Area Forecast Discussion
Issued by NWS Honolulu, HI

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914
FXHW60 PHFO 200615
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
815 PM HST Thu Oct 19 2017

.SYNOPSIS...
Strong and gusty trades will continue due to a surface high
located far north of the islands. The winds are expected to start
trending down Friday night as the high begins to weaken as it
moves southeastward. Winds will continue to decrease through the
weekend, with light southeast to south winds expected on Sunday.
Clouds and showers will remain focused over windward and mauka
areas into this weekend, with the trades carrying some brief
showers to leeward sections of the smaller islands through Friday
night. Southerly winds ahead of a potentially vigorous front may
bring much wetter weather to parts of the state early next week.

&&

.DISCUSSION...
A 1030 mb surface high near 33N 156W, or about 850 miles north of
Honolulu, is moving toward the southeast at about 10 mph.
Elsewhere, a north-northeast to south-southwest oriented surface
trough about 800 miles east of Hilo is moving slowly westward.
The tight pressure gradient between these features is maintaining
very strong and gusty trade winds across that main Hawaiian
Islands, especially the eastern end of the state, early this
evening. Therefore, a Wind Advisory remains in effect through
Friday afternoon for Lanai, Kahoolawe, and the Kohala section of
the Big Island. We are also monitoring observations on Mount
Haleakala where windy conditions have also been occurring during
the past 24 to 36 hours. However, winds continue to remain below
the summit wind advisory criteria.

Loops of satellite imagery continue to show scattered to broken
low clouds streaming into the islands in the robust low-level
trade wind flow. Radar reflectivities indicate numerous showers
are embedded within many of these clouds. Most of the showers are
impacting the windward facing slopes and higher terrain on the
islands. However, the strong low-level trade wind flow continues
to transport some of this precipitation over to the leeward
sections of some of the smaller islands. Clouds that developed
over the leeward Big Island are dissipating as nighttime cooling
occurs and local drainage flow develops. Note that although there
are frequent showers over many areas of the state, rainfall totals
are relatively light.

By late Friday and Saturday, the surface high is expected to
weaken as it shifts to a position far northeast of the islands. As
a result, the trade winds will begin to become lighter as we head
into the weekend. A potentially vigorous surface front
approaching the islands from the northwest will cause the winds to
shift out of the southeast and south by Sunday. The southerly
flow will transport additional moisture into the state from late
Sunday into early next week. In addition, volcanic haze (or
"vog") will likely be transported up over most of the state from
sources on the Big Island.

In addition to the front, a sharpening upper level trough will
move down toward the islands. These features will act to
destabilize the atmosphere, so the enhanced moisture will be
available for widespread rainfall. Some of the rain could be
heavy, so we can not rule out the possibility of flooding early
next week. Unfortunately, the forecast models have been changing
significantly from run to run, so we are unable to determine the
exact details of this event. For example, 24 hours ago the
guidance suggested the surface winds would be light in advance of
the front. Now, there are indications that strong south to
southwest surface winds may develop over parts of the state early
next week ahead of this front. As a result, future model runs
will need to be monitored for further details.

&&

.AVIATION...
Trade winds will remain strong and gusty into Friday. Thus, AIRMET
Tango will remain in effect through at least 210400 UTC.

Satellite imagery shows a field of loose cumulus based clouds
upwind of the islands between 150 to 200 nm. The strong trades will
be pushing these showers ashore of all the islands through out the
night. And thanks to the firm trades, many of these showers will
reach the lee districts of the smaller islands. Brief MVFR ceiling
and vis are expected with these showers. Based on surface obs, the
ceilings associated with these showers are mainly above the MVFR
value of 3k feet. Thus AIRMET Sierra is not necessary at this
time. The tops of these showers are around 9k feet.

&&

.MARINE...
A dynamic weather pattern is expected over the next several days,
with the current strong to gale force trade winds diminishing
steadily over the weekend. Light and variable winds are expected by
Sunday, as the high to the N of the area weakens and moves E, and a
low develops in the same area. This low will send a front toward the
islands early next week, with low confidence as to what impact the
front will have on the local wind and weather. Latest GFS guidance
indicates the potential for light to moderate S to SW winds as the
front approaches Monday, and confidence decreases further thereafter
as a deep-layer cut off low develops N or NW of the islands. There
is the potential for deep southerly flow over the area, which could
fuel heavy showers next week.

With high pressure currently fueling strong trade winds, a Gale
Warning remains in effect through Friday for the Pailolo and
Alenuihaha Channels, while a Small Craft Advisory (SCA) remains
posted elsewhere. The diminishing winds will likely allow the Gale
Warning to expire Friday night, and the SCA will likely be lowered
for all areas early this weekend. Although short-period wind waves
will be easing, building NNW and S swells Sunday and Monday are
expected to keep combined seas elevated, but below the SCA criteria
of 10 feet.

A High Surf Advisory for E facing shores is posted through Friday,
with surf expected to diminish below advisory levels by the weekend.
A relatively small NNW swell will build Friday and lower Saturday,
with a larger swell on its heels for Sunday and Monday. This second
swell will likely result in advisory-level surf along N (and
possibly W) facing shores - the first High Surf Advisory of the
"winter" surf season. This swell will diminish Monday, with the
potential for a larger NW swell Tuesday and Wednesday. Additionally,
another advisory-level S swell is possible Sunday and Monday. A
small long-period W swell is possible for much of next week,
generated by large and slow-moving Typhoon Lan in the W Pacific. See
the Oahu Surf Discussion (SRDHFO) for details on the swell sources
affecting the islands.

&&

.FIRE WEATHER...
The KBDI remains elevated, but below the 600 threshold. Strong
and gusty winds with low relative humidities are also expected
Friday afternoon. With minimal rainfall expected at HNL during
the next couple of days, we expect the KBDI to continue to slowly
climb. The gusty winds and low afternoon relative humidity values
are sufficient for a Red Flag Warning, but the limiting factor is
the below threshold KBDI. Note that some leeward areas may still
be experiencing critically dry conditions. The gusty trade winds
should decrease on Saturday. We also expect gradual improvement
later this weekend as the winds continue to weaken and relative
humidities increase.

&&

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

Wind Advisory until 6 PM HST Friday for Lanai-Kahoolawe-Big
Island North and East-Kohala.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Maalaea Bay-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

Gale Warning until 6 PM HST Friday for the Pailolo and
Alenuihaha Channels.

&&

$$

Discussion/Fire Weather...Houston
Aviation...Lau
Marine...Birchard



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