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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 221941

Area Forecast Discussion
National Weather Service Honolulu HI
941 AM HST Thu Oct 22 2020

A trough aloft will produce light winds and humid conditions over
the main Hawaiian Islands through at least early next week.
Through tomorrow, the trough will bring the chance of heavy
showers and a slight chance of thunderstorms for Kauai and Oahu,
with drier and more stable weather over Maui County and the Big
Island. Over the weekend, an area of moisture moving over the
islands from east will bring wetter weather to Maui County and the
Big Island.


A front about 175 miles northwest of Kauai is moving southeast
slowly. A pre-frontal convergence band near Oahu is also moving
southeast slowly. Showers along this band are mainly offshore, but
a few showers are starting to move over Oahu. A low aloft is
keeping the atmosphere unstable, especially near Kauai County and
Oahu. Moisture and instability over Kauai County and Oahu will
bring scattered showers and a slight chance of thunderstorms
through tomorrow. The atmosphere is drier and more stable over
Maui County and the Big Island of Hawaii. Afternoon clouds and
showers are expected over those islands today with clearing
tonight, then afternoon clouds and showers again tomorrow.

There may be a temporary lull in the wet weather tomorrow night
before a new area of moisture moves over the islands from the
east. This moisture and the lingering trough aloft will likely
bring more rainy weather through the first half of next week. The
global models show a new front moving toward the islands from the
northwest around the middle of next week but there are large
differences between the ECMWF and GFS, so there is low confidence
in the longer range forecast.


A nearly stationary band of clouds and showers along with a few
embedded thunderstorms lies in the vicinity of Oahu. The airmass
is rather unstable. This combined with daytime heating will allow
for clouds and showers to form over Oahu and possibly Kauai
through this evening. Expect periods of MVFR conditions with local
IFR conditions possible in heavy showers over Kauai and Oahu.
Maui county and the Big Island will see a few showers during the
afternoon hours though VFR conditions will prevail.

No AIRMETs are currently in effect. An AIRMET for tempo mountain
obscuration may be issued later this afternoon for low cloud ceilings
over the islands of Kauai and Oahu.


A surface ridge extends from near the islands to a surface high
far to the northeast. A front located about 150 nm northwest of
Kauai is moving southeast slowly. This pattern will produce light
to moderate southeast winds near the Big Island and light and
variable winds around the smaller islands.

A trough aloft is keeping the atmosphere unstable in the vicinity
of the western islands. This instability combined with abundant
moisture will allow locally heavy downpours and thunderstorms to
develop through early Friday morning over portions of the coastal
waters, especially in the vicinity of Kauai, Niihau and Oahu. The front
is expected to remain northwest of Kauai this weekend. A new
front will reach the far northwest offshore waters on Monday. This
scenario would keep light winds over most of the state early next

Surf produced by the current small, short period northeast swell
will continue to gradually subside through Friday. A new long
period northwest swell will arrive late Saturday and build on
Sunday. Surf may approach advisory threshold along north facing
shores of some of the smaller islands Sunday. This swell will
subside from Monday into the middle of next week.

A small, medium period south swell will maintain small background
surf along south facing shores through this weekend. In addition,
a small background southeast swell will produce small surf along
exposed shorelines, such as the south Big Island. Otherwise,
expect surf along the other shorelines to remain tiny.

Peak monthly tides and higher than normal sea levels may produce
local coastal flooding this morning. See the Special Weather
Statement for additional information about the potential for
nuisance coastal flooding through noon today.





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