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RH15 %
WindENE 25 mph
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Hawaiian Islands Synoptic Discussion and Guidance

060
FXHW60 PHFO 250651 CCA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Fri Mar 24 2017

.SYNOPSIS...
Trade winds will prevail through the week, with a slight increase
into the locally breezy range expected during the next couple of
days. Trade winds will focus clouds and showers along windward
slopes, especially at night and in the mornings. An area of moisture
will move west of the islands tonight, leading to a decrease in
humidity.

&&

.DISCUSSION...
A surface ridge extending east to west along 28N, to the north of
the main Hawaiian islands, keeps moderate to locally strong
easterly trade winds blowing across local waters this evening.
Remnant moisture from an old frontal boundary has passed west of
the islands for the most part, leaving a typical trade wind
weather pattern of clouds and showers locally. Earlier deep
convection across interior sections of the Big Island has subsided
as evening cooling set in.

Satellite loop shows low clouds lingering mainly across windward
shores and slopes of the smaller islands. Low clouds persist
across all lower slopes on the Big Island. However, we expect Big
Island cloud cover will decrease as the evening wears on,
especially across leeward areas. Radar shows minimal shower
activity, mainly across windward Maui and across local waters 40
miles or more lee of Maui and Lanai.

Models show the ridge will remain north of the islands through the
forecast period, with no areas of enhanced moisture or disturbed
weather expected through the weekend and into early next week.
A 1026 mb high will move west along the ridge tonight and
Saturday, tightening the pressure gradient a bit across the
islands and causing trades to increase Saturday across the
eastern half of the main island chain. Clouds and showers will
favor windward and mauka areas, with leeward areas of the Big
Island expected to cloud up each afternoon and evening, then
clearing each night. Deep troughing forecast during the middle of
next week west of 170W will weaken the pressure gradient across
the islands, causing trade winds to decrease slightly from next
Tuesday onwards. The next area of enhanced moisture could lead to
an uptick in windward rainfall on Tuesday or Wednesday, as ridging
aloft is eroded by a trough skirting by to the north.

&&

.AVIATION...
Lingering clouds and showers will continue to impact mostly the
windward sides of the islands overnight. Increasing trade winds on
Saturday will help to push this moisture out of the area. AIRMET
SIERRA continues for mountain obscuration over the windward sides
of all the islands through 08Z. With clouds starting to thin, will
likely be able to drop the AIRMET for all islands by 08Z or
shortly thereafter.

As trade winds strengthen Saturday, AIRMET TANGO may be issued
for moderate turbulence leeward of the mountains.

&&

.MARINE...
A large incoming west-northwest swell is forecast to arrive at
the far northwest buoys (51001/51101) overnight and near the
islands on Saturday. Surf near Kauai is expected to reach warning
levels (25 ft north/20 ft west) by Saturday afternoon before
peaking Saturday night. Warning level surf should continue through
the day Sunday, before gradually subsiding early next week. The
more westerly direction of the swell will cause Kauai to shadow
the other smaller islands. The current forecast holds surf for
exposed north and west facing shores of Oahu and Maui County just
under warning levels. This will be monitored as observations start
coming in from the northwest buoys, and a slightly larger or more
northwest swell could bring warning level surf to those areas as
well. Surf along west facing shores of the Big Island is also
forecast to reach advisory level (8 ft) and possibly up to warning
level (12 ft) level surf late Saturday into Monday. There
continues to be about a 12 hour timing difference between the
faster Wave Watch III and the slower ECMWF wave model. The current
forecast will continue to follow the faster Wave Watch III, but
we will wait to issue any advisories or warnings until we start to
see the initial forerunners arrive at the northwest buoys late
tonight.

Otherwise, small long period south swells will continue through
this weekend and into next week, with a slightly larger long
period south swell forecast for Wednesday night through Friday.
Another large west-northwest swell will be possible from next
Friday into the following weekend.

A Small Craft Advisory (SCA) is posted for the typically windier
waters around the Big Island and Maui County. High pressure
strengthening northeast of the state will provide locally breezy
conditions through the weekend. The SCA will very likely be
expanded to cover the remaining waters on Saturday, as seas build
above 10 ft due to the large incoming west-northwest swell.

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...M Ballard
MARINE...Jacobson

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