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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 202001
Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Mon Nov 20 2017
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.
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
rather dry and stable. Balloon soundings at 200 AM showed strong
temperature inversions near 8000 feet at Lihue and 7000 feet at
Hilo. The north winds are carrying isolated showers over north
sections of the islands but rainfall has been quite light.
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
gradually shift out of the northeast. The northeast trade winds will
carry some showers over windward areas, especially during the nights
and mornings, while leeward areas will remain mostly dry.
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.
VFR conditions will prevail, but showery clouds moving south over
the area will produce brief MVFR ceilings and visibility over north
sections. No AIRMETs are in effect and none are expected for today.
A High Surf Advisory (HSA) remains in effect for north facing shores
of all the islands except Lanai. Surf along the north facing shores
of the state remains elevated this morning. Although the buoy data
continue to show a downward trend in both swell size and period,
wave heights remain higher than the Wave Watch III model guidance.
Surf may remain above the advisory threshold beyond the 600 PM
expiration time of the current HSA. A final decision on extending
the HSA will be made later today after assessing additional
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 SCA
may need to be extended beyond the current expiration time of 600 PM
if buoy observations don't show a more significant decrease during
Winds over the coastal and offshore waters are generally from the
northwest to north direction due to a low pressure system about 900
miles northeast of the state. The low is in the process of weakening
and moving northeastward away from state. 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 SCA levels Wednesday with the possibility of
gales 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 Wednesday night 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.
High Surf Advisory until 600 PM today for north facing shores of
Niihau, Kauai, Oahu, Molokai, Maui and the Big Island of Hawaii.
High Wind Advisory until 600 PM today for the summits of Mauna Kea
and Mauna Loa on the Big Island.
Small Craft Advisory until 600 PM today for all coastal waters
except Maalaea Bay and waters south and west of the Big Island.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office