Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 290202

Area Forecast Discussion
National Weather Service Honolulu HI
402 PM HST Tue Mar 28 2017

A surface ridge north of the state will maintain light to
moderate east to east-southeast surface winds across the islands
through Friday. An upper level trough, which will dig east of the
island chain later tonight and Wednesday, is expected to bring an
increase in showers early Wednesday morning through Saturday. A
surface high building far northeast of the area starting late
Friday will produce breezy trades this weekend.


A nearly stationary 1031 mb surface high is centered far east-
northeast of Hawaii this afternoon. A surface ridge extends west-
southwest from this high passing about 300 miles north of Kauai.
The pressure gradient south and southwest of these features is
maintaining light to moderate east to east-southeast wind flow
across the state. Water vapor imagery shows a shortwave trough
near Oahu this afternoon digging towards just east of the state.
Afternoon soundings at Hilo and Lihue show dry and stable
midlevels, strong inversions and below normal precipitable water
values of 0.8 to 1 inch. Radar shows that much of the showers
from this morning have dissipated this afternoon. Due to the light
wind flow over the state, sea breezes have resulted in cloud build
ups over the leeward sides of the islands this afternoon. These
clouds should gradually dissipate this evening.

Water vapor imagery shows a shortwave trough currently passing
north of Oahu, and is expected to merge into a much stronger
trough aloft east of the state by Wednesday morning before slowly
tracking away to the east through this weekend.

The surface ridge is expected to remain north of the islands through
Thursday night, which will continue to drive the light to moderate
east to east-southeast flow. With trade winds upstream, models
show an increase in showers expected to push in from the east
early Wednesday morning. The presence of this trough aloft east of
the area will enhance the low clouds and precipitation tracking in
from upstream through Saturday. The light winds will also continue
on Friday.

Therefore, expect wetter trade wind type weather after tonight
with the relatively weak low level flow focusing most of these low
clouds and showers over windward and southeast facing slopes.
While leeward areas should stay relatively dry at night, sea breezes
will continue to dominate over the next several days and produce
cloud build ups and showers. Given the relative atmospheric stability
over Hawaii, showers enhanced by the upstream trough will likely
be loosing their intensity as they reach the islands. Therefore,
while it will likely be wetter than it has been, we do not
anticipate a flooding-type situation.

The surface ridge to our north is expected to erode some time on
Thursday, as a cold front approaches from the northwest. This
front will likely stall and weaken two to three hundred miles
north-northwest of Kauai by late Friday, so it is not expected to
ever reach the state. However, the forecast models continue to
suggest a band of clouds and showers will form near the western
end of the state to the southeast of the front. These low clouds
and showers might produce moderate rains over parts of the state,
particularly the western islands, late Thursday and Friday.

Surface high pressure is expected to build far northeast of the region
this weekend, which will likely produce breezy trade winds across
the Hawaiian Islands beginning Friday night through early next
week. Clouds and showers will continue on Saturday, but a slightly
drier air mass is expected over most of the state beginning on


AIRMET SIERRA for mountain obscuration was recently cancelled as
lingering clouds and showers have continued to thin after a brief
uptick in coverage. The Big Island has the most clouds overhead
this afternoon, but expect clouds to thin overnight. The clouds
are topping out around 9000 ft. The north to south oriented jet
forecast to be over the islands this afternoon has not been as
strong as the models indicated, so have not issued a SIGMET. Will
continue to monitor the situation.


The west-northwest (290-310 deg) swell that peaked Sunday into
Monday will hold through mid week due to a slight reinforcement
tonight into Wednesday. A combination of this swell and moderate
to fresh breezes over the channels and waters around the Big
Island will continue to generate rough boating conditions.
Although the winds have dropped slightly below Small Craft
Advisory levels (25 kt), seas will hold around 10 ft through the
day today across windward waters between the Big Island and Oahu.
As a result, the Small Craft Advisory will remain up across these
areas for seas today. Surf associated with the west-northwest
swell should remain just below advisory levels along exposed
north and west facing shores through mid week.

Winds should remain below advisory levels through Friday as the
ridge of high pressure north of the state slightly weakens in
response to a two weak cold fronts passing north of the area.
Advisory- level winds will likely return across portions of the
waters over the weekend as high pressure builds north of the state
behind the second front.

Surf along east facing shores will remain small and choppy through
the week due to persistent moderate to fresh onshore winds, but
increase over the weekend as winds strengthen again locally. A
slight increase in surf along south facing shores will be possible
Thursday through Saturday due to a long-period swell (190 deg)
associated with recent activity across the southern Pacific.

The main forecast challenge will be centered on another, slightly
larger, west-northwest (290-310 deg) swell that will impact the
islands Thursday night through the weekend. This is due to a powerful
storm-force low currently tracking east-northeastward across the
northwest Pacific. Recent ASCAT and altimeter obs support latest model
model runs with regard to wind speeds and resulting sea heights.
The large west-northwest swell associated with this system could
reach the islands as early as Thursday night, peak through the day
Friday and Friday night, then slowly ease over the upcoming
weekend. Warning-level surf and advisory-level seas are expected
Friday into the weekend before trending down through the beginning
of next week. The southern shores could see impacts from this
swell again at select spots due to wrapping around the islands.





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