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Forecast Discussion for Honolulu, HI

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000
FXHW60 PHFO 171400
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Sat Nov 17 2018

.SYNOPSIS...
Breezy trade winds will persist into early next week. A weakening
frontal boundary moving down across the state will bring numerous
showers and the possibility of a heavy downpour across windward
sections this weekend. In addition, the strong trades will carry
brief showers to leeward sections of the smaller islands through
tonight. Showers are expected to be most active over Kauai, Oahu
and Molokai today, across Maui through this evening, and on the
Big Island from tonight into Sunday. The trades may become
locally windy next Tuesday and Wednesday.

&&

.DISCUSSION...
The tight pressure gradient south of a 1026 mb surface high near
32N 163W, or about 750 miles north-northwest of Lihue, is
producing breezy northeasterly trade winds across the state early
this morning. The high is also pushing a band of low clouds and
showers associated with a weakening frontal boundary down across
western end of the island chain. Mid- and upper-level troughs near
the western islands is causing some atmospheric instability. As a
result, the showers along and ahead of the frontal boundary are
somewhat enhanced. This is resulting in widespread rainfall over
windward and mauka areas, especially on Kauai, Oahu, Molokai and
the windward Big Island. In addition, the strong trades are
transporting brief trade showers over to some leeward sections of
Kauai, Oahu and Molokai. Also, high clouds continue to stream
across most of the islands east of Oahu.

The forecast guidance indicates the surface high will continue to
move east through this weekend. This will push the weakening
frontal boundary over Oahu this morning, and across portions of
Maui County into this evening. The boundary will eventually move
across the Big Island from tonight through Sunday, before moving
south of the Big Island by Monday. The troughs aloft will
gradually propagate eastward and weaken through Sunday. This will
maintain slight instability across the area, which will continue
to enhance the low clouds and showers, especially along the
remnant frontal band. This could result in brief locally heavy
downpours over some windward areas. We do not anticipate
significant flooding, but we will closely monitor radar data and
rain gages in case some areas have localized ponding of water on
roads or brief periods of elevated water levels in streams or
drainage systems. The breezy trades will also continue to
transport a few showers over to leeward sections of some of the
smaller islands through tonight.

As the troughs aloft continue to shift slowly eastward, the high
clouds will eventually move east of the state. In the meantime,
moisture associated with the frontal boundary combined with the
cold pool aloft due to the mid-level trough has the potential to
produce brief periods of winter weather on the Big Island Summits,
mainly on Sunday. At the moment, we do not have significant
accumulations of ice or snow in the forecast.

The surface high will track slowly east to a position far north
of the islands Monday. The low-level east-northeast trade flow
will continue to deliver passing low clouds and showers to
windward areas from Monday through mid-week. A new surface high
building far north of the state will likely cause additional
strengthening of the trades starting Tuesday. This may produce
locally windy trade winds Tuesday and Wednesday. Later next week,
the winds may diminish and veer around to the southeast. In
addition, showers may increase again as an upper-level low
develops west of the islands.

&&

.AVIATION...
High pressure will build northwest of the state resulting in
moderate to fresh trade winds at the low levels over the next
several days. Low clouds and showers will move in with the trades
along a weakening frontal boundary this weekend, resulting in MVFR
conditions and mountain obscurations for windward sides of the
islands. The band of clouds and showers will track west to east
tonight through the weekend. AIRMET Sierra is posted for tempo
mountain obscurations along windward sides of most of the main
Hawaiian Islands through today.

High level clouds cover the central and eastern portions of
Hawaii this morning along the subtropical jet. These high clouds
(FL200-350) will gradually shift eastward as an upper level trough
moves over the islands through the weekend. Strong directional
and speed shear persists over the island chain due to the jet.
AIRMET Tango remains posted for tempo moderate turbulence in
FL250-400 entire area. The upper trough will cause some
instability in the atmosphere resulting in some enhancement in the
showers.

&&

.MARINE...
Trades are forecast to hold in the fresh to strong category
through the weekend as high pressure builds north of the state in
the wake of a dissipating front pushing south across the islands.
Overnight satellite data showed this frontal boundary extending
west-southwest across the Oahu and Kauai waters with wind speeds
up to 25 knots surrounding it. Trades may reach the strong
category across most waters Tuesday through Wednesday with near
gales possible across the typically windier waters between Maui
County and the Big Island. Seas will remain around the advisory
level (10 ft) over most waters into tonight before lowering Sunday
through Monday as the large north-northwest swell lowers. As a
result, a small craft advisory will remain up through tonight
across most waters due to a combo of winds and seas, then across
the windier zones through the day Sunday.

Surf along north and west facing shores will hold around advisory
levels at least through the first half of the day before
gradually lowering through the rest of the weekend as the swell
eases. Observations at the PacIOOS nearshore buoys are remaining
steady in the 9-12 ft range this morning, which is well above
predicted levels. As a result, the high surf advisory has been
extended through 6 pm today. Expect this source to shift out of a
more northerly direction through the weekend as it fades.

A reinforcement out of the northwest is expected to fill in late
Monday, peak Monday night into Tuesday, then fade through midweek.
This source will be from a storm-force low that was depicted in
the Thursday evening analysis over the far northwest Pacific near
the western Aleutians. ASCAT data during this time showed a
decent sized area of strong- to gale-force westerly winds
associated with it focused at the islands within the 300-320
directional band. Guidance remains in decent agreement and shows
this feature tracking eastward to the Date Line today. Surf
associated with this source should remain below the advisory
levels along north and west facing shores as it fills in and peaks
early next week.

Surf along east facing shores will gradually rise over the weekend
and early next week as the trades increase into the fresh to strong
category, especially next Tuesday as trades reach the strong
category across most waters. Limited fetch upstream of the state,
however, should keep the surf below advisory levels for east
facing shores through this time.

Small surf will continue along south facing shores with mainly
background southern Pacific pulses moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening 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 AM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters.

&&

$$

DISCUSSION...Houston
AVIATION...Powell
MARINE...Gibbs

Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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