. This
trend will likely prevail through the latter half of next week as
the ridge remains weak to the north.

A fresh north-northwest swell that arrived Friday will hold and
shift out of a more northerly direction through the day, then
ease Sunday into Monday. A medium period north-northeast swell
arriving late Sunday night into Monday from low pressure centered
around 1000 NM northeast of the islands will drive the surf up
along northerly exposures trough the day Monday. Surf associated
with this swell should peak below the advisory level by sunset
Monday, hold through Tuesday, then fade Wednesday.

For the extended, guidance shows low pressure moving eastward
over the western Aleutians Sunday into Monday, then dropping
south-southeastward after it passes east of the Date Line Tuesday
through Wednesday. A large batch of north-northwest winds focused
at the islands down the 350 degree directional band relative to
the area on its backside as it drops southward should send
another northerly swell through the islands, with an arrival
centered around Thursday night. If this materializes, surf will
peak around the advisory level for north facing shores Friday,
then ease next weekend.

A small, long-period south-southwest swell arriving through the
day Sunday will lead to increasing surf along southern exposures.
This should peak Monday, then fade by midweek. A similar south-
southwest swell is possible next weekend. For the extended,
guidance shows an active trend continuing with a large area of
gales expanding northward within Hawaii's swell window Tuesday
through midweek (seas climbing to around 30 ft) on the backside of
low pressure passing south of New Zealand. If this unfolds, a
solid south-southwest swell will be possible by mid month.

Surf along east facing shores will remain small and well below
normal through the upcoming week due to a lack of strong trade
winds over and upstream of the islands.






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