Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 201335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Mon Nov 20 2017

.SYNOPSIS...
Cool northerly breezes will continue today, and then turn toward the
northeast Tuesday and Wednesday. Low clouds will focus along north
and northeast facing slopes and coasts, dropping a few showers that
will be most active during nights and mornings. Gusty trade winds
are expected Thanksgiving and Friday before easing slightly by next
weekend, with windward showers occasionally spreading leeward.

&&

.DISCUSSION...
Currently, the low-level gradient between a surface high to the
distant NW and a low to the NE is supporting light to moderate N
winds. This flow continues to usher a cool air mass over the
islands, with dew points in the mid- to upper-50s. Varying amounts
of low clouds are streaming toward the islands from the N, with
broken to overcast low clouds over the area from Molokai westward,
and much fewer low clouds over the remainder of Maui County and the
Big Island (except for the Hamakua coast where low clouds remain
anchored to the terrain by the wind).

Low clouds near the islands have shown a strong diurnal trend over
the past day or so, with cloud and shower coverage increasing at
night and diminishing during the day in response to cloud-top
cooling/heating. This is expected to continue, and the PoP/Sky
forecast attempts to capture this over the next day or two.

Now that a shortwave moving through the trough aloft near the
islands has moved E of the area, winds over the summits of the Big
Island have steadily diminished overnight, and the High Wind Warning
has been cancelled. However, winds remain sufficiently strong to
warrant a Wind Advisory, and this is in effect through today, with
winds expected to continue weakening. Although a trough aloft is
expected to remain over the area this week, strong winds are not
expected to return to the summits.

Over the next 72 hours, the low to the NE will move gradually move
away to the NE, in response to a new low developing to the distant N
of the islands. The shortwave supporting this new low is not
expected to dig as far S as this weekend`s low, which will allow the
high to the NW to build E slightly. Net result is expected to be a
subtle veering of low-level winds to the NE Tuesday and Wednesday.
ENE winds will increase Thursday and Friday as the high moves E,
increasingly exerting its influence on island winds. As the high
continues to moves E and SE toward the end of the week, E trades are
expected, with a slight decrease in wind speeds possible by next
weekend. Showers will be focused along windward slopes, though some
may periodically spread leeward on the brisk trade winds.

A trough aloft will extend over the area from the NE for the bulk of
the next week, but warming mid-level temperatures (and relatively
low PWAT) should act to limit shower coverage and intensity over the
next couple of days. A shot of cooler air moving into the trough
around Thanksgiving may act to make trade showers more active.

&&

.AVIATION...
Cool northerly winds and unstable cloud bands drifting into Hawaii
will bring periods of showers. The highest coverage for shower
activity will favor mountain areas and the northern slopes of all
islands. VFR conditions will prevail with only brief periods of MVFR
conditions in showers.

No AIRMETs are in effect and none are expected.

&&

.MARINE...
The subsiding process has started for the north swell affecting the
islands. The High Surf Warning for the north shores of Kauai and
Niihau is to be downgraded shortly to a High Surf Advisory. This
advisory will be good to until 6 pm HST this afternoon. The High
Surf Advisory governing the north facing shores of the other islands
will continue through 6 pm HST this afternoon as well. There is the
possibility that this advisory will be extended through tonight. We
will see how this swell trends as the day progresses, but we
anticipate that this swell will dip below advisory criteria for the
north facing shores by 6 pm HST this afternoon. All near-shore buoys
are running around 10 feet significant wave height this past hour.

A Small Craft Advisory (SCA) is in effect for most of the waters
this early Monday morning, mainly due to the large north swell
causing the combined seas to be above the 10-foot limit. The SCA is
good to until 6 pm HST this afternoon, and there is a good chance
that this will happen.

This SCA flag will likely be hoisted again on Wednesday as the next
large open ocean swell rolls in from the north. This new swell is in
the works now as a storm-force low forms in the Gulf of Alaska. Its
size is likely to be a few feet larger than the current fading swell
which maxed out at around 14 feet. It will have a longer period at
14 to 16 seconds. Warning level surf will likely affect the north
facing shores of all islands exposed to this swell well into Friday.

The source of the fading north swell, a low pressure area 800 nm
northeast of the islands, is forecast to move on after today. In its
place, a moderately strong high pressure cell (1032 mb) will settle
in about a thousand nm north-northwest of the islands. This will
result in strengthening trades to moderate to strong across the
Hawaiian Islands area between Tuesday night and Thursday. Therefore,
the SCA will not only pertain to the combined seas from the large
north swell, but also the strong and gusty trade winds. More so, the
trades may be strong enough to warrant gale warnings for some of the
channels between Wednesday night and Friday night. Lastly, the
locally generated trade winds will build a swell large enough to
prompt issuance a high surf advisory for the east facing shores as
early as Thursday. The trades will soften slightly over the upcoming
weekend.

A series of small but long period swells of at least 15 seconds will
arrive this week. These swells will keep the surf from going flat
along the south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Today for North facing shores of
Niihau, Kauai, Oahu, Molokai, Maui and the Big Island.

Wind Advisory until 6 PM HST Today for Big Island Summits.

Small Craft Advisory until 6 PM HST Today for all waters except
Maalaea Bay and areas south and west of the Big Island.

&&

$$

Discussion...Birchard
Aviation...Bohlin
Marine...Lau



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