Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 221406
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Mon May 22 2017

.SYNOPSIS...
Trades will continue to increase and become quite brisk through
Tuesday, as a strengthening high pressure passes far north of the
islands. Showers embedded in the winds may be enhanced as a weak
upper level trough is expected to move across the state later today
through Tuesday. Trades will gradually weaken during the latter
half of the week, becoming light and variable by the start of the
holiday weekend as a surface trough is expected to develop near the
islands.

&&

.DISCUSSION...
Moderate trade winds prevail across the islands early this morning
as a high pressure far northwest of the area moves east-northeast.
There is a weakening frontal cloud band just northwest of the
islands, and it will stay northwest of the area through Tuesday.
Latest satellite imagery revealed not much low clouds in the island
vicinity early this morning, except for the Big Island where an area
of low clouds has reached that island earlier, bringing a little
more showers. There are also some thin high clouds passing over the
southern islands, keeping the skies a little cloudier at times.
Early morning Lihue sounding revealed a stable airmass, though the
usual trade wind inversion is rather weak, while Hilo sounding has a
much well-defined trade wind inversion. Surprisingly, Hilo sounding
is rather dry considering the present of clouds and showers in its
vicinity.

As the aforementioned high passes far north of the islands through
Tuesday and strengthens, trades will firm up quite a bit across the
area, becoming locally windy later today through Tuesday. The rather
breezy trade wind weather is expected for the next couple of days,
with passing low clouds and showers affecting mainly windward areas.
Winds will be strong enough to carry low clouds and showers to the
lee areas of the smaller islands. In the meantime, a weak upper
level trough is expected to pass over the state later today through
Tuesday. The cooler temperatures aloft associated with this feature
will allow the airmass in the island vicinity to become a little
more unstable, with passing showers becoming more enhanced. On the
other hand, forecast models do not indicate much increase in the low
level moisture. Coupled with the rather robust trade winds, these
enhanced showers are not expected to pose any hydrological problem
for the islands.

The trades will very gradually decrease toward the later part of the
week as the latest models still indicated a sharp upper level trough
will move in from the northwest, leading to the possible development
of a weak surface trough near the islands by the start of the
holiday weekend. Moreover, pressure gradient across the area is
expected to ease enough for the synoptic winds to become quite
light, allowing more widespread daytime sea breezes to develop.
Moisture from the old frontal cloud band to our north may also
finally to spread to the islands, especially the northern ones,
while the airmass in the island vicinity may become unstable.
Therefore, weather may become more active for the state toward the
next weekend, with more enhanced showers and less winds. The
unsettled weather may linger in the area through the Memorial Day
weekend.

&&

.AVIATION...
Trade winds will continue to strengthen today and remain breezy
through mid week, as an area of high pressure well northwest of the
state strengthens and shifts eastward. Clouds and showers will favor
windward and mauka areas, but may occasionally spread leeward due to
the strength of the trades. Predominantly VFR conditions are
expected in most areas through early this evening. The exceptions
are windward Big Island and possibly windward Maui, where an area of
enhanced clouds and showers will bring several hours of MVFR
cigs/vsbys later this morning.

AIRMET Sierra may be needed for windward Big Island and possibly
windward Maui later this morning as the area of enhanced clouds and
showers moves in with the trades.

AIRMET Tango will likely be needed for low level turbulence over and
downwind of the terrain with the 22/16Z AIRMET issuance due to the
strengthening trade winds.

&&

.MARINE...
With trade winds on the increase, a small craft advisory (SCA) is
now in effect for the typically windier zones around Maui and the
Big Island. More zones are likely to be added, possibly later today
or tonight, as high pressure strengthens across the area. Winds will
begin to weaken by midweek.

The current south swell will gradually lower through Wednesday. Wave
model guidance continues to show a larger south-southwest swell
arriving later in the week, persisting through most of the Memorial
Day weekend. This swell has the potential of boosting surf along
south-facing shores to near 8 feet, which is at the threshold for a
high surf advisory. In addition, the combination of swell energy and
greater than normal astronomical tides may result in minor coastal
flooding due to wave run-up along south-facing shores this coming
weekend, especially during each high tide.

The next moderate northwest swell has been arriving this morning. It
is forecast to peak today before slowly subsiding and swinging
around to a north-northwest direction through midweek. This swell is
expected to increase surf along north- and west-facing shores, but
remaining below advisory level. A small, short-period north swell is
expected to follow Thursday and Friday, before shifting around to a
north-northeast swell early this coming weekend. Also, short-period
choppy wind waves will return along east-facing shores as the trade
winds gradually strengthen.

For more detailed information on surf for Oahu, see the latest
Collaborative Nearshore Swell and Wind Forecast
(www.prh.noaa.gov/hnl/pages/SRF.php#Discussion).

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

Aviation...Jelsema
Marine...Kinel
Rest...Hui



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