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Hawaiian Islands Synoptic Discussion and Guidance

069
FXHW60 PHFO 150158
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
358 PM HST Sat Sep 14 2019

.SYNOPSIS...
Trade winds will be on the lighter side over the next couple of
days as a trough of low pressure north of the area disrupts the
wind flow across the state. Enhanced moisture along with an
unstable airmass will allow for increased chances for afternoon
clouds and showers across portions of the area through Monday. A
drier and more typical trade wind weather pattern is expected
Tuesday through Thursday night as a new high builds in to our
north. Another upper trough is forecast to move over the area
toward the weekend with increased shower activity a possibility.

&&

.DISCUSSION...
High pressure is centered far north of the area while a weak low
level trough is located a few hundred miles north of the state.
The trough has disrupted the pressure gradient across the area
resulting in light easterly flow. Winds are light enough allowing
for daytime sea breeze and nighttime land breeze to occur across
many areas.

Water vapor imagery shows an upper level low centered a few
hundred miles to the north of the state. This has destabilized the
airmass across the area as indicated by the 00Z soundings. The
airmass is rather moist as indicated by dew points in the upper
60's to mid 70's and satellite derived precipitable water values
of around 1.9 inches. Shower coverage has increased over the
islands due to daytime heating along with local low level wind
convergence. The airmass is unstable enough that a few thunderstorms
have formed as well. Shower activity will likely taper off somewhat
overnight but is expected to increase once again Sunday afternoon
since the weather scenario is expected to be about the same as
today.

The upper low along with the weak low level trough will move to a
position a few hundred miles north northwest of the area by
Monday. Some drier and more stable air will begin to move in from
the east but enough moisture and instability will linger near or
across the area to allow for another round of afternoon showers.

By Tuesday, the influence of the upper low and the low level
trough will have moved off to the northwest of the area as high
pressure builds in to our north. This will allow for a return of
moderate trade winds across the area. Under this scenario, clouds
and passing showers will favor windward and mauka areas with just
a few passing showers expected over select leeward locations.
These conditions are expected to continue through Thursday night.

Toward the end of the week, an upper trough is forecast to move
into the area from the northeast. This will likely cause the
airmass to become less stable once again with an increase in
shower activity expected as we head into next weekend.

&&

.AVIATION...
Trade wind speeds are diminished as a diffuse low-level trough
lies between the islands and a surface high far north of the area,
and winds will remain that way into early next week. Winds were
light enough to allow sea breezes to drive cloud and shower
formation over leeward and interior areas this afternoon, with a
somewhat unstable and moist atmosphere fueling a few heavy showers
and isolated thunderstorms. The bulk of the leeward and interior
heavy showers will diminish shortly after sunset, while the light
trade winds will help to focus overnight showers over windward
areas.

Heavy showers and possibly a thunderstorm will develop over the
islands once again on Sunday as little overall change to the
weather pattern is expected. Most TAF sites will be prevailing
VFR. AIRMET SIERRA is posted for leeward Big Island due to
mountain obscuration. This will likely be dropped later this
evening, but may be required again tomorrow afternoon.

&&

.MARINE...
A weak surface trough north of the state is starting to drift
west and away from the state. This has led to an increase in
trades winds to moderate to locally fresh speeds across the
eastern half of the coastal waters. Meanwhile, gentle to moderate
easterly trades will continue for the western half of the area
through Sunday. As the trough continues to moves to the west to
northwest, moderate to locally strong trades are expected to
return late by Tuesday, but could be as early as Monday afternoon
for the typical windy areas around Maui and the Big Island. The
current forecast reflects winds just under the Small Craft
Advisory through the week, but winds are expected to be borderline
advisory levels through the week. Stay tuned for updates as we get
more high resolution model data available.

A combination of small south-southeast and south-southwest swells
will continue to bring a modest increase in surf along south
facing shores today. These swells are expected to peak today and
gradually weaken over the next few days. Another modest south-
southwest swell will be possible towards the end of next week.

Only small surf is expected along north, east and west facing
shores through next week. A small moderate period swell is
possible along east facing shores from Tropical Cyclone Kiko
and may arrive as early as Wednesday of next week. Latest model
guidance is showing a swell of 2 to 3 feet with periods of 11 to
13 seconds.

&&

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

&&

$$

DISCUSSION...Burke
AVIATION...Birchard
MARINE...Kino

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



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