Area Forecast Discussion
Issued by NWS Honolulu, HI
FXHW60 PHFO 190620
Area Forecast Discussion
National Weather Service Honolulu HI
820 PM HST Sat Feb 18 2017
A dissipating cold front moving down the island chain will bring passing
clouds, brief showers, and gusty north winds. The front will reach
Molokai this evening, Maui overnight, and the Big Island Sunday
morning, focusing clouds and showers over windward areas and
northern slopes. After the front passes, a mostly dry and gusty
trade wind weather pattern will prevail into Monday, with
northeast winds stacking low clouds along windward slopes. Breezy
trade winds and cool weather will continue into the middle of next
week, with clouds and brief showers favoring windward areas,
especially during nights and mornings.
A weak, shallow front is moving through the smaller islands
tonight with strong and gusty north winds following the front.
Clouds and showers will mainly affect north slopes and windward
areas of the islands with only light to moderate rainfall totals
expected. A few showers will reach leeward locations as the strong
winds push moisture over the terrain but these will be brief in
nature. Some clouds and showers will get hung up along the
northern slopes as the low level winds push perpendicular into
those slopes allowing for some higher rainfall totals in those
The front has passed Oahu and will cross Molokai this evening,
Maui overnight, and reach the Big Island Sunday morning, before
moving south of the Big Island Sunday night. Gusty north winds
are already occurring over Kauai and exposed sections of Oahu as
high pressure builds in from the north. Winds will accelerate over
and through terrain and a wind advisory may be necessary for
those areas that enhance the winds the most, such as over Lanai,
the Maui central valley, Kahoolawe, and areas within the Kohala
mountains on the Big Island. Winds in those areas are expected to
be strongest Sunday, and if the front remains on track, an advisory
will be issued with the 4am forecast package.
High pressure behind the front will push in dry air from the north
with associated precipitable water values in the 0.6 - 0.8 inch
range. This dry air will limit most clouds and showers to the
windward slopes Sunday and Monday as the front pushes south of the
Big Island. Gusty trade winds will remain through Wednesday as the
high travels north of the islands, with limited showers focused
over windward and mountain areas. While another high is expected
to build NW of the islands by the middle/end of next week, a
developing low E of the high may act to limit any local increase
in wind speeds. Latest GFS and ECMWF guidance indicate that
light to moderate winds will back to the NE, keeping a cool and
relatively dry weather pattern in place.
A weakening cold front will continue to move southeastward down
the island chain tonight and Sunday, moving through Maui County
later tonight and through the Big Island on Sunday. A band of
lower clouds and showers will move through the islands in
association with the front, with windward and mauka areas the most
likely to see any restriction to ceilings and visibilities.
Behind the front, strong north to northeast winds will develop and
remain in place through the day on Sunday.
AIRMET Sierra is in effect for mountain obscuration across
windward sections of Oahu, Molokai and the Big Island. The AIRMET
will likely need to be expanded to include windward sections of
Maui later tonight as well. Conditions should improve by 19/1000z
in Oahu, but will likely continue beyond 19/1000z across Molokai
and the Big Island.
AIRMET Tango is in effect for moderate low level turbulence over
and downwind of the mountains on Kauai and Oahu. This AIRMET will
likely be expanded down the island chain tonight into Sunday as
winds pick up behind the cold front.
Strong to gale-force winds associated with strong high pressure
building north of the state in the wake of a cold front are expected
Sunday through mid week. The best chance for gales will be across
the Pailolo and Alenuihaha Channels and Maalaea Bay beginning
Sunday, which are expected to persist into the upcoming week before
trending down. Local winds will quickly fill in out of the north
through the overnight hours tonight, then veer to the northeast
Sunday and toward the east by Monday night. The evening observations
at the Barking Sands station on Kauai have already begun to reflect
these strong northerly winds with readings ranging from 20 to 25 kt.
These winds will generate very rough/short-period seas across the
local waters. A Gale Warning for the aforementioned channels and
Maalaea Bay will be issued later tonight.
In addition to the winds and seas, a large north-northwest (330-350
deg) swell will fill in across the waters Sunday, peak Sunday
evening/overnight, then gradually ease Monday through Tuesday. This
swell is associated with a storm-force low that was analyzed just
under 1000 nautical miles north of the state this evening. Provided
the proximity of this storm to the local area, wave periods will
come in within the 11-13 second energy band, which are shorter
periods than typically observed from our large northwest swells.
Surf along exposed north and west facing shores will quickly respond
and reach advisory levels Sunday, which should persist through
Monday before trending down. Later packages may need to include the
northern shores of the Big Island as the storm passes well to the
north Sunday night through Monday.
Surf along east facing shores will gradually rise through the week
and near advisory levels Tuesday through Wednesday as the winds veer
out of the east.
High Surf Advisory from 6 AM Sunday to 6 PM HST Monday for
Niihau-Kauai Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-
Molokai-Maui Windward West-Maui Central Valley-Windward
Small Craft Advisory from 6 AM Sunday to 6 PM HST Monday for Big
Island Windward Waters-Big Island Leeward Waters-Big Island
Gale Watch from Sunday afternoon through Monday afternoon for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel.
Small Craft Advisory until 6 PM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maalaea
Bay-Pailolo Channel-Alenuihaha Channel-Maui County Windward
Waters-Maui County Leeward Waters.