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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 191410

Area Forecast Discussion
National Weather Service Honolulu HI
410 AM HST Thu Oct 19 2017

Strong and gusty trades will continue today 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
southeastward. 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 brief showers to
leeward sections of the smaller islands through Friday night.


The tight pressure gradient south of a 1033 mb surface high
centered near 37N 159W, or about 1100 miles north of Honolulu, is
maintaining very strong and gusty trade winds across the main
Hawaiian Islands early this morning. Water vapor imagery shows
significant subsidence aloft directly above the islands, which is
indicative of stable atmospheric conditions. This is primarily
due to a mid-tropospheric ridge, which is nosing in from the
northwest across the state. Loops of infrared satellite imagery
show scattered to broken low clouds being transported into the
islands by the robust low-level trade wind flow. Radar
reflectivity data indicate scattered showers embedded within many
of these clouds. Some of these low clouds and showers are being
transported over to leeward sections of the smaller islands.

The very strong and gusty trade winds will require the wind
advisory to remain in effect for portions of Maui County and the
Big Island through this afternoon. 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

The forecast guidance indicates the surface high will move toward
the southeast, and gradually weaken from tonight 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 east
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. Also, volcanic haze (or "vog"),
will likely spread up from sources on the Big Island across most
of the state starting later this weekend, and continuing into
early next week.

By Tuesday the front may push down over the western islands, with
cloudy skies and widespread rainfall ahead of and along the
boundary. Since an upper level trough is expected to pass
somewhere north of Hawaii from Monday through Tuesday night, there
may also be increased atmospheric instability. These unstable
conditions combined with the enhanced moisture may result in
periods of locally heavy rainfall early next week.


Windy conditions will continue today and tonight, then will
weaken through the weekend. AIRMET TANGO is in effect for tempo
moderate low level turbulence across leeward portions of all
islands and will continue through at least tonight.

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.


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 all
waters due to a combination of winds and seas (10 ft). Gale
conditions are expected to hold over the Pailolo and Alenuihaha
Channels through the day today.

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
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. Surf will possibly reach advisory
levels once again Sunday through Monday.

Up north, trade wind energy wrapping into the typical locations are
expected to continue to generate surf into the weekend. A long-
period north-northwest (340 deg) swell is expected to fill in and
peak tonight 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 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. Additionally, a long-period westerly swell from
Typhoon Lan in the far western Pacific can't be ruled out by
midweek. More on the details in later packages as conditions


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
afternoon is expected to remain through the end of the work week,
before winds ease and humidities begin to rise somewhat this


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

Wind Advisory until 6 PM HST this evening 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 this evening for the Pailolo and
Alenuihaha Channels.



Discussion/Fire Weather...Houston

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