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Hawaiian Islands Synoptic Discussion and Guidance

606
FXHW60 PHFO 241941
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
941 AM HST Fri Mar 24 2017

.SYNOPSIS...
Trades 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 morning. An
area of increased moisture will continue to move from east to west
across the islands today, leading to an uptick shower activity.

&&

.DISCUSSION...
A surface ridge sitting about 275 miles north of Kauai is driving
easterly trade winds across the state. The recent run of rather
dry weather has been interrupted, mainly over windward slopes. A
diffuse area of low level moisture, the remnants of a long ago
dissipated front, has been drawn northward over the islands and
enhanced by a shallow upper level trough passing overhead. The
overnight Hilo sounding revealed no inversion and precipitable
water values of 1.75 inches confined mainly below 13,000 ft. This
has resulted in wet conditions over windward Big Island,
especially the Kau District where a few thunderstorms were
observed this morning. The moisture has spread northward to Oahu,
where mainly a half an inch or less of rainfall has been recorded
along windward slopes.

As the area of moisture moves along the trade wind flow, windward
areas of the smaller islands will experience some increase in
rainfall today, especially across the western end of the state.
The upper level trough is not expected to significantly enhance
the showers over the smaller islands, where leeward areas will
likely only see an increase in afternoon clouds and only a few
showers. However, lingering instability on the leeward slopes of
the Big Island could trigger an isolated thunderstorm this
afternoon. The pocket of low level moisture will move west of the
islands overnight. The surface ridge will remain parked about 275
miles north of Kauai, close enough to keep easterly trade winds
slightly lighter across Kauai and Oahu.

A stable, moderate to locally breezy trade wind flow will prevail
during the weekend. The enhanced low level moisture should move
west of Kauai before day break on Saturday, and the upper level
trough will drift south of the Big Island. As a result, the
inversion is expected strengthen and come back down below 5000 ft.
With no other areas of significant low level moisture seen in the
short term, expect limited shower activity to be confined to
windward slopes and rather dry conditions leeward through at least
Monday.

In the extended, trade winds will hold through next week. The next
area of enhanced moisture could lead to an uptick in windward
rainfall on Tuesday or Wednesday, but confidence in this is low at
this time.

&&

.AVIATION...
Clouds and showers from an old frontal boundary are embedded
within the relatively light trade wind flow. Showers are being
enhanced near the Big Island by an upper trough and may result in
a few thunderstorms through the day. AIRMET Sierra is in effect
for tempo mountain obscurations over windward sections of all
islands except Kauai due to the extensive cloud cover banked
against windward slopes. The AIRMET may extend to Kauai or to
some leeward sections this afternoon depending on if localized sea
breezes in the more trade-wind protected areas produce extensive
cloud build ups.

Turbulence and icing impacts will be minimal and occur in the
vicinity of thunderstorms on the Big Island. Given the easterly to
east southeasterly background flow direction, low level wind shear
may occur during takeoff/approach to the Kahului airport (PHOG)
today. Please see PHOG TAF for more information.

&&

.MARINE...
Winds should gradually strengthen into the weekend. A Small Craft
Advisory (SCA) has been posted for the typically windier waters
around the Big Island and Maui County through at least Sunday
afternoon.

A large west-northwest swell is forecast to arrive Saturday and
peak late Saturday into Sunday. The more exposed areas along the
north and west facing shores of Kauai and Niihau could see High
Surf Warning (25 ft north/20 ft west) conditions, while warning
conditions for the other smaller islands remains borderline.
Island blocking will be the main factor, due to the more westerly
nature of the swell. High end advisory (15 ft+ north/12 ft+ west)
should be expected. The western shores of the Big Island will also
likely see advisory (8 ft) to possible warning (12 ft) level surf
from this swell, starting late Saturday night. The swell will then
gradually subside through the upcoming week.

This large swell combined with the wind waves will produce
combined seas above the SCA threshold (10 ft), and by Saturday
afternoon all waters should see SCA conditions either from seas or
winds.

Another large west-northwest swell is forecast for next weekend.
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 into Friday.

&&

.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.

&&

$$

Public...Wroe
Aviation...Foster
Marine...Eaton

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