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RH9 %
WindNE 10 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180635

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Wed Jan 17 2018

Strong and gusty trade winds are in store through Thursday as
robust surface high pressure passes north of the state. Clouds and
showers will favor windward and mauka areas, though a few showers
will periodically spread leeward on the gusty trades. Winds will
gradually decrease from Friday into the weekend as the surface
high is pushed far northeast of the islands.


Strong and gusty trade winds continue to develop this evening.
A 1036 mb surface high is located about 1,100 miles north of
Kauai, while a decaying front is stalling about 50 miles north of
Kauai. A rather strong ridge aloft over the region is maintaining
a solid inversion based at 3,500 to 5,500 ft, according to the
afternoon soundings. Winds have been steadily ramping up into this
evening, with numerous stations on Maui County and the Big Island
reporting advisory level winds.

A further increase in winds is expected into Thursday as the
surface high passes north of the state and builds to 1037 mb. The
inversion will remain unchanged in most areas, causing the trade
winds to be accelerated over and around island terrain. As a
result, the Wind Advisory remains in effect for all islands. With
models showing winds near the inversion level increasing another
5 kt or so, we will have to closely monitor locations such as the
leeward Kohala districts for the potential for High Wind Warning
conditions. The Wind Advisory currently runs through Thursday
afternoon, though it will likely need to be extended through
Thursday night for the state and possibly into Friday for portions
of the Big Island and Maui.

Showers will focus across windward slopes during the next couple
of days. The atmosphere will remain stable as the inversion holds
firm. The above mentioned decaying frontal band will sag
southward and likely reach Kauai tonight, making windward portions
of the Garden Isle wetter than most areas. This band of moisture
is expected to slowly lift northward late Thursday and Friday. A
separate area of poorly-defined low level moisture is expected to
fill in across windward Big Island tonight, then spread up the
island chain on Friday.

Trade winds will begin to decrease on Friday and continue
decreasing into the weekend. During this time, the surface high
will weaken and move to the northeast of the state, leaving an
associated surface ridge about 300 miles north of Kauai by Friday
night. A North Pacific trough will erode the surface ridge during
the weekend. During this time, trade winds will veer out of the
east to east- southeast and may become light enough for daytime
sea breezes to dominate across the western end of the island

Beyond the weekend, models are not in good agreement on the
details of a developing low near Midway Atoll. However, models do
show moderate easterly trades hanging on over the Big Island with
southeasterly flow across the smaller islands. The mid level
ridge currently over the state will likely erode significantly,
leading to some instability and the potential for a few heavier


Robust high pressure passing N of the area will support strong
trade winds through Thursday while a ridge aloft will maintain a
stable island atmosphere. This will lead to strong low-level
winds, with AIRMET TANGO in effect for moderate to severe turbulence
near and downwind of the terrain, and for surface wind speeds.

Two bands of low clouds embedded within the trade flow will make
for locally cloudy conditions, with a stalled frontal band moving
over Kauai from the E, and a more diffuse cloud band near and
upwind of the Big Island, with fewer clouds over Maui county and
Oahu. Radar is detecting isolated to scattered light showers
within these cloud bands, and thus far AIRMET SIERRA for mountain
obscuration is not anticipated, and observations will be monitored
for its possible issuance.


For the evening update, the high surf warning for north and west
facing shores has been lowered to an advisory this evening based on
the latest buoy observations and trends. Surf will dip below
advisory levels early Thursday for north and west facing shores.
Along east facing shores, a high surf advisory is now in effect
from 6 AM HST Thursday through 6 PM HST Friday due to strong
trades locally and upstream.

The ASCAT pass earlier today showed strong- to gale-force winds over
the local waters (gales were shown across the Alenuihaha Channel),
which is in line with the current forecast package. Seas at the
exposed nearshore PacIOOS buoys reflect this and show 8 to 12 ft due
to a combination of rising wind waves and a moderate northwest swell
that is trending down. Strong- to gale-force winds are forecast to
hold through Friday across the waters as strong high pressure shifts
eastward to the north. As a result, the small craft advisory for all
waters remains in effect through Thursday night and will likely be
extended for most waters through Friday in later packages.

For the upcoming weekend, forecast confidence begins to lower due to
significant differences shown between the ECMWF and GFS solutions.
The GFS advertises light and variable winds, while the ECMWF depicts
moderate to fresh trades holding. The current forecast is more in
line with the GFS solution, but is subject to changes over the
coming days. Despite these differences, winds will likely trend down
over the weekend, which will allow surf along east facing shores to
begin lowering, as well as seas across all waters.

A small northwest swell is expected Sunday, that will be reinforced
with a moderate northwest swell by Monday, potentially reaching
advisory levels. This reinforcement will peak by late Monday, then
gradually lower through midweek.


Wind Advisory until 6 PM HST Thursday for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala-Kona-
South Big Island-Big Island North and East-Kohala-Big Island

High Surf Advisory until 6 AM HST Thursday for Niihau-Kauai
Leeward-Waianae Coast-Oahu North Shore-Molokai Leeward-Maui
Central Valley.

High Surf Advisory until 6 PM HST Friday for Kauai Windward-Oahu
Koolau-Molokai Windward-Maui Windward West-Windward Haleakala.

High Surf Advisory from 6 AM Thursday to 6 PM HST Friday for
Olomana-South Big Island-Big Island North and East.

Gale Warning until 6 AM HST Friday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

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-Big Island
Windward Waters.




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