Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 161408

Area Forecast Discussion
National Weather Service Honolulu HI
408 AM HST Tue Jan 16 2018

A high pressure ridge just north of the Hawaiian islands will
slowly lift northward today increasing chances for windward and
mauka showers over the next 24 hours. Trade wind speeds will also
increase each day 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 near the islands lifts north and stronger winds help
enhance windward and mauka showers. Expect 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.


A trade wind weather pattern continues through the week with
typical windward and mauka clouds and showers along with isolated
showers over leeward sections. A west to east high pressure ridge
just north of the Hawaiian Islands will slowly drift northward
today. Strong downward vertical motions (subsidence) under this
ridge will continue to suppress windward and mauka shower activity
over the smaller islands with trade wind temperature inversion
heights ranging from 4000 to 5000 feet.

Chances for precipitation will increase this afternoon over the
windward Big 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 later tonight
into tomorrow morning. Leeward sections will see isolated showers
as strong winds cause a few showers to drift into the drier south
and western sections of each island.

Stronger trade winds are expected on Wednesday and Thursday with
forecast guidance consistently indicating near wind advisory
conditions on Wednesday over many wind favored locations of Hawaii
and Maui Counties. On Thursday, wind advisory conditions will
likely spread across the entire island chain affecting most of the

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 the trade winds. Due to these model run consistency
challenges we continue to blend both the American and European
model guidance from Friday onward. The latest 16/00Z European
model run shows a deeper low over the Central Pacific than
previous model runs and the European model appears to be trending
towards the American GFS solution over time.


High pressure building north of the state will strengthen the
trade winds to moderate levels today and locally breezy levels
tonight. Fairly dry conditions are expected to remain in place
through the day. Some brief MVFR cigs will be possible across
windward areas, but prevailing VFR conditions are expected at all
terminals. MVFR cigs and/or vsbys will be more common in passing
showers for windward areas tonight, with VFR conditions expected
in leeward areas.

No AIRMETs are currently in effect. AIRMET Tango may be needed for
moderate low level turbulence over and downwind of the terrain late
this afternoon or tonight.


A High Surf Advisory (HSA) is in effect through Wednesday for most
north and west facing shores on the smaller islands. The current
northwest swell is forecast to slowly subside with surf dropping
below the advisory threshold. However, a low about 1200 miles
north of Oahu this evening is producing hurricane force winds.
The swell from this low will boost surf back up above the
advisory threshold late tonight. After that, no large northwest
swells are expected through the weekend.

The Small Craft Advisory (SCA) for waters around Kauai and Oahu
due to the large northwest swell has been dropped. The SCA for
waters around the Big Island and Maui has been extended for
today and tonight due to strengthening trade winds. The SCA will
be expanded tonight to include additional waters around Maui and
Oahu. The SCA will likely be expanded to all waters, with gale
force winds possible in the windiest zones. A Gale Watch is now in
effect for waters around the Big Island and Maui. 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.


High Surf Advisory until 6 PM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward

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

Gale Watch from Wednesday morning through late Thursday night
for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 AM HST
Wednesday for Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters.



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