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Area forecast discussion
National Weather Service Honolulu HI
346 PM HST Mon Nov 20 2017

Synopsis...
a surface low about 1000 miles northeast of the islands will move
away to the northeast. A surface high far north of the area will
move southeast through Thanksgiving day, then dissipate. A strong
new surface high will build far northwest of the islands on
Wednesday, then move southeast through early next week. A trough
aloft will remain over the area.

&&

Discussion...
the pressure gradient between a surface high far north of the area
and a low to the northeast is producing gentle to fresh north winds.
Satellite water-vapor imagery shows a trough aloft extending north
from over the islands. Despite the trough, the atmosphere remains
dry and stable. Balloon soundings at 200 PM showed strong temperature
inversions near 6900 feet at Lihue and 5000 feet at Hilo. The north
winds are carrying isolated showers over north sections of the
islands but rainfall has been quite light. Rainfall should
generally remain light, but the north winds could boost rainfall
over the northeast slopes of The Big Island.

The trough aloft continues to produce strong west winds over the
summits of Mauna Kea and Mauna Loa on The Big Island of Hawaii. A
Wind Advisory is in effect for the summits. The winds are expected
to gradually subside as the trough weakens.

Over the next couple of days, the surface low northeast of the area
will move northeast and weaken while the surface high moves
southeast and weakens. Winds over the islands will shift out of
the northeast by Tuesday. The global models show a low-cloud band
moving over Kauai County from the northwest Tuesday afternoon,
moving over Oahu and Maui County Tuesday night, and pushing down
over The Big Island by Wednesday evening. This cloud band will
bring an increase in clouds and showers to windward areas.

Starting Wednesday night, a strong new surface high far northwest of
the islands will start moving southeast. The northeast trade winds
over the islands will strengthen and become breezy. Wind speeds will
likely decrease again starting Saturday, but trade winds will likely
persist through at least early next week.

&&

Aviation...
cool northerly winds will push a band of clouds and showers over
the islands tonight. This band is currently about 50 nm north of
the island chain this afternoon. VFR conditions prevail at this
time, but an airmet for tempo mountain obscurations may be issued
later tonight as the band of clouds approaches. The highest
coverage for shower activity will occur over mountain areas and
the northern slopes of all islands.

&&

Marine...
surf along the north-facing shores of the state remains elevated.
Buoy observations show a slow downward trend but heights remain
above wave watch iii guidance. The effect of the low bias in the
guidance is that surf height will remain above high surf advisory
(hsa) criteria through Tuesday morning. The north swell is
expected to diminish Tuesday afternoon.

A Small Craft Advisory (sca) remains in effect for all Hawaiian
waters except Maalaea Bay and waters south of The Big Island due
to the large north swell causing the combined seas to be above the
10-foot threshold. With wave heights running above guidance, the
Small Craft Advisory has been extended until Tuesday morning.

Winds over the coastal and offshore waters are generally from the
northwest to north direction due to a low pressure system northeast
of the state. The low is in the process of weakening and moving
northeastward away from the island chain. As this occurs, a high
pressure ridge will slowly build north of the forecast area
through the week. Winds will slowly veer toward a trade wind
direction and increase, reaching Small Craft Advisory levels Wednesday with near
gale-force winds in some of the channels by Thursday.

In addition to the increasing trades, a new north-northeasterly
swell is forecast to arrive later this week. The swell is
currently projected to be a larger than the current swell with a
slightly longer period. This will translate to possible warning
level surf for north facing shores building late Wednesday and
peaking on Thanksgiving day. The swell direction will result in
some wrap into the east facing shores so warning level surf is a
possibility for these areas as well.

&&

Hfo watches/warnings/advisories...
high surf advisory until noon Tuesday for north facing shores of
Niihau, Kauai, Oahu, Molokai, Maui and The Big Island of Hawaii.

High Wind Advisory until noon Tuesday for the summits of Mauna
Kea and Mauna Loa on The Big Island.

Small Craft Advisory until noon Tuesday for all coastal waters
except Maalaea Bay and waters south and west of The Big Island.

&&

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