Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 180632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Tue Oct 17 2017

.SYNOPSIS...
High pressure north of Hawaii will keep a windy trade wind
pattern in place through much of the work week, with the trades
trending down slightly Friday and weakening further over the
weekend. Clouds and showers will favor windward and mauka areas
through the period, but showers will reach leeward areas from
time to time as well due to the strength of the trades.

&&

.DISCUSSION...
Windy conditions will continue to be the main weather player
across the state over the rest of the work week. High pressure
around 1030-1034 mb expected to remain to the north of the state
will continue to result in a pressure gradient supportive of
strong winds, locally up to Wind Advisory conditions with gusts
over 50 mph possible in the windiest areas around Maui County and
the Big Island through late Thursday. Along with the winds,
pockets of showers riding along in the trades will continue to
pass over the state. The showers will focus over windward areas,
but occasionally carry to leeward areas due to the strong winds.
Rain totals will remain generally on the light side for both
windward and leeward areas.

The pattern is expected to shift as we go into the weekend. The
surface ridge to the north shifts eastward and is replaced by a
trough, which carves out from the mid latitude westerlies
somewhere to the north or northwest of the state by Sunday. Trades
become weak and begin to shift more southeasterly by Sunday and
more southerly on Monday. Most areas should be fairly dry
Saturday, but tropical moisture is expected to come up from the
southeast and spread over the state on Sunday (Big Island) and
Sunday night (smaller islands). The additional moisture will
provide more shower activity around the state, particularly as
sea breezes direct the showers onto the islands Sunday and Monday
afternoons. In addition, an upper trough will dig towards Hawaii
Monday through Tuesday night resulting in unsettled weather in the
state during that time period.

Confidence in this pattern shift happening is higher than normal
as the GFS ensemble spread is relatively small compared to typical
for Day 6 and 7 digging trough, and with ECMWF in good agreement
with GFS.


&&

.AVIATION...
High pressure north of the state will keep windy trades in place
across the island chain tonight and Wednesday. Clouds and showers
will favor windward and mauka areas, but will also spread leeward
due to the strength of the trades. Brief MVFR cigs/vsbys will be
possible in passing showers, but predominantly VFR conditions are
expected through 06Z Thursday.

AIRMET Tango is in effect for moderate upper level turbulence
between FL250 and FL370 due to a jet stream in the vicinity of the
islands. The upper level turbulence is expected to continue
overnight, with some improvement expected on Wednesday.

AIRMET Tango is also in effect for moderate low level turbulence
over and downwind of the terrain of all islands. This AIRMET will
likely continue through Thursday night and possibly Friday.


&&

.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 11 ft range. Heights within the 12 to 15 ft range are
likely occurring over the windier locations, especially across
the Pailolo and Alenuihaha Channels and waters south of the Big
Island where the strongest winds are. Satellite altimetry data
within the past 12 hrs even showed wave heights up to 14 ft west
of Kauai. Small craft advisory headlines will remain likely
through Friday over most waters. Gale conditions will likely hold
over the Pailolo and Alenuihaha Channels through Wednesday night
and may need to be extended through the day 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
Wednesday and likely need to be extended through Thursday. 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 continue to trend down through
Thursday as the south swell that peaked Monday eases. Mostly shorter-
period and small southeast energy will be expected Wednesday through
Friday. An upward trend is expected once again over the upcoming
weekend due to the ongoing active pattern over the southern
Pacific within Hawaii`s swell window. 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 shown long-period energy
arriving locally by Saturday, peaking Sunday, then slowly trending
down early next week out of 190-200 deg.

Mainly trade wind energy wrapping into the typical locations are
expected for surf along north facing shores into Thursday. A
long-period north-northwest (340 deg) swell associated with a
storm-force low passing from west to east across the far northern
Pacific 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.


&&

.FIRE WEATHER...
Gusty winds and low afternoon relative humidities will continue to
result in the possibility of rapid fire spread in areas where
fuels have dried sufficiently. KBDI has crept up closer to
critical red flag thresholds at Honolulu, but remains just below
levels required for red flag warnings to be issued. Will continue
to closely evaluate fuel conditions. However, local leeward areas
across the state may already be experiencing critically dry
conditions. The combination of dry and windy conditions in the
afternoons is expected to remain through the work week, before
winds ease and humidities rise somewhat over the weekend.


&&

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

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

Gale Warning until 6 AM HST Thursday for Pailolo Channel-
Alenuihaha Channel.

Small Craft Advisory until 6 AM HST Thursday for all remaining
Hawaiian coastal waters.


&&

$$

DISCUSSION/FIRE WEATHER...Foster
AVIATION...Jelsema
MARINE...Gibbs



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