High Wind in Hawaii, December 6, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
849 PM HST Tue Dec 5 2017

.SYNOPSIS...
Strong northerly winds will rapidly diminish overnight and Wednesday
as a ridge of high pressure approaches the islands. The ridge will
settle over the islands by Wednesday night, and remain through
Friday, leading to light winds, cool weather, and only a slight
chance of a light sprinkle. Another cold front is expected on
Saturday, bringing a brief period of showers, increasing winds, and
a reinforcing shot of cool dry air. Winds will diminish Sunday and
Monday as mostly dry weather persists.

&&

.DISCUSSION...
N to NNE Winds are rapidly diminishing this evening as a surface
ridge of high pressure is approaching from the NW. Strong cold air
advection is bringing surface dew points in the upper 40's, making
it feel quite chilly by island standards. While stable low clouds
moving over the islands (primarily focusing over windward slopes and
coasts) have increased somewhat since this afternoon, local radars
are in clear air mode and precipitation is completely absent. PoPs
were reduced for tonight and early Wednesday, as measurable precip
is not expected.

A Wind Advisory is in effect until 10 PM this evening for the entire
state. While winds have diminished below advisory-levels in most
areas this evening, winds on Lanai, Kahoolawe, and portions of the
Big Island have been at or above advisory levels, with sustained
winds near 30 mph. With winds expected to continue diminishing, the
plan is to allow the Wind Advisory to expire, and a forecast update
will follow soon thereafter.

&&

.PREV DISCUSSION ISSUED 402 PM HST...
A dry, stable post-frontal airmass has overspread the islands this
afternoon. The 00z soundings and the UW-CIMSS MIMIC layer
precipitable water (PW) product show PWs between 0.5 and 0.75 inch
statewide, well below normal values. Sea level dewpoints statewide
are in the upper 40s to mid 50s, giving a little chill to those
strong northerlies.

The winds have exceeded advisory levels at quite a few gauges this
afternoon. Since the winds are more northerly than usual around
the Big Island, places like Hilo, Puna, and Keahole Point/PHKO on
the Big Island are actually getting those impact level winds--
whereas from more typical trade wind directions, those spots are
not usually exposed to the strongest winds and associated impacts.
We do expect the winds to settle down quickly overnight as a
ridge of high pressure behind the front will push S toward the
islands.

The ridge will settle over the islands Wed and Wed night, with
gentle northerlies giving way to mostly sea and land breezes. The
airmass over us will be quite dry with PWs from 0.5-0.9 inch, so
humidity will not be a problem, and showers--if any--will be very
few and far between. With the light winds, Wed night and possibly
Thursday night should be quite cool indeed, with lows in the 50s
at some sea level stations. The ridge will remain over the islands
with quiet weather through Friday with lots of sunshine except
for a few afternoon interior clouds and maybe a sprinkle or two.

Models are in good agreement in the medium range with the synoptic
pattern repeating itself starting on Saturday. Another mostly dry
cold front will push in with a reinforcing shot of windy, cool
dry air. The following surface ridge will build over the islands
with light winds on Sunday. The ridge quickly moves S of the
islands on Monday and the winds turn to SW or W ahead of the next
front.

&&

.AVIATION...
High pressure aloft and a dry stable environment will keep
prevailing VFR conditions through Wednesday. Strong surface winds
overnight will weaken overnight, these winds will remain strong
enough to produce moderate mechanical turbulence along downwind of
mountain ranges overnight with AIRMET TANGO continuing until
Wednesday morning. Dry and stable air aloft will minimize any shower
activity.

&&

.MARINE...
Strong north-northeast winds will quickly taper off tonight as the
ridge axis nears the islands. The Gale Warning will be lowered to a
Small Craft Advisory (SCA) for the Alenuihaha Channel, Leeward Maui
Waters and Maalaea Bay with the SCA remaining in effect for all
other coastal waters. The SCA for all areas will go through the day
Wednesday for a combination of swells from the north northwest and
northeast.

A large north northwest swell will peak around Wednesday morning,
bringing warning level surf to exposed north facing shores, as well
as advisory level surf to exposed west facing shores. See the
Coastal Hazard Message (CFWHFO) on our website for more specific
information for what is in effect at each coastline. East facing
shores will be exposed to a short period northeast swell that will
bring advisory-level surf tonight and diminish by morning.

Large surf associated with the aforementioned swells coinciding with
strong northerly winds, peak monthly tides, and beach erosion from
recent large surf events could translate to some overwash onto
vulnerable roadways. The best chances for overwash will be centered
around the pre-dawn high tide cycle on Wednesday.

The north northwest swell will slowly trend down through the end of
the week, but advisory surf heights and SCA may still continue into
Thursday or Thursday night for some north and west facing shores.
The forecast then calls for an even larger north northwest swell to
start building Saturday night.

Small long period swells will keep surf above flat for the south
facing shores this week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Wednesday for N and W facing shores
of Niihau and Kauai, and N facing shores of Oahu, Molokai, Maui, and
the Big Island.

High Surf Advisory until 6 PM Wednesday for W facing shores of Oahu
and Molokai, and for E facing shores of Kauai, Oahu, Molokai, Maui,
and the Big Island.

Small Craft Advisory until 6 PM HST Wednesday for all waters.

&&

DISCUSSION...Birchard
AVIATION...Bohlin
MARINE...TS