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Hawaiian Islands Synoptic Discussion and Guidance

528
FXHW60 PHFO 250151
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
351 PM HST Mon Apr 24 2017

.SYNOPSIS...
A ridge north of the islands will maintain moderate east
southeast winds into midweek. Limited shower activity is expected
through Tuesday. From late Tuesday onward, more windward and
mauka showers are expected, with the potential for a period of
unsettled weather this weekend.

&&

.DISCUSSION...
A 1027 mb surface high is centered about 1400 miles east
northeast of Hawaii with a ridge axis passing about 400 miles
north of Kauai. This weather setup is driving moderate east
southeast winds over Hawaii this afternoon.

Water vapor imagery shows an upper level low over Kauai that is
slowly tracking eastward. The subtropical jet along the south and
eastern flank of the low will bring thin cirrus clouds over the
Big Island this evening, then clearing late. Afternoon soundings
shows a dry and stable midlayer of the atmosphere effectively
limiting the influence of the upper low on our weather. Satellite
derived precipitable water show the fetch of dry air extending
400 miles east of the Big Island. Thus, we anticipate the dry and
pleasant weather to continue tonight and through at least Tuesday
morning.

By Tuesday late in the day Tuesday, clouds and showers will begin
to increase, particularly for the windward and mauka areas as they
are funneled in by the east southeast winds. It's possible a few
stray showers could track over the leeward areas of the smaller
islands as well. These clouds and showers will continue streaming
in over windward slopes on Wednesday and Thursday.

A cold front will approach the state from the northwest on Thursday
night. Winds will weaken and briefly shift out of the southeast
ahead of the front on Thursday. The moisture tracking in with the
east southeast flow will form a pre-frontal band over the Big
Island, Maui County and Oahu Thursday night and Friday and begin
to dump heavier rain over these islands. The front will reach
Kauai on Friday with strong north winds filling in behind it.

Forecast details into this weekend are a bit hard to pinpoint as
long range forecast models continue to show differences (significant
in terms of how it affects our weather). Both GFS and ECMWF track
a cold and deep low pressure system towards Hawaii from the north
that would destabilize the atmosphere with a lot of moisture
already in place over the islands. However, location of the
weather features are bouncing in different places model to model
run. In general, we anticipate a wetter than normal and unsettled
weather through the weekend over the islands. We will focus on
more details as the event time draws closer.


&&

.AVIATION...
VFR conditions will dominate through Tuesday under a rather dry
and stable trade wind flow. A low to mid level ridge is generating
an inversion around 5,000 to 6,000 ft, and a surface ridge about
350 nm north of Kauai is producing generally moderate easterly
trade winds. Since low level moisture is limited, expect isolated
MVFR ceilings to be confined to windward terrain, as well as
leeward slopes of the Big Island during afternoon hours.

Border line conditions for turbulence AIRMETs will hold through
Tuesday over some areas. Easterly winds near and just below the
inversion will hover in the 20 to 25 kt range and will produce
some low level turbulence to the west of terrain, especially
around the Big Island and Maui. An upper level trough overhead
will generate diminishing high clouds over the eastern end of the
state into tonight and may produce moderate turbulence aloft,
roughly between FL300 and FL400 along and north of the band of
high clouds. We will closely monitor pilot reports.


&&

.MARINE...
There will be a series of small, short period northwest swells
through Friday with open ocean swell heights of 3 to 4 feet with
periods in the 11 to 13 second period range. Easterly trade winds
will continue to produce short period choppy surf along east facing
shores through Thursday.

A series of small, long period south swells can be expected
through Thursday with open ocean swell heights of 1 to 2 feet
with periods in the 15 to 20 second period range. A slightly
larger, long period south swell is expected to arrive late
Thursday or Thursday night. Open ocean swell heights are expected
to be at least 3 feet. Surf heights may reach High Surf Advisory
levels during the peak of the swell. This swell will linger on
through the weekend. Another moderate size south swell is on tap
for early next week.

A Small Craft Advisory is currently posted for the waters
surrounding The Big Island as winds are generally running in the
20 to 25 kt range. We expect these conditions to continue for the
next couple of days with winds speeds dropping off during the
second half of the week as a front approaches the area from the
northwest.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Wednesday for Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$

DISCUSSION...Foster
AVIATION...Wroe
MARINE...Burke

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