Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 280709

Area Forecast Discussion
National Weather Service Honolulu HI
909 PM HST Mon Mar 27 2017

A surface ridge north of the state is expected to maintain
moderate east to east-southeast flow across the islands through
late Thursday. An upper level trough will dig east of the island
chain Tuesday night, and will bring an increase in showers over
windward facing slopes of the state during the second half of the
work week. A band of clouds and showers ahead of a front may
impact parts of the state Thursday night and Friday, but the front
is not expected to reach the islands. A surface high is forecast
to build northeast of the area this weekend, which will produce
breezy trades starting Saturday.


A nearly stationary 1031 mb surface high is centered nearly 1800
miles east-northeast of Honolulu early this evening. A surface
ridge extends west-southwest from the high through a point about
280 miles north of Lihue. The pressure gradient south and
southwest of these features is maintaining light to moderate east
to east-southeast winds across the state. The weaker background
surface winds, which are mainly over the western end of the state,
are allowing local land breezes to become established over some of
the islands. especially parts of Kauai and Oahu. Also, a mid-
tropospheric ridge, which remains nearly stationary above the main
Hawaiian islands, continues to provide stable atmospheric
conditions across the region. Satellite imagery shows low clouds
over some parts of the state early this evening. These clouds
appear to be tinning over many areas, except over parts of Oahu,
where low clouds and showers continue along the Koolau and Waianae
mountain ranges. Additonal low clouds and showers are over east
Molokai and east Maui, as well as near the eastern Puna District
on the Big Island.

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 winds across the state. The mid-
tropospheric ridge will also keep the atmosphere stable through
Tuesday night, so rainfall will remain light. However, the
forecast models show an increase in precipitation starting early
Wednesday morning as an upper level trough digs in east of the
state Tuesday night and Wednesday. The presence of this trough
aloft east of the area will potentially enhance the low clouds
and precipitation tracking in from upstream. Therefore, expect
slightly wetter trade wind conditions during mid-week. The low-
level flow will focus most of the low clouds and showers over
windward and southeast facing slopes. Leeward areas may also have
brief showers.

The western end of the surface ridge to our north is expected to
erode by late 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 show a band of clouds and showers forming over the
western end of the state southeast of the front, which might
produce additional wet weather over parts of the state late
Thursday and Friday.

An upper level ridge will likely to build over the state from
late Friday through this weekend. This will result in more stable
atmospheric conditions. Also, a surface high is expected to build
northeast of the region this weekend, which will likely produce
breezy conditions across the Hawaiian Islands. We also expect a
drier air mass over most of the state this weekend, so typical
windward and mauka showers will be brought in by the trades.


High pressure northeast of the state will keep a light to
moderate trade wind flow in place through Tuesday. Convective
showers over leeward areas resulting from daytime heating should
end over the next hour or two, with showers then favoring
windward and mauka areas overnight. Localized sea breezes are
expected to develop on Tuesday, and this will lead to convective
showers over leeward areas once again. Brief MVFR cigs/vsbys will
be possible in showers, but predominantly VFR conditions are
expected over the next 24 hours.

No AIRMETs are currently in effect.


The west-northwest (290-310 deg) swell that peaked Sunday into
Monday will slowly ease into Tuesday, then hold through mid week
due to a slight reinforcement Tuesday night 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 overnight and possibly into Tuesday across windward
waters exposed to northwest swells and over the waters south of
the Big Island. As a result, the Small Craft Advisory will remain
up overnight across these areas for seas. Winds will likely remain
below advisory levels through Friday as the ridge of high pressure
north of the state slightly weakens in response to a passing cold
front well north of the area. Another cold front is forecast to
approach the region later in the week, then stall and weaken north
and northwest of the area into the upcoming weekend. Advisory-
level winds will likely return across portions of the waters over
the weekend as high pressure builds north of the state.

Surf along east facing shores will remain small and choppy through
the week due to moderate to fresh onshore winds persisting. A slight
increase will be possible over the weekend as the winds strengthen
locally and upstream of the islands.

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. In
addition to this southerly swell, another large west-northwest
swell(discussed below) expected by Friday could impact the southern
shores once again at select spots due to wrapping around the

Later in the week, the main forecast challenge will become centered
on another, potentially larger, west-northwest (290-310 deg)
swell that will impact the islands Thursday night through the
weekend. A powerful storm-force low that may have briefly reached
hurricane force earlier this morning off the coast of Japan is
currently tracking east-northeastward across the northwest
Pacific. The latest surface analysis showed this storm-force low
centered just under 1000 nm east of Tokyo with a central pressure
of 988 mb. An ASCAT pass earlier today reflected this system and
showed a large batch of gale to storm force winds (34-50 kt) out
of the west focused toward the region. Model consensus remains in
decent agreement over the upcoming few days and depicts this
system continuing to deepen to around 970 mb as it approaches and
moves across the Date Line Tuesday night into Wednesday. The
westerly wind field associated with this system is forecast to
expand through this period and remain at gale to storm force
levels over a large area with seas reaching as high 30 to 40 ft. A
large west-northwest swell associated with this system will
result across the region, that could reach the islands as early as
Thursday night, peak through the day Friday, then slowly ease over
the upcoming weekend. Some uncertainty regarding the details this
far out still exists due to some model differences depicted
between the ECMWF- Wave and GFS- WAVEWATCH III solutions.
Regardless of these subtle differences, warning-level surf and
advisory-level seas will be likely Friday into the weekend before
trending down. More details will follow in later packages this
week with regard to specifics on arrival times and impacts.


High Surf Advisory until 6 AM HST Tuesday for north and west
facing shores of Niihau, Kauai, Oahu and Molokai, north facing
shores of Maui, and west facing shores of the Big Island.

Small Craft Advisory until 6 AM HST Tuesday for Oahu Windward
Waters-Kaiwi Channel-Maui County Windward Waters-Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.



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