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Menu of Text Products for the Hawaiian Islands and the Tropical Pacific/Atlantic Oceans:
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Hawaiian Islands Synoptic Discussion and Guidance

572
FXHW60 PHFO 181338
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 AM HST Wed Jan 18 2017

.SYNOPSIS...
High pressure northeast of the state will keep moderate to
locally breezy trade winds in place through tonight. The trades
will ease a bit on Thursday, then become light and variable
Thursday night and Friday as a front approaches from the
northwest. A dry and stable airmass will keep showers limited to
mainly windward and mauka areas through the work week. Big changes
in the weather are in store over the weekend, with a front pushing
southward down the island chain Friday night through Saturday
night, then stalling out near the Big Island through early next
week. The front will bring increasing showers to windward and
mauka areas with some of the showers reaching leeward areas as
well. Additionally, very strong trade winds are expected to
overspread the state this weekend, before easing early next week.

&&

.DISCUSSION...
Currently at the surface, a ridge of high pressure is oriented
east-west to the north of the island chain, with the ridge axis
around 225 miles to the north of Kauai. This is resulting in
moderate to locally breezy trade winds across the state early this
morning. Infrared satellite imagery shows stable stratocumulus
trapped below a sharp low level inversion as evident in the 12z
soundings from PHTO and PHLI, with cloud cover more prevalent in
windward locales. Meanwhile, radar imagery and rain gauge
observations show light showers primarily limited to windward and
mauka areas. Main short term concern revolves around the timing
and size of the incoming west-northwest swell.

Today through Friday,
The high pressure ridge axis north of the state will keep a
moderate to locally breezy trade wind flow in place through
tonight. The trades will ease on Thursday as an approaching front
well northwest of the islands begins to slacken the gradient. As
the front draws closer Thursday night and Friday, the winds will
become light and variable, with local land and sea breezes
expected. A dry and stable airmass is expected to remain in place
through the period as mid-level ridging north of the island keeps
inversion heights suppressed, and precipitable water values remain
near or below 1 inch. As a result, showers will primarily be
limited to windward and mauka areas with rainfall amounts
remaining light.

Winds on the Big Island summits are running just below advisory
levels early this morning, but an increase in 600 mb winds is
expected to push winds back up into the advisory range later
this morning. As a result, the Wind Advisory remains in effect
through 6 pm this evening. Winds at the summits will continue to
flirt with advisory levels into Thursday, so the advisory may
eventually need to be extended in time.

Friday night through next Tuesday,
Big changes are in store for the weekend as a cold front moves
down the island chain, bringing an increase in showers and strong
trade winds to the Aloha state. In fact, the trades will likely be
strong enough to warrant a Wind Advisory over portions and
possibly the entire island chain Saturday through Sunday. The
front is expected to first reach Kauai late Friday night, then
continue to push southward down the island chain through Saturday
night. Showers will be most prevalent across windward and mauka
areas, but with the increasing trades and rising inversion
heights, some of the showers will reach leeward areas of the
smaller islands as well. There remains some uncertainty regarding
the southward push of the front, with indications in the latest
model solutions that it may hang up near or over the Big Island
Sunday into early next week. As a result, will show a drying
trend across the smaller islands Sunday into early next week,
while keeping windward sections of the Big Island and Maui a bit
more unsettled.

&&

.AVIATION...
Locally breezy trade winds will continue to transport low
clouds and isolated showers mainly along windward facing slopes
and shorelines of the islands today. Brief periods of MVFR CIGS /
VIS are possible over windward sections due to these passing low
clouds and showers. Otherwise, VFR conditions are expected to
prevail.

AIRMET Tango remains in effect for TEMPO moderate low-level
turbulence leeward of the mountains on all islands due to the
breezy low-level trade wind flow. This AIRMET will likely
continue into tonight, but may not be necessary on Thursday,
since the trade wind speeds are expected to decrease within the
next 24 hours.

In addition, AIRMET Sierra has recently been issued for MTN
Obscuration over the windward facing higher terrain on Kauai.
Conditions may improve on that island later today, so Sierra
may be cancelled later this morning or this afternoon.

&&

.MARINE...
The Tuesday evening ASCAT pass missed the Hawaiian coastal
waters, but the latest high-resolution model guidance continues
to indicate stronger trade winds over the waters through today as
an area of high pressure races eastward to the north of the
islands. Expect winds of 25 kt or greater across many areas around
Maui County and the Big Island today into this evening, with winds
forecast to decrease somewhat by early Thursday morning. Although
an incoming large west-northwest swell has yet to appear at the
buoys northwest of the state and is now running several hours late,
we still expect this swell to build in during the day with seas
rising above 10 feet across most waters during the afternoon.
Therefore, a Small Craft Advisory remains posted for almost all
waters today and tonight.

Uncertainly with the incoming high surf event remains greater than
normal, due to the late arrival of the swell and continued
disagreement between the higher GFS-based Wavewatch guidance and
lower ECMWF-based wave guidance. With no hard data at this point
showing how large the swell will eventually become, have made no
major changes with the early morning forecast package. Therefore
a High Surf Warning remains posted for exposed north and west
facing shores from Kauai to Maui, while the west shore of Oahu
remains under a High Surf Advisory. Large surf will reach west
facing shores of the Big Island tonight, but with the swell not
yet showing up at the buoys we have held off from issuing an
advisory for that area at this point.

Trade winds will gradually diminish on Thursday and Friday as the
area of high pressure to the north is eroded by an approaching
front. The front is expected to produce strong northeast winds
and large, short-period seas beginning Friday night and continuing
through the weekend, leading to widespread SCA conditions and
local gale warnings. A large, short-period north-northeast swell
will likely produce advisory-level surf along north and east
facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from noon today to 6 AM HST Thursday for
Niihau-Kauai Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory from noon today to 6 AM HST Thursday for
Waianae Coast.

Wind Advisory until 6 PM HST this evening for Big Island Summits.

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

Small Craft Advisory from 6 AM this morning to 6 AM HST Thursday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-
Kaiwi Channel-Maui County Leeward Waters.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay.

&&

$$

DISCUSSION...Jelsema
AVIATION...Houston
MARINE...Jacobson

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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