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Temp5.0 C
RH9 %
WindNE 26 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

806
FXHW60 PHFO 190658
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
858 PM HST Wed Oct 18 2017

.SYNOPSIS...
Windy trades will continue through Thursday due to a surface high
located far north of the islands. The winds are expected to start
trending down on Friday as the high begins to weaken as it moves
eastward. Winds will continue to decrease through the weekend,
with light winds expected Sunday and Monday. Clouds and showers
will remain focused over windward and mauka areas through the
weekend, with the trades carrying some showers to leeward
sections of the smaller islands through Friday.

&&

.DISCUSSION...
The tight pressure gradient south of a 1034 mb surface high
centered near 38N 161W, or about 1185 miles north of Honolulu,
is maintaining very strong and gusty trade winds across the main
Hawaiian Islands early this evening. Aloft, water vapor imagery
shows significant subsidence aloft directly above the islands,
which is indicative of stable atmospheric conditions. This is due
to middle and upper tropospheric ridging, which is nosing in from
the northwest. Loops of satellite imagery show broken low clouds
being transported into the islands by the robust low-level trade
wind flow. Radar reflectivity data show scattered showers embedded
within many of these clouds, especially over and upstream of the
islands from Oahu to Maui. Some of these low clouds and showers
are being transported over to leeward sections of the smaller
islands. Clouds, which were over portions of the leeward Big
Island are continuing to dissipate due to nighttime cooling and
downslope flow.

The gusty trade winds will require the wind advisory to remain in
effect for portions of Maui County and the Big Island through
Thursday. The strongest winds and gusts are expected through
passes, across ridges and over higher terrain. The winds on the
Big Island summits remain relatively light, but we will be
monitoring observations from Mount Haleakala in case they reach
the summit wind advisory criteria later tonight or early Thursday
morning.

The forecast guidance indicates the surface high will move toward
the southeast, and gradually weaken from late Thursday through
Friday. This will likely cause the trades to weaken to breezy
speeds by the end of the work week. Shower bearing clouds
transported by the trades will continue to be focused over
windward and mauka areas through Friday. The low-level flow will
continue to carry a few showers to leeward sections of some of
the smaller islands. The leeward Big Island will have clouds and
a chance of showers each afternoon and evening, with clearing at
night. Rainfall totals are expected to remain relatively light.

As we head into the weekend, the weakened surface high moves to a
position far northeast of the state. This will result in
additional weakening of the pressure gradient over the islands
through Saturday night. By Sunday, a front approaching the
islands from northwest will likely cause the winds to shift to a
southeasterly direction. On Monday as the front moves closer, the
winds may turn more southerly. Additional moisture is expected to
spread over the islands in this low-level southeasterly and
southerly flow. As the winds shift, they will also weaken,
allowing for some local afternoon sea breezes and nighttime land
breezes to become established.

An upper level trough is expected to pass just north of Hawaii
from Monday through Tuesday night. This will likely cause
increased instability. These unstable conditions combined with
increased moisture may result in heavier rainfall early next
week.

&&

.AVIATION...
Windy conditions will continue into Thursday. AIRMET TANGO is in
effect for tempo moderate low level turbulence across leeward
portions of all islands and will continue on Thursday.

Scattered showers embedded in the trade winds will stream over the
state. Most clouds and showers will favor the windward and
mountain areas, but some showers will make it to the leeward areas
under the strong trades. Expect primarily VFR conditions, with
brief MVFR CIGS/VIS within the passing showers.

&&

.MARINE...
Hazardous marine conditions are expected to continue through Friday
due to strong- to gale-force trade winds and rough seas. Latest
buoy observations around the islands reflect this and are holding
within the 8 to 10 ft range. Even higher seas up to 10 to 13 ft
are likely occurring over the windier channel waters. Small craft
advisory headlines will remain likely through Friday over most
waters. Gale conditions are expected to hold over the Pailolo and
Alenuihaha Channels through Thursday.

The latest guidance lines up well with the current pattern and
depicts a weakness within the ridge axis developing north of the
islands over the upcoming weekend. This weakness will be in response
to a frontal boundary approaching from the northwest and a weak
surface trough approaching from the east. Trade winds are forecast
to respond and gradually trend down late Saturday through Sunday.
Seas will drop below advisory-levels (10 ft) through this time.

Surf along east facing shores will remain rough through the rest of
the week due to strong trades locally and upstream of the islands.
The high surf advisory for east facing shores will hold through
Thursday and may need to be extended through Friday. A downward
trend is expected over the upcoming weekend due to the
aforementioned weakness developing within the ridge and a weak
trough moving into the area from the east.

Surf along south facing shores will remain small through Friday with
mainly short-period and small south-southeast energy continuing. An
upward trend is expected once again over the upcoming weekend due to
the recent active trend over the southern Pacific within Hawaii's
swell window near New Zealand. Altimetry and ASCAT data showed a
gale- to storm-force low passing quickly from west to east south
of New Zealand this past weekend with seas peaking southeast of
New Zealand around 40 ft on Sunday. WAVEWATCH III and ECMWF-Wave
solutions reflect this and show long-period energy arriving
locally Saturday, peaking Sunday, then slowly trending down early
next week out of 190-200 deg.

Up north, trade wind energy wrapping into the typical locations are
expected to continue to generate surf through the remainder of the
week. A long-period north-northwest (340 deg) swell is expected to
fill in and peak late Thursday through Friday before slowly easing
over the upcoming weekend. Surf heights are expected to remain below
advisory levels through the peak Friday.

A larger reinforcement out of the northwest (330 deg) is forecast to
arrive Sunday and peak Sunday night into Monday. Recent satellite
data showed a good sized area of gale- to storm-force west-northwest
winds focused toward the islands within the 310-320 deg directional
band over the far northwest Pacific near the Kuril Islands.
Altimetry data from a pass directly over this batch of storm-force
winds showed peak seas up to 26 ft today. No major differences
are shown with regard to the swell arrival time Sunday (ECMWF-
Wave advertising a slightly later arrival Sunday by a few hours).
Surf heights may end up reaching advisory levels along north and
west facing shores through the peak of this swell.

For the long range or outlook through next week, the active pattern
across the northern Pacific is expected to continue, which should
correspond to back-to-back north-northwest swell events Tuesday
through midweek. More on the details here in later packages as
conditions evolve.

&&

.FIRE WEATHER...
The KBDI at Honolulu remains at 576, which is still below the 600
critical threshold. With minimal rainfall expected at HNL over
the next several days, the KBDI will continue to slowly climb.
Gusty trade winds are expected to continue with low afternoon
relative humidity. While the gusty winds and low relative
humidity may meet the Red Flag Warning criteria, the KBDI is
currently the limiting factor. That being said, some leeward areas
across the state may already be experiencing critically dry
conditions. The combination of dry and windy weather each
afternoons is expected to remain through the end of the work
week, before winds ease and humidities begin to rise somewhat
this weekend.

&&

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

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

Small Craft Advisory until 6 AM 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 Thursday for the Pailolo and
Alenuihaha Channels.

&&

$$

Discussion/Fire weather...Houston
Aviation...Foster
Marine...Gibbs

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