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Hawaiian Islands Synoptic Discussion and Guidance

797
FXHW60 PHFO 190155
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Sun Aug 18 2019

.SYNOPSIS...
A ridge north of the state will continue to weaken overnight,
maintaining a lighter trade wind pattern through Monday. Expect
the trades to build again during the second half of the week as
high pressure rebuilds north of the area. Showers riding in on
the trades will favor the windward and mountain areas, especially
during the nights and early mornings. Rainfall amounts are
expected to be lighter than normal through Tuesday.

&&

.DISCUSSION...
No significant changes to the forecast or forecast philosophy with
the afternoon package. Have made some tweaks to the winds Monday
night through Tuesday night to trend a little higher in line with
the high resolution models. PoPs and associated elements have
been tweaked from Monday night through Saturday to better
represent the latest blend of the models. This starts to shift the
increase in showers for Tuesday/Wednesday to south of the islands
in line with the latest model runs. And trends the showers up a
bit during the latter part of the week with increased moisture
coming in.

The ridge to the north of the islands continues to weaken, and
winds around the state have been light to moderate. Little change
in winds are expected over the next 24 hours. Some areas will
start to see some winds picking up on Tuesday, with more areas
feeling the increase Tuesday night and Wednesday. The high
pressure system building north of the islands will help then to
maintain a moderate to locally breezy trade wind pattern into the
weekend.

An old front northeast of the islands as we head into the middle
of the week will help to send more moisture to the region on the
trade wind flow. Trade wind showers could be a bit more frequent
as that moisture rides in. There is still some model disagreement,
both between models and from run to run, with the amount of
moisture and its placement, so the forecast currently reflects a
blend of models. Further refinement to the forecast will come as
we get closer.

&&

.AVIATION...
Elongated surface high pressure north of the Hawaiian islands
will help to maintain light to moderate easterly flow through this
evening. Thus, isolated showers will mainly be limited to
windward and mauka regions. Shower activity is expected to
increase after sun down along the windward slopes. MVFR conditions
will be possible at times due to lowered visibility and mountain
obscuration.

No AIRMETs are currently in effect.

&&

.MARINE...
High pressure north of the state will continue to weaken tonight
and will result in light to moderate northeast trades across the
area waters for Monday. As high pressure builds north of state
Monday night through the middle of the week, moderate to fresh
east-northeast trades will return Monday night and will likely
increase to fresh to locally strong speeds by Wednesday. Winds are
expected to reach Small Craft Advisory levels as early as Tuesday
night for the typical windy areas around Maui County and the Big
Island. A weak trough passing near the state during the second
half of the week, may bring a slight decrease in wind speeds and a
slight change in wind direction, with northeast winds expected
ahead of the trough on Thursday and east winds expected behind the
trough on Friday.

Surf along south facing shores will remain small through Monday,
with mainly background energy continuing out of the southwest and
southeast. An upward trend is anticipated beginning late Monday,
as one of the largest south swells of the season begins to build
across the Hawaiian waters. Surface analysis charts showed a
compact storm-force low bottoming out at 956 mb southeast of New
Zealand last Monday through Tuesday within Hawaii's swell window.
ASCAT passes reflected this and showed a pocket of storm-force
winds nosing northward through this period around the 190 deg
band relative to the islands. Altimeter passes within this area
showed seas ranging from 35 to 42 ft. This swell has already moved
through the nearshore PacIOOS buoy at Pago Pago, which showed
seas and energy levels slightly higher than our recent July 14th
warning-level event. The possible limiting factor to this swell
was that the storm was slightly more compact compared other high
surf events. As a result, surf heights will likely near/reach
warning levels along south facing shores through the peak of the
event Tuesday through Tuesday night and may not drop below
advisory levels until Thursday.

The small westerly swell associated with recent typhoon activity
in the western Pacific will steadily trend down through Monday.
Surf will respond and lower along the exposed coasts from this
source.

Surf along north and west facing shores may trend up through the
second half of the week as a small northwest swell builds down the
island chain. Guidance remains in good agreement and depicts a
gale evolving around 2000 miles northwest of the islands near and
just west of the Date Line early this week.

Surf along east facing shores will trend up by midweek as the
trades strengthen locally and upstream of the islands.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...M Ballard
AVIATION...Thomas
MARINE...Kino

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