Area Forecast Discussion
Issued by NWS Honolulu, HI

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500
FXHW60 PHFO 160635
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Mon Jan 15 2018

.SYNOPSIS...
Trade winds will increase through Thursday as a strong high
pressure system moves eastward across the Central Pacific basin.
Wind speeds will peak on Wednesday and Thursday into the 15 to 30
MPH range with higher gusts. Shower activity will also increase as
the ridge lifts north and stronger winds help enhance windward
and mauka showers. Expect more isolated shower activity over
leeward sections of each island. Wind speeds are forecast to
weaken from Friday onward as the high pressure center drifts away
from the state into the Eastern Pacific and a low pressure system
moves into the Central Pacific.

&&

.DISCUSSION...
A return to a trade wind weather pattern starts this week with a
west to east high pressure ridge lingering just north of the
Hawaiian Islands this evening. Strong downward vertical motions
(subsidence) under this ridge will continue to suppress windward
and mauka shower activity over the smaller islands for one more
day with trade wind temperature inversion heights ranging from
4000 to 5000 feet.

This ridge will slowly drift north over the next two days and
chances for precipitation will begin to increase tomorrow over the
windward Big Island as a result of decreasing subsidence over the
island with trade wind inversion heights lifting into the 5000 to
7000 foot range. Scattered showers will then spread to the
windward slopes of the smaller islands by Tuesday night into
Wednesday morning. Leeward sections will see only isolated showers
as strong winds cause a few showers to drift into the drier
south and western sections of each island.

Stronger trade winds will develop on Wednesday and Thursday with
forecast guidance consistently indicating near wind advisory
conditions over many wind favored locations of Hawaii and Maui
Counties starting on Wednesday, with strong trade winds spreading
to the entire island chain with wind advisory conditions possible
across much of the state on Thursday.

On Friday, a low pressure system far to the northwest, near 32N
latitude, will move across the Central Pacific. This system will
help weaken the ridge north of Hawaii from Friday through Sunday
producing weaker easterly wind speeds across the islands. The
European (ECMWF) and American (GFS) model solutions diverge a bit
with the depth and track of this system leading to differences in
the strength of the ridge and trade winds. Due to these model run
consistency issues we are blending both the American and European
model guidance for the extended forecast time period.


&&

.AVIATION...
High pressure building north of the state will keep a light to
moderate trade wind flow in place tonight, with trades
strengthening to moderate levels on Tuesday. Fairly dry conditions
are expected to remain in place through the TAF forecast period,
with prevailing VFR conditions at all terminals.

No AIRMETs are currently in effect. AIRMET Tango may be needed for
moderate low level turbulence over and downwind of the terrain of
all islands, late Tuesday or Tuesday night.


&&

.MARINE...
A High Surf Advisory (HSA) is in effect through tonight for most
north and west facing shores on the smaller islands. The current
northwest swell is forecast to slowly subside with surf dropping
to borderline advisory levels on Tuesday.

A low about 1200 miles north of Oahu this evening is producing
hurricane force winds. The low is moving rapidly northeast, so the
swell generated by the low will be rather short-lived, with
advisory level surf expected for north and west facing shores of
the smaller islands from late Tuesday night through Wednesday
night. After that, no large northwest swells are expected through
the weekend.

A Small Craft Advisory (SCA) remains in effect through tonight
for 10 ft seas in waters exposed to the northwest swell. The SCA
for waters around Kauai and Oahu will likely be dropped by morning
as the swell subsides. The SCA for waters around the Big Island
and Maui has been extended for Tuesday and Tuesday night due to
strengthening trade winds. Trade winds will become even stronger
on Wednesday and Thursday. The SCA will likely be expanded to all
waters, with gale force winds possible in the Alenuihaha Channel.
The strong trade winds will produce large seas that will likely
require an HSA for east facing shores late Wednesday or Thursday
through at least Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 AM HST Wednesday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Big Island Windward Waters.

Small Craft Advisory from 6 AM Tuesday to 6 AM HST Wednesday for
Maalaea Bay-Pailolo Channel.

&&

$$

Discussion...Bohlin
Aviation...Jelsema
Marine...Donaldson



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