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RH40 %
WindSE 1 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180646

Area Forecast Discussion
National Weather Service Honolulu HI
846 PM HST Wed Aug 17 2022

A ridge of high pressure north of the state will lift northward
and allow for a gradual strengthening of the trade winds during
the next couple days. Showers will favor windward slopes and
coasts tonight and early Thursday, before transitioning over to
leeward and interior areas Thursday afternoon. A more typical, but
drier than normal trade wind pattern will return Thursday night
and hold through the weekend, with moderate to locally breezy
conditions prevailing. A mid and upper level trough will bring
some deeper moisture and an increase in trade wind showers as it
moves into the region during the first half of next week, while
the trades gradually ease into the light to moderate range.


Currently at the surface, a 1022 mb high is centered around 1600
miles northeast of Honolulu, with a ridge axis extending west-
southwest to a location around 300 miles north of Kauai. The
resulting gradient is producing light trades over the western
islands, while moderate trades prevail over the eastern end of
the state. Infrared satellite imagery shows partly to mostly
cloudy conditions, with cloud cover decreasing in most areas with
the loss of daytime heating. Radar imagery shows isolated to
scattered showers moving into windward areas, a few lingering
showers over leeward sections of the Big Island, and mainly dry
conditions elsewhere. Main short term focus revolves around trade
wind trends and rain chances.

The ridge of high pressure north of the state will lift northward
during the next couple days, resulting in an increase in trade
wind speeds across the island chain. Moderate trades will prevail
tonight and Thursday over the eastern islands, while light to
locally moderate trades hold over the western end of the state.
The trades will likely be light enough over Kauai and Oahu for
some land and sea breezes to develop in the more sheltered
leeward areas during this time. The trades will increase to
moderate and locally breezy levels Thursday night, then hold
steady Friday through the weekend. A front dropping southward
toward the islands early next week, should ease the trades to
moderate levels on Monday, and into the light to locally moderate
range Tuesday and next Wednesday with some land and sea breezes
possible in the more sheltered leeward areas.

As for the remaining weather details, we should see a gradual
increase in trade wind showers overnight, with the coverage the
greatest over the eastern end of the state. Most leeward areas
will remain dry, although a few showers will remain possible over
leeward Big Island through the night. Showers will continue to
favor windward areas Thursday morning, before transitioning over
to leeward and interior locales Thursday afternoon. Lingering
instability over the Big Island could allow for the development
of a thunderstorm or two Thursday afternoon as well. The daytime
convective showers should end quickly Thursday evening, with a
more typical trade wind pattern then taking hold featuring
primarily windward and mauka showers. Rather dry trade wind
weather will then prevail Friday through the weekend, with showers
limited primarily to windward slopes and coasts. We should see an
increase in trade wind showers during the first half of next week
as a mid and upper level trough and some associated deeper
moisture moves in from the east.


The trade winds are trying to strengthen over the eastern end of
the island chain. However, the background flow remains weak
enough to allow local land breezes to become established over most
of the islands. These downslope winds are expected to clear out
most of the residual clouds and showers over leeward and interior
sections of the islands this evening. The weak trade wind flow
will transport some low clouds and brief showers into windward
facing coasts and slopes through Thursday morning. Brief MVFR
conditions may develop over some windward sections overnight, but
VFR conditions will likely prevail.

Expect Thursday to be a transition day, with the trade winds
gradually returning to the entire island chain. The strongest
trade winds will initially be over the eastern islands Thursday
morning. The weaker trades near Kauai and Oahu may allow some
local sea breezes to develop starting late Thursday morning over
the leeward sections of the western islands. However, expect
moderate to locally strong trade winds over most areas by late
Thursday afternoon or evening. The trades will carry some low
clouds and showers into windward sections of the islands. This
may produce brief MVFR conditions, but VFR will be the general
rule for most areas on Thursday.

No AIRMETs are currently in effect, and none are anticipated
through Thursday morning. There is a possibility that as the low-
level trade wind flow strengthens late Thursday, AIRMET Tango for
mechanical low-level turbulence may eventually be required for
portions of the state starting Thursday afternoon or evening.


A weak ridge persists north of the islands and will sustain light
to moderate trade winds. The ridge will strengthen and shift
north, causing trade winds to increase beginning Thursday morning
through the afternoon. A Small Craft Advisory will likely be
posted across the typically windier waters around Maui County and
the Big Island when warranted.

Small south and southeast swells are expected to produce below
typical summertime surf heights along south facing shores into
Friday. Forerunners from a new, long-period south swell are
expected to arrive late Friday night. This swell will likely
produce near to slightly above average surf along south facing
shores from this weekend into early next week. Surf will remain
small along east facing shores until trade winds strengthen. As
they do, modest, choppy surf will return along east facing
shores, lasting from this weekend into early next week. Expect
nearly flat conditions to persist along north facing shores
through tonight. Small, short-period north-northwest swells
beginning Thursday may provide a slight boost in surf heights
along exposed north facing shores through this weekend. Models
show a slightly larger and longer period northwest swell arriving
early next week.






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