Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 230139
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 PM HST Mon May 22 2017

.SYNOPSIS...
Trades will increase through Tuesday as a surface high passes far
north of the islands. Showers embedded in the trades may be
enhanced as a weak upper trough moves across the state tonight
through Tuesday. Trades will weaken during the latter half of the
week, becoming light and variable by the start of the holiday
weekend as a surface trough develops near the islands.

&&

.DISCUSSION...
Trade winds across local waters have begun to increase as a 1037
MB high far north northwest of the main Hawaiian Islands
continues to strengthen and drift eastward. Satellite loop shows a
stationary frontal cloud band about 110 miles northwest of Kauai
as well as patchy broken low clouds embedded in the trades moving
across windward areas of most of the islands. A veil of high
clouds is streaming eastward across the Big island and adjacent
waters. Afternoon soundings show our airmass remains rather dry
and stable, with neutral to positive LI values and PW near 1.2
inches.

As the high far northwest of the islands strengthens and passes
eastward through Tuesday, trades will increase across the area,
becoming locally windy later tonight through Tuesday. Stronger
trades are expected for the next couple of days, with passing low
clouds and showers expected to favor windward and mauka areas.
Winds will be strong enough to carry low clouds and showers
leeward at times. A weak upper trough is expected to pass over the
state tonight through Tuesday which will destabilize our airmass
a bit and enhance trade showers. However, models do not move in
much additional low level moisture for a few days and we do not
expect widespread heavy rainfall.

Trades will gradually decrease toward the later part of the week
as models indicate a sharp upper level trough will move in from
the northwest, inducing a weak surface trough near the islands by
the start of the holiday weekend. This will allow daytime sea
breezes to develop by Thursday or Friday. Moisture from the old
frontal cloud band to our north may also spread to the northern
islands late this week, fueling more widespread showers. This
wetter weather may linger in the area through the Memorial Day
weekend.

&&

.AVIATION...
Moderate to strong northeasterly trade winds will remain in the
forecast this week. Moisture associated with an old frontal
boundary north of the islands will slowly drift into the state
after midweek. Expect moderate low level turbulence over all
mountains and south through west sections of all islands.
Otherwise isolated MVFR conditions are possible in showers.

AIRMET Sierra in effect for mountain obscurations over north
through east sections of the Big Island as cloud cover and showers
continue to move in with the trade winds.

AIRMET Tango in effect for low level turbulence over all mountains
and downwind south through west sections all islands.

&&

.MARINE...
High pressure continues to strengthen across the area, increasing
the trade winds along with it. This trend will continue over the
next day or so, with strong east to northeast winds expanding
westward as well. Therefore, the small craft advisory (SCA)
remains in effect for the typically windier zones around Maui
County and the Big Island through Wednesday, but waters around
Oahu and leeward Kauai have been added to the product as well.
Winds will begin to weaken by midweek.

The current south swell will continue to 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 the 8-foot 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.

A moderate northwest swell arrived earlier this morning. It is
forecast to peak tonight 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 Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-
Kaiwi Channel-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big
Island Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Bohlin
MARINE...Stall



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