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Temp-0.6 C
RH26 %
WindE 4 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

179
FXHW60 PHFO 221408
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
408 AM HST Sun Jan 22 2017

.SYNOPSIS...
A strong high pressure north of the state will bring strong,
gusty and potentially damaging trade winds this morning. Winds
will gradually weaken beginning later today through Monday. A
front will stall and weaken near the Big Island through Monday,
while the trades focus showers over the windward Big Island
slopes. Moderate trade wind weather will persist through the
week.

&&

.DISCUSSION...
Shortwave infrared satellite shows a slowing front near the Big
Island this morning. Clouds and showers associated with this front
is pushing over windward sections of Maui and the Big Island, with
drier and more stable air across the rest of the state. A strong
surface high far north of the state is driving the current strong
trade winds and gusts. Surface observations around the state
currently report sustained winds in the 20 to 40 mph range with
gusts to 60 mph in exposed areas and downwind of the mountain
ranges. A High Wind Warning has been extended for the entire
state (except for Big Island summits and Haleakala summit) through
noon today. Models agree with an expected down trend in wind
speeds by this afternoon, however, some particularly windy areas
of the state will likely see this High Wind Warning extended
this afternoon, with other areas will likely see a downgrade to a
Wind Advisory. We will monitor the next model runs and wind
observations through the rest of the morning to determine if, and
where, a split may be necessary. Either way, we are anticipating
windy conditions to continue today, just not quite as strong as
Saturday / Saturday night.

An upper level trough with an axis about 350 miles west of Kauai
this morning will track east over the area today and tonight.
Cirrus clouds just east of the trough axis are already over Kauai
and should push east over the rest of the islands as the trough
approaches. The upper trough will raise the inversions today and
induce a subtle surface trough over the state on Monday which
will help weaken the trade winds. More importantly, the high
pressure system to the north will quickly move east today and
begin to track away from us. Aside from the winds, expect the
showers over Big Island to linger today through Monday night,
with drier conditions for the smaller islands. However,
stratocumulusclouds associated with the high will bring partly
sunny to mostly cloudy conditions for the smaller islands over the
next day or so, though not much rain is expected from these
clouds.

The trade winds will continue dropping on Monday as the high (now
to the northeast) weakens as a front passes north of our area.

From Tuesday to the end of the week, a broad high pressure system
will set up far to our northeast with a ridge passing north of
Hawaii. This setup with drive generally moderate trade winds with
occasional windward and mauka showers moving through. The
atmosphere should remain stable through this period. In the long
range, models are hinting at another weak cold front reaching the
state next weekend.


&&

.AVIATION...
Somewhat of a diffused front lies over northern Big Island this
hour...moving south at around 10 mph. Remnant moisture lags
behind still draped over windward areas of Maui and Molokai. Lanai
has cleared out and is now being sheltered by Maui. AIRMET SIERRA
for Molokai and Maui will continue until they clear out which can
happen before 16z, the next scheduled issuance.

AIRMET SIERRA covering parts of windward Big Island will also
stay in effect beyond 16z, and perhaps for most of the today as
the front makes it way southward across the Big island. With this
said, AIRMET SIErRa may need to be expanded to cover the Kau
district perhaps as early as 19z. The cloud tops stretched from 7k
feet on Kauai to 11k feet on the Big Island.

A scattered to broken layer of stable stratocumulus (SC) clouds
lies over Oahu and Kauai, affecting their windward sections.
A very large field of SC is noted on satellite slowly approaching
the islands from the northeast at near 10 kt. The base of this
field is presumably VFR at around 5k feet. So, AIRMET SIERRA is
not needed at this time but we will be monitoring closely. Some
light showers or drizzle are expected from these clouds.

Besides the low level turbulence due to the very strong and gusty
winds, there is an inversion present across the island chain
sloping from 65 hundred ft on Kauai to 11k feet over the Big Island.
It is also quite strong over Kauai with almost a 4 degree C
difference within a thousand feet. This thermal inversion is strong
enough to cause some light to mod turbulence. Therefore, the
turbulence layer was increased to 10k feet.

Also, we are monitoring for any reports of high level turbulence
associated with an upper level trough located about 75 miles west
of Kauai. The trough is moving eastward at 12 kt. A layer of cirrus
clouds, above 20k feet, lies between the trough and Oahu.


&&

.MARINE...
The Saturday evening ASCAT pass sampled eastern and central parts
of the coastal waters, and found large areas of gale force winds,
with estimated winds up to 45 kt in the Alenuihaha Channel.
Overnight surface observations also show very strong northeast
trade winds continuing across the state. The Gale Warning has been
extended through noon today for all Hawaiian coastal waters, and
through midnight tonight for the windiest areas around Maui
County and the Big Island. Small Craft Advisory conditions will
continue across all other waters this afternoon and tonight after
the winds diminish below gale force.

Very strong trade winds and a short-period north swell will
combine to produce surf above the advisory level on east facing
shores of all islands. The north swell will decrease through
Monday, and winds will be gradually easing as well starting later
today, but it will take a while for seas to calm down, and the
High Surf Advisory has been extended for east facing shores
through Monday afternoon.

A new large northwest swell will likely produce surf well above
the advisory threshold along north and west facing shores of the
smaller islands from Tuesday through Thursday morning. Another
large northwest swell may produce advisory-level surf along north
and west facing shores of the smaller islands starting Friday
night and continuing into next weekend. Otherwise, a minor bump
in surf in south facing shores is possible today due to a small
but long period south swell.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until noon HST today for all portions of the
state except for Big Island summits and Haleakala summit.

High Surf Advisory until midnight HST tonight for Niihau and Lanai.

High Surf Advisory until 6 PM HST Monday for east facing shores of
Kauai, Oahu, Molokai, Maui and Big Island.

Gale Warning until midnight HST tonight for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Gale Warning until noon HST today for all remaining coastal
waters.


&&

$$

DISCUSSION...Foster
AVIATION...Lau
MARINE...Jacobson

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