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Temp4.5 C
RH24 %
WindNNE 3 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 181323

Area Forecast Discussion
National Weather Service Honolulu HI
323 AM HST Thu Aug 18 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 this morning, before transitioning over to leeward and
interior areas this afternoon. A more typical, but drier than
normal trade wind pattern will return tonight 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 350 miles north of Kauai. The
resulting gradient is producing light to moderate trades early
this morning, with land breezes present in many of the more
sheltered locales. Infrared satellite imagery shows clear to
partly cloudy conditions in most areas, with a bit more cloud
cover in some windward and mauka areas, as well as leeward Big
Island. Radar imagery shows scattered showers moving into windward
areas, with a few decaying showers or sprinkles making it into
leeward locales. A few showers also continue to hug the leeward
Big Island coast. 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. Light to moderate trades
will hold over the western end of the state today, while moderate
trades will become locally breezy by late in the day. The trades
are expected to be light enough over Kauai and Oahu for sea
breezes to develop in the more sheltered leeward areas. The trades
will increase to moderate and locally breezy levels statewide
tonight, with moderate to breezy conditions then holding in place
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 Wednesday, with some land and sea breezes possible in
the more sheltered leeward areas once again.

As for the remaining weather details, scattered showers will
continue to favor windward areas this morning, before
transitioning over to leeward and interior locales this
afternoon. Lingering instability could allow for the development
of a thunderstorm or two over the Big Island this afternoon as
well. The daytime convective showers should end quickly this
evening, with a more typical trade wind pattern then taking hold
featuring primarily windward and mauka showers. Rather dry trade
wind weather will 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 local weather pattern will be in a transition period today,
as the recent light wind regime is gradually replaced by
strengthening trade winds across the entire island chain. The
strongest trade winds will initially be in the vicinity of the
eastern islands this morning. The weaker trades near Kauai and
Oahu may allow local sea breeze circulations to develop starting
late this morning over the leeward sections of the western
islands. However, expect moderate to locally strong trade winds
to spread over most areas by late this afternoon or evening. The
trades will carry some low clouds and showers into windward
sections of the islands, which may produce brief MVFR conditions.
If the sea breezes form later today, there may also be clouds and
spotty showers over some leeward sections. Otherwise, expect VFR
conditions to prevail through this evening.

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


A weak ridge has persisted north of the islands and has been
sustaining light to moderate trade winds. However, the ridge has
begun strengthening and shifting north, causing trade winds to
increase from east to west across the island chain. A Small Craft
Advisory will be posted for the typically windier waters around
Maui County and the Big Island by noon HST today.

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
further. As they do, modest, choppy surf will return along east
facing shores, lasting from this weekend into early next week.
Expect nearly flat conditions along north facing shores. However,
small, short-period north-northwest swells beginning later today
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.


Small Craft Advisory from noon today to 6 PM HST Sunday 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