High Surf in Hawaii, May 13 to 17, 2017

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
333 AM HST Sat May 13 2017

CORRECTED POSITION OF HIGH IN AVIATION SECTION

.SYNOPSIS...
Moderate to locally breezy trade winds will persist through early
next week as strong surface high pressure remains far northeast
of the state. Clouds and showers will favor windward and mauka
areas, but will occasionally spread leeward. Rather wet conditions
are expected for the second half of the weekend and early next
week as an upper trough enhances bands of moisture embedded in the
trade wind flow. A drier weather pattern with decreasing trade
winds is forecast to develop by the middle of next week.

&&

.DISCUSSION...
Overall shower coverage has decreased somewhat during the past
few hours, with showers most abundant at this time over Kauai,
and scattered showers over windward areas of the other islands.
Mainly dry conditions prevail over leeward areas. Rainfall
amounts during the past six hours have been generally light,
with almost all gauges recording less than a quarter inch of
rain during that time period. Surface analysis depicts a 1040 mb
high centered about 1400 miles north-northeast of Honolulu, which
is driving locally breezy trade winds over the state. Aloft,
water vapor imagery shows a well-defined mid/upper level low
centered about 750 miles northwest of Kauai, with a weaker upper
trough to the east of the Big Island. MIMIC-TPW imagery shows a
band of somewhat higher moisture moving over Kauai and passing
southwest of the other islands, with a large area of deep moisture
associated with the upper trough located west of Kauai. The
leading edge of another area of enhanced moisture is located just
northeast of Maui, with a narrow wedge of drier air just ahead of
this feature over the central part of the state and the Big Island
at the moment. The 12Z soundings show little change during the
past 12 hours, with elevated/weak inversions at 9-10K feet and PW
values ranging from 1.14 to 1.32 inches.

Windward showers are likely to increase once again this morning
and remain rather abundant through the day as the next area of
enhanced moisture embedded in the trades moves through. The upper
trough to our northwest will change little during this time, so
conditions aloft will not be especially unstable. Therefore expect
mainly typical trade showers with just a few brief and somewhat
heavier showers possible. Showers will remain focused over
windward/mauka areas, but some will spill into leeward areas,
especially those areas just to the lee of the higher terrain,
as locally breezy trade winds continue.

Showers will probably remain active especially over windward
areas this evening, but may decrease again somewhat late tonight
into early Sunday morning, at least for the smaller islands, as
another slot of drier air moves in.

Sunday through Monday, mid/upper level troughing is forecast to
dig down across the state from the north and east. This will
provide a more unstable airmass with cooling temps aloft. Models
show a couple more bands of enhanced moisture moving through in
the trades during this time, although exact timing remains
uncertain. With a strong surface high remaining anchored over the
Northeast Pacific, expect locally breezy and rather wet trades to
prevail. There is some potential for heavier downpours and perhaps
even isolated thunderstorms during this time period, but models
continue to differ as to the degree of instability aloft we will
experience during this time, and confidence remains too low to
mention heavy rains or thunder in the forecast at this time.

Models are showing a gradual transition back to a drier and more
stable pattern beginning on Tuesday, as the mid/upper trough
weakens and ridging builds in from the west. At this time, longer
range guidance indicates fairly dry trade wind weather prevailing
through the second half of next week, with the trades weakening
somewhat as a weak front passes by to our north.

&&

.AVIATION...
High pressure far to the NE will maintain moderate to locally breezy
northeast to easterly trade winds across the Hawaiian Islands. An
upper level trough over the state will enhance showers and bring
cloud tops to 10-20 KFT. Expect periods of MVFR ceilings with
scattered showers mainly along northern and eastern slopes of all
islands.

AIRMET Tango continues for low level turbulence over mountains and
SW through NW sections of all islands.

&&

.MARINE...
A strong high pressure system remains far northeast of the
islands, and is expected to remain nearly stationary into the
first half of next week. This will maintain fresh to strong trade
winds over the coastal waters. A Small Craft Advisory (SCA)
remains posted for the channels east of Oahu, Maalaea Bay, the
windward Maui County waters, and the waters south of the Big
Island through Sunday, with other zones fairly close to SCA winds.
Anticipate the SCA to be extended into the first part of next
week as the winds will likely not change much, and it may need to
be extended for additional areas. Will be continuing to monitor.

A bump on the south swell is expected today as additional long
period energy is expected to arrive. This should maintain heights
along south facing shores similar to what was seen yesterday. The
incoming swell will likely wrap to some west facing shores.
Additional impulses along the south shore are possible during the
end of next week. The trade wind swell will continue to bring
surf near advisory levels to the east facing shores into early
next week. Will continue to monitor the need for an advisory.

Surf along the north facing shores will remain small through the
start of the new week. A gale east of Japan is expected to track
to the northeast, and then stall near the western Aleutian
islands. This could generate a small northwest swell that would
reach the islands during the second half of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Monday for Kaiwi Channel-
Maui County Windward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

DISCUSSION...Jacobson
AVIATION...Bohlin
MARINE...M Ballard