Current Conditions
Temp6.8 C
RH39 %
WindNE 22 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 211942

Area Forecast Discussion
National Weather Service Honolulu HI
942 AM HST Fri Sep 21 2018

Moderate to breezy trade winds will build across the state today
as a weak trough near Kauai lifts away. Clouds and showers will
favor windward slopes of all islands. A few afternoon showers
will flare up along leeward slopes of the Big Island and on Kauai,
where deeper moisture and higher humidity will linger. Winds will
shift out of the southeast and south on Sunday and Monday,
bringing humid conditions and potentially wet weather across the
western end of the state through the middle of next week.


A brief return to typical late summer trade wind weather is
underway. The weak surface trough that passed overhead during the
past two days is lifting north, allowing a surface high far
northeast of the state to exert greater influence. As a result,
easterly trade winds are building in from the east this morning
and will strengthen through the day. Rainfall has been modest
this morning and confined to windward slopes as stable conditions
build in from the east. As is typical for summer, clouds and a
few showers will develop across windward slopes this afternoon,
and with deeper moisture lingering near Kauai, we cannot rule out
an isolated heavier shower on the Garden Isle. Humidity will
remain somewhat elevated on Kauai, while the rest of the island
chain sees a drop in humidity as dew points fall back to typical
September levels of around 70 degrees F.

Stable trade wind weather will persist tonight and Saturday.
Expect typical late summer conditions for the equinox. Easterly
trades will hold at moderate to locally breezy strength. Ridging
aloft will produce stable conditions with an inversion around 4000
to 6000 ft, keeping modest rainfall confined largely to windward

A humid southeast to south flow will develop on Sunday, with wet
conditions possible for the western end of the state into the
middle of next week. A broad slow-moving area of low pressure
will strengthen several hundred miles northwest of Kauai on
Saturday night and Sunday. Locally, winds will initially shift
out of the southeast, then veer out of the south for the western
half of the island chain by late Sunday. This will draw up a very
humid air mass from the deep tropics. Dew points will to rise
several degrees above normal into the mid 70s, creating very humid
conditions across all islands.

A convergence band with deep moisture will set up just west of
the state and may bring enhanced rainfall to parts of the island
chain. There is still some disagreement between the GFS and ECMWF
on the position of this feature, with the GFS bringing the deep
moisture as far east as Kauai by Sunday night. As a result, there
will be a chance of enhanced rainfall with heavy showers over
Kauai, and as far east as Oahu, through the middle of next week.
That said, uncertainty remains elevated. Across the rest of the
state, mid level ridging should remain dominant, allowing modest
showers along southeast facing terrain and spotty afternoon
showers elsewhere.


A stable easterly trade wind flow will build in from the east
today as a trough near Kauai lifts away. Isolated MVFR conditions
will be confined to windward slopes of most islands, and VFR
conditions will dominate over most areas. However, with deeper
moisture lingering near Kauai, more extensive clouds and showers
will be possible for the Garden Isle through the afternoon.

As the trades strengthen and the inversion builds around 4000 to
5000 ft, low level turbulence will increase over and west of
terrain today. If conditions warrant, AIRMET Tango may be needed.


High pressure northeast of the state will keep a moderate to
locally strong trade wind flow in place through Saturday. As a
result, a Small Craft Advisory (SCA) remains in place for the
typically windy waters around Maui County and the Big Island. An
area of low pressure is expected to develop several hundred miles
northwest of Kauai over the weekend, allowing the winds to weaken
below SCA levels and shift around to the southeast Sunday through
early next week.

Surf will remain below advisory levels along all shores through the
middle of next week. The current south swell will slowly decline
through Saturday. Rough, short-period surf will remain around the
summer average along east facing shores through tonight, then
decline over the weekend. The first northwest swell of the season
could arrive late next week, possibly reaching advisory levels
along north and west facing shores.


Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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