Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 162010

Area Forecast Discussion
National Weather Service Honolulu HI
1010 AM HST Fri Nov 16 2018

High pressure building far north of the islands will bring
strengthening northeast trade winds this weekend, with breezy trade
winds continuing into next week. The high will push a weakening
frontal boundary over the islands this weekend, while an upper-level
trough brings some instability. An increase in windward showers is
expected, with showers most active over Kauai tonight, Oahu and Maui
county on Saturday, and on the Big Island Saturday night and Sunday.
Fewer windward showers are expected early next week.


A surface high building N of the area the next couple of days will
push a dissipating frontal boundary over the islands from the N and
NE, which is expected to bring an increase in windward showers while
also bringing strengthening NE trade winds. An upper trough axis
currently near Kauai will gradually slide E and weaken the next
several days, taking the layered mid- and high-level clouds of the
last couple days with it. In the meantime, instability associated
with the upper trough has brought isolated thunderstorms to waters
around Kauai this morning, and a slight chance of thunderstorms has
been included today for windward portions of Kauai and Oahu and
adjacent waters.

Latest GOES-17 visible imagery shows limited low-level moisture
immediately upstream of most islands, except for scattered to broken
showery low clouds near and NE of Kauai. This moisture is associated
with an ENE-WSW oriented diffuse band of low clouds, marking a
weakening frontal boundary associated with a surface low near the
Gulf of Alaska. While predominantly comprised of shallow low clouds
near the islands, a few thunderstorms developed along this boundary
near the islands early this morning. Stronger NE winds and slightly
cooler air lie N of the boundary, which will be moving down the
island chain through Sunday night.

Latest models indicate the boundary will be over Kauai tonight,
spreading to Oahu and portions of Maui county on Saturday as the
high currently to the NW moves to a position N of the islands. The
boundary will spread to the Big Island Saturday night through Sunday
night, moving S of the Big Island by Monday. Although the upper
trough will progressively move E through the weekend, the mid-level
reflection (in the form of a weak closed low at 500 mb) will linger
SW of the islands. The associated mid-level cold pool and trough
will extend over the islands, bringing modest instability. The
increasing NE winds will ensure that most rainfall will occur along
windward slopes and coasts, with showers increasing in coverage as
the boundary moves across (and the instability may allow for a few
brief downpours).

The mid-level trough will dissipate early next week as the surface
high tracks slowly E well to the N of the islands. Locally breezy
ENE trade winds will deliver passing clouds and showers to windward
areas. Winds may diminish and veer to the SE by the end of the week,
and showers may increase again as another low aloft develops nearby.


High pressure will build northwest of the state resulting in
strengthening trade winds today. High level clouds along the
subtropical jet will gradually shift eastward as an upper level
trough moves over the islands. These high clouds range from FL200-
350. Strong directional and speed shear persists over the island
chain at these heights. AIRMET Tango remains posted for tempo
moderate turbulence in FL200-350 entire area.

The atmosphere will become more unstable as the upper trough
approaches. Combined with increasing trade winds, this will cause
showers to become more frequent over windward sections, with areas
of MVFR conditions and mountain obscuration possible. AIRMET SIERRA
may be required for mountain obscuration later today and/or tonight.


Trade winds will continue to strengthen today through Saturday as
high pressure builds in far north northwest of the area. A Small
Craft Advisory is in effect over the typically windy waters around
Maui County and the Big Island. These strong winds are expected to
continue into early next week. A new northwest swell is producing
seas above the 10 foot advisory threshold over exposed waters around
Kauai, Oahu and Maui County. Thus a Small Craft Advisory is in
effect for those waters as well.

A High Surf Advisory is in effect for shores exposed to the new
northwest swell. This swell is expected to peak today, then lower
gradually tonight and Saturday while swinging around to a more
northerly directions. Another small to moderate size northwest swell
is expected to gradually build on Monday, peak Monday night and
early Tuesday, then lower gradually into Wednesday. Small, mainly
background south swells are expected through the weekend and on into
the middle of next week. An uptick in short period choppy surf can
be expected along east facing shores beginning Friday as the trade
winds strengthen.

An upper level trough centered just southwest of the state will
allow for a slight chance of thunderstorms today over waters around
Kauai and Oahu.


High Surf Advisory until 6 AM HST Saturday for North and West facing
shores of Niihau, Kauai, Oahu, and Molokai, and for North facing
shores of Maui.

Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Kaiwi Channel-Maui County Windward Waters.



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