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RH15 %
WindW 14 mph
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Hawaiian Islands Synoptic Discussion and Guidance

622
FXHW60 PHFO 250648
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Sun Sep 24 2017

.SYNOPSIS...
A gentle to moderate trade wind flow will be with us for the new
work week as a surface ridge of high pressure remains north of the
islands. The trades will be light enough to bring on daytime sea
breezes favoring trade wind shelter areas such as the leeward
coast. This will lead to some afternoon clouds over the lee and
interior areas. Meanwhile, the trades will carry in a few showers
to the windward areas. A boost in the showers is likely Wednesday
through Thursday as a surface trough passes through the islands.

&&

.DISCUSSION...
The trades are expected to become a tad lighter overnight with
gentle to moderate strength persisting between Monday through
Friday.

The air mass is currently rather stable with a low level
inversion sloping from 6k feet over Kauai to 8k feet over
windward Big Island. It is not a robust inversion, likely weakened
by an upper level trough draped across the area. This weakness in
the inversion could be responsible for some the evening and late
night showers along the Kona coast. This trough is forecast to
fill through the next 24 hours, with upper ridging taking over for
the rest of the week.

As the trades bring in a few showers during the next 2 days, the
winds will be weak enough to allow for daytime sea breezes,
favoring the lee and interior areas of the islands. Stable mass
will be enough to ward off any widespread afternoon showers.

On Wednesday, we anticipate a boost in the trades showers as
a trough in the trades passes through the islands from east to
west. The ECMWF model differs from the GFS where the EC is faster
by a half a day, and is the model of choice. The moisture will
linger through Thursday before becoming slightly drier on Friday.
With a little more moisture, the chances for afternoon showers
increases on both Wednesday and Thursday.

Over the weekend, the trades give way to SE winds over the western
half of the island chain. This change is brought on by a front
stalling 330 miles NW of Kauai, and has the support of an upper
level trough. The old EC run has the front further west than the
GFS, as well as the upper trough. But should the GFS pan out, the
humidity will be near or at uncomfortable level, and we may have
vog.

&&

.AVIATION...
VFR will predominate across the island chain. Only brief MVFR
conditions are possible over windward and mauka sections in
passing showers, being carried by the light to moderate trade
winds. Low ceilings and showers are also possible over leeward
parts of the Big Island through the evening hours.

No AIRMETs are currently in effect and none are anticipated.


&&

.MARINE...
Light to moderate trades are expected to persist through the first
half of the week as a weak surface trough approaches the islands
from the east. The wind flow will be light enough for land/sea
breezes to setup over some leeward areas through this time. A
return of moderate to fresh trades are forecast through the second
half of the week as the trough moves through from east to west.
Some of the typically windier channel waters may even reach
advisory levels Thursday through Friday.

Surf along south facing shores will remain small through the week
with mainly a mix of background southeast and southwest swell energy
expected. The highly zonal pattern with systems racing eastward
across the southern Pacific continues, with no significant sources
shown setting up within Hawaii's swell window over the next several
days. The next long-period pulse out of the southwest (210-220 deg)
from the Tasman Sea should fill in Tuesday night and continue
through midweek, which will lead to a slight increase in surf. This
source is from a strong- to gale-force low that moved across the
Tasman Sea last week.

Small, short period (10-12 seconds), northwest swell energy will
continue and should be enough to keep the surf from going flat along
north facing shores through the week. The latest surface analysis
and satellite data showed a 998 mb low centered around 1200 nm north-
northwest of the islands with a pocket of near gale-force winds
focused toward the islands within the 330-340 deg band and seas up
to around 10 to 13 ft. This reinforcement out of the north-northwest
should reach the local waters late Wednesday and hold through
Thursday. A compact storm-force low currently east of Japan is
forecast to continue eastward over the upcoming few days toward the
Date Line around 35N. Although the GFS/WAVEWATCH III solution
suggests a favorable setup for a west-northwest (310-320 deg) swell
to fill in across the local waters this coming weekend, the ECMWF
is not as aggressive with this system. As a result, forecast
confidence for the upcoming weekend remains low at this time.
Guidance should come into better agreement over the upcoming
couple of days.

Surf along east facing shores will remain small through the first
half of the week as the trades locally and upstream of the state
remain light. Once the aforementioned surface trough moves through
from east to west by midweek, small and choppy surf will likely
return as the trades fill back in through the second half of the
week.

&&

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

&&

$$

Public...Lau
Aviation...Kinel
Marine...Gibbs

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